Deposition of Eosin Y / ZnO film

Before EY / ZnO films were deposited on textile Au electrodes, the deposition of EY / ZnO films on Au electrodes was tested since the activity of Au is different from ITO. The total amount of charge during the deposition is shown in Table 3 for the film deposited at -0.9 V vs.SCE for 30 min. The growth of the film is strongly influenced by the substrate.

Table 3; The total amount of charge (mC cm^-2) during the deposition on different substrates. The films were deposited at -0.9 V vs.SCE for 30 min in the electrolyte contains 0.1 M Zn(NO3)2, 50 µM EY. Zn rod was used as counter electrode.

Substrate ZnO EY / ZnO

SnO2 202.5 150

Au 840 510

Textile Au 8901.5 6347

After the electrochemical deposition of ZnO or EY / ZnO hybrid thin films, the deposition of the films were observed. Since SnO2 is more transparent, the milky color of ZnO film could be observed clearly. However, the deposition of the film was not so clear since Au and Au textile electrode had shiny Au color. Some depositions were confirmed by observing at the edge part of the film; the milky color of the ZnO could be observed at the border between bare Au electrode and ZnO deposited Au electrode. In the case of EY / ZnO, the reddish color of EY was clearly observed from both Au and Au textile electrode.

During the deposition of ZnO, 202 and 840 mC cm^-2 of charges through SnO2 and Au conductive substrate respectively were passed. More than 3 times higher amount of charge was found for Au electrode. It is caused by the different activity between fluorine doped SnO2 and Au electrode. Similar results were seen during the deposition of one-step EY / ZnO films. 150 and 510 mC cm^-2 of charges through SnO2 and Au conductive substrate respectively were passed. Smaller amount of charges comparing with the deposition ZnO is caused by the disturbance of the reaction by dyes in the deposition bath.

６０ More than 10 times larger amount of charges were passed during the deposition in the case of textile electrode. It indicates the higher surface area of the electrode. The benefit of the electrochemical deposition can be seen that the electrochemical deposition can be applied to any shape of conductive electrode, even on textile, porous substrate, since film preparation is carried out in the soft process. In dye-sensitized solar cells, the porosity of the electrode gives the strong influence to its photoelectrochemical properties. The improvement of the dye-sensitized solar cells have been achieved by increasing the surface area simply.^{６０,７１} The electrochemical deposition method can prepare thin layers of films on porous electrodes. The preparation of dye-modified ZnO thin films on textile electrodes confirms those benefits of the electrochemical deposition method.

-1.1 -1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 0

10000 20000 30000 40000 50000 60000 70000

Passed charge during deposition / mC cm-2

Deposition potential / vs.Ag/AgCl

Fig. 4.5; The total amount of charge passed during the deposition at different potential

In Fig. 4.5, the total amount of charges passed through the electrode during the deposition at different potentials is shown. The amount of charges is significantly higher than in the preparation of EY / ZnO hybrid thin film on SnO2 substrate as described above. The charges of about 350 and 63000 mC cm^-2 were found after the deposition at -0.46 and -1.06 V vs. Ag/AgCl respectively. Approximately 180 time larger amount of charges were found during the deposition at negative potential. The higher activity of Au electrode reflects as the exponentially increased charges. However, it should be noted that degassing of oxygen from the deposition bath was not carried out before the deposition. At negative potential, there are four possibilities for the reaction; the reduction of 1, nitrate, 2, oxygen, 3, Eosin Y and 4, hydrogen. From these reactions, it is not clear how much reaction from oxygen, hydrogen and EY are contributing the current during the deposition.

But concerning the concentration of nitrate, it can be assumed that the reaction of nitrate dominates the ZnO deposition.

The amount Zn atoms in the film was estimated by AAS measurement. (Fig. 4.6) 2.4 x 10^-6 and 8.4 x 10^-4 mol cm^-2 of Zn atoms at the deposition potential of -0.46 and -1.06 V vs.

Ag/AgCl respectively were detected from EY / ZnO textile electrode. The ratio of the difference has a nice agreement with the difference in the amount of charge. It was confirmed by this measurement that Zn atoms were on the textile electrode since it was not clear from the observation by eyes whether there was ZnO on the Au textile electrode due to its transparence property except for EY. The Faraday efficiency was calculated from the amount of Zn atoms comparing with the amount of charge passed during the deposition and more than 60 % of Faraday efficiency was found from all cases. However, it is not clear whether the amount of Zn atoms corresponds to the amount of ZnO molecules or not, since crystalline ZnO has not been proven by qualitative analysis like XRD. The deposition of ZnO on Au textile electrode can be assumed since those electrodes show the

properties to work as photoelectrode by generating the photocurrent (shown in section 4.2.3) although it still does not mean that all Zn atoms exist as ZnO on the film, there is a possibility to deposit as Zn(OH)2 on the film.

-1.1 -1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4

0.0 1.0x10^-4 2.0x10^-4 3.0x10^-4 4.0x10^-4 5.0x10^-4 6.0x10^-4 7.0x10^-4 8.0x10^-4 9.0x10^-4

the amount of zinc / mol cm-2

Voltage / vs.Ag/AgCl

Fig. 4.6; The amount of Zn atoms in the film prepared at different potential

-1.1 -1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4

0 1 2 3 4 5 6 7 8

The amount of dye loaded / 10-8 mol cm-2

Deposition Voltage / vs.Ag/AgCl

Fig. 4.7; The amount of dye loaded to the film prepared at different potential

The amount of dye on the film was estimated by measuring absorption spectroscopy of the NH3 aqueous solution in which the film was dissolved. Rather high amount of dye molecules were found in the range of 10^-8 mol cm^-2 in all films. Surprisingly, there is no correlation between the amount of dye loaded and the amount of Zn atoms in the film. A relatively high concentration of EY was found in the film deposited at the positive potentials such as -0.46 and -0.56 V vs. Ag/AgCl.

Taking into account the molar ratio between the amount of Zn and amount of dye in the film, the molar ratio is 10000:1 and 100:1 in the film deposited at -1.06 and -0.46 V vs.

Ag/AgCl respectively. In both case, it was found that relatively high amount of Zn was detected compared with the amount of dye loaded when the ratio of 1:1 was expected as ideal. In EY / ZnO hybrid thin films deposited at less than -0.9 V vs. SCE, it was reported that EY molecular produced a complex with Zn and precipitation of ZnO occur with the complex.^６０ In this case, the deposition of the EY / ZnO hybrid thin film takes place with enclosing the EY molecular in the film. Simply, EY molecules are not on the surface of ZnO.

On the other hand, if the potential is more negative than the redox potential of EY, EY molecular attaches after ZnO is created.^６０ In this case, EY molecules stay at the surface of ZnO since the growth of ZnO continues from the site where EY has not adsorbed. From those previous works, it can be expected also for those textile electrodes that the dyes are in the film deposited at -0.46 V vs. Ag/AgCl and the dyes are on the surface of ZnO film prepared at -1.06 V vs.Ag/AgCl. It has been reported the positive shift of the redox potential of EY was caused by the existence of Zn²⁺ ions.^６０ Such shift was observed in the concentration of Zn²⁺ larger than approximately 10^-4 M in the electrolyte and the slope of the shift is approximately 64 mV / decade. This was measured with ITO. It can be expected that same phenomenon takes place with textile electrodes. Nevertheless, the deposition potentials of -0.46 and -1.06 V vs. Ag/AgCl are crossing the EY redox potential even if the shift of the EY redox potential for textile electrode is bigger than the case of ITO. Relatively higher amount of dye in the film prepared at positive deposition potential can be understood by the fact that ZnO precipitate following the formation of a complex by Zn²⁺ ions and EY molecules. On the other hand, relatively small amount of the dye in the film deposited at a negative potential can be understood by considering that the adsorption rate of EY is slower than the growth rate of ZnO particles. The higher activity of Au than SnO2 leads the faster growth of ZnO. It can be seen from much bigger amount of the charge during the deposition. Then the creation of ZnO particles occurs mainly earlier than the adsorption of EY on the surface. Those different mechanisms of the deposition and different kinetics cause the different ratio of dye and Zn in the film significantly.