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3. Experimental

4.2. Electrochemical deposition of dye / ZnO hybrid thin film on Au and conductive

4.2.3. Photoelectrochemical characterization

From Fig. 4.10 (A), the film was observed at low resolution of x 100. In such a low resolution, the trace of the textile electrode can be seen slightly. It is observed that the film covers the electrode fully. Some holes can be seen in this image and it indicates the gas creation at the Au textile electrode. The reaction to produce the gas at the electrode is competing with the other reactions, mainly with the deposition of ZnO. It can be expected that the reduction of nitrate which leads the creation of ZnO dominates the reaction at the electrode and at some parts the reduction of hydrogen can take place. The point, where the reduction of hydrogen occurs, continues the reaction and such points stays as it is during the deposition and be left as hole after the deposition.

Rather flat structure can be observed from high resolution image (Fig. 4.10 (C)). And each particle sticks strongly each other, a crack to break such connections is observed. When the creation of the gas does not occur, the continuous deposition can be expected from such places. Such homogenous structure, however, might decrease its photoelectrochemical properties since it does not seem to have large surface area. Generally it was found that one-step electrodeposited films fill their fine pores by adsorbed dyes.69 Relatively small surface area can be expected form the image shown here.

The deposition on the textile electrode was confirmed by the analysis such as AAS and SEM. Characteristic structures were seen from the films prepared at different deposition potential and it seems that the deposition potential will influence strongly their porosity of the film, which influences also their photoelectrochemical properties. The photoelectrochemical properties of those films are studied in the next section.

0.0 0.1 0.2 0.3 0.4 0.00

0.01 0.02 0.03 0.04 0.05

Current / mA cm-2

Time / sec

Fig. 4.11; Time-resolved photocurrent measured for EY / ZnO hybrid thin film deposited on an Au textile electrode at a potential of -0.76 V vs. Ag/AgCl for 60 minutes. A mixed solvent of ethylene carbonate : acetonitrile =4:1 (in volume) which contains 0.5M KI was used as an electrolyte.

Time-resolved photocurrent measured for EY / ZnO thin film deposited at -0.76 V vs.

Ag/AgCl for 60 minutes on an Au coated textile electrode are shown in Fig. 4.11. The electrode shows the photocurrent response for the illumination, the current is increased quickly as the shutter opened. It is confirmed that this film works as a photoelectrode even if the film was prepared on the textile. The magnitude of the photocurrent was almost stable during the illumination for 200 ms. It indicates that no recombination is taking place in the device. The photocurrent of approximately 47 µA cm-2 is detected. This is a relatively high photocurrent in the study of one-step electrodeposited film although it is not high enough for the application to dye- sensitized solar cells. The reason of this high photocurrent are following; the high photochemical property of Eosin Y as reported before

60 and the relatively high surface area due to the textile structure comparing with conductive glass substrate. However, it should be mentioned that the electrode size of textile for the measurement might be underestimated since the electrode size was defined from its two dimensional size.

The surface morphology of this film is shown in Fig. 4.8. As mentioned above, the porous film was deposited on the textile electrode. Since no recombination evidence observed in the time-resolved photocurrent, it can be considered that such structure is one of the ideal as a photoelectrode. And moreover, considering the molar ratio of Zn atoms and dye in the film, it was found as approximately 1000:1. Such ratio can also be a reason for the ideal shape of the time resolved photocurrent.

0.0 0.1 0.2 0.3 0.4 -0.002

-0.001 0.000 0.001 0.002 0.003 0.004 0.005

Current / mA cm-2

Time / sec

Fig. 4.12; Time-resolved photocurrent measured for EY / ZnO hybrid thin film deposited on Au textile electrode at a potential of -0.46 V vs. Ag/AgCl for 60 minutes. A mixed solvent of ethylene carbonate : acetonitrile =4:1 (in volume) which contains 0.5M KI was used as an electrolyte.

The film deposited at -0.46 V vs.Ag/AgCl for 60 minutes showed a characteristic shape of time- resolved photocurrent. (Fig. 4.12) An overshoot could be observed in the beginning and the end of the illumination and the relaxation of the photocurrent were observed during the illumination. Such shape of the time- resolved photocurrent was observed earlier.64,83 After the absorption and excitation of electrons in sensitizer molecules and the injection of the electrons to the conduction band of the semiconductor, the oxidized sensitizer can not be supplied with an electron from the redox electrolyte since the oxidized sensitizers have only a poor contact with the redox electrolyte. Generally, the overshoot in the beginning can be understood by the electrons injected initial moment of the illumination and the relaxation is caused by decreasing number of the photoactive dyes in the film. Such oxidized dyes after the injection once stay as oxidized states during the illumination and those dyes require the electrons to recover from the oxidized condition. Since those dyes do not have a contact with the electrolyte, the electrons are taken from ZnO, such movement of the electrons is detected as an anodic peak at the end of the illumination, which was also observed in the Fig. 4.12. These phenomena might be related with the extra amount of dye loaded in the film. The molar ratio of Zn atoms and dye molecules are 100:1 in this film. Relatively high amount of dye molecules are in the film. And it can be expected that such extra amount of dye molecules fills the pores in the film. And also, since this film is prepared at more positive potential than the redox potential of EY, it can be expected that the dyes are enclosed in the film.6 0 The photocurrents of 5 and 3 µA cm-2 were detected at the peak and after the relaxation respectively. The enclosed dyes in the film and the rather flat structure of the films as observed in the SEM image (Fig. 4.9 (A)) can be the reason for the poor photoelectrochemical property.

0.0 0.1 0.2 0.3 0.4 0.000

0.002 0.004 0.006 0.008 0.010 0.012 0.014

Current / mA cm-2

Time / sec

Fig. 4.13; Time-resolved photocurrent measured for EY / ZnO hybrid thin film deposited on an Au textile electrode at the potential of -1.06 V vs. Ag/AgCl for 60 minutes. A mixed solvent of ethylene carbonate : acetonitrile =4:1 (in volume) which contains 0.5M KI was used as an electrolyte.

The slow increase of the photocurrent was observed in the photocurrent transient measurements for the film prepared at -1.06 V vs. Ag/AgCl for 60 minutes. (Fig. 4.13) The textile electrode was almost covered after the deposition as shown in Fig. 4.10. Since this film was deposited at more negative potential than the redox potential of EY, it could be expected that the dyes were on the surface of the film. The photocurrent of 12 µA cm-2 was detected from the figure. The rather poor extent of the photocurrent is caused by the small number of the dyes adsorbed as monomer state on the surface of ZnO.69 A slow increase of the photocurrent after the shutter open and the slow decrease of the photocurrent after the closer can be explained by the electron transport property of ZnO and the traps in ZnO film. The electron transport in electrodeposited ZnO films can be assumed fast enough as reported before.60,64,83 Then, the slow response of the photocurrent is caused by time to fill the traps in the film. Since some kinds of the reactions take place during the deposition at such negative potential as mentioned above, it can be supposed that the film is deposited as the results of mixed reaction routes such as the reduction of oxygen, hydrogen and nitrate. It can be expected that the film has more traps than the one prepared by only the reduction of nitrate.

From the shape of the time- resolved photocurrent, no recombination was seen. It indicates the efficient photoelectrochemical activity of the device. However, aggregated dye molecules on the surface of the film depress the photoelectrochemical activity of the electrode. Such dyes do not contribute for the photocurrent although those dyes absorb the photons. Increasing of the amount of dye adsorbed as monomer states will be the key issue to increase the photoelectrochemical property.

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

10 20 30 40 50

Current / 10-6 A cm-2

Deposition potential / vs.Ag/AgCl

Electrolyte:

AN : EC = 1:4 0.5M KI 0.3M KI 0.1M KI 0.5M TBAP

Fig. 4.14; The stable photocurrent values detected from the time-resolved photocurrents measured with different kinds and concentrations of solute in the electrolyte for EY / ZnO hybrid thin film deposited on an Au textile electrode.

The photocurrents were measured for the films prepared at several deposition potentials.

And 0.5, 0.3, 0.1 M KI and 0.5 M TBAI were used for the electrolyte. (Fig. 4.14) It can be seen that the highest photocurrents have been achieved at the film prepared at -0.76 V vs.

Ag/AgCl. At the other deposition potentials, the photocurrents are relatively low. The porosity of the film influences the photoelectrochemical property of the films. The film deposited at -0.76 V vs. Ag/AgCl has the most porous structure as observed in the SEM image and other films have a rather dense structure. By this reason, the photocurrent values is low in the film prepared at the negative potential more than -0.76 V vs. Ag/AgCl.

At the potential less than -0.76 V vs. Ag/AgCl, the enclosed dye molecules do not contribute to the photocurrent and hence low photoelectrochemical properties. And those dyes cause the recombination and it is observed in the photocurrent transient measurement. The conditions of the dye in the film and the porosity are the key points to decide their photoelectrochemical properties.

When TBAI was used for the electrolyte, relatively larger photocurrents were detected except for the deposition potential of -0.76 V vs. Ag/AgCl. The bigger size of cation than potassium might influence the photoelectrochemical properties. The porous structure of the film prepared at -0.76 V vs. Ag/AgCl might not be facile for the big cation to enter. But the magnitude of the photocurrent at this deposition potential is still highest compared to the other potentials. It indicates the importance of the porosity for such devices even if solute is not suitable for its structure.