Pre-deposition electrolysis and deposition of Eosin Y / ZnO films

When an EY / ZnO hybrid thin film is electrodeposited, the increase of the current in the initial stage of the deposition is observed in a current-time curve. It does not only mean the surface area of the substrate is increased three- dimensionally due to the creation of ZnO particles on the substrate. If this was the only factor, the current would decrease after a short while, corresponding to the time that the ZnO particles cover the whole substrate. It was found that the increase of the current was also caused by the fact that the activity of the SnO2 conductive substrate was increasing by the applied potential.^１６４ To obtain the films with a nice reproducibility, the condition of the substrate should be prepared identically. Before starting the electrochemical deposition of EY / ZnO hybrid thin film, the “pre-deposition electrolysis” should be introduced to activate the surface of the SnO2 conductive glass substrate, and it leads to increase the number of the seeds of ZnO particles on the substrate in the initial stage of the film precipitation.^１６４

-200 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 -3.0

-2.8 -2.6 -2.4 -2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2

Time / s

Current / mA cm-2

Fig. 5.2; The current-time curve during the pre-deposition electrolysis. The potential of -1.0 V vs. SCE was applied to the electrode for 30 minutes in the solution of 0.1 M KCl with the bubbling of oxygen in the bath. Pt electrode was used as counter electrode. A rotating disk electrode was set at 500 rpm. The flow rate of oxygen and nitrogen was 200 and 400 ml / min. respectively.

The current-time curve during the pre-deposition electrolysis is shown in Fig. 5.2. The current of 2.6 mA cm^-2 was reached at the end. The current is caused by reduction of the oxygen bubbled in the deposition bath. The Zn rod electrode must not be used for pre-deposition electrolysis because the precipitation of ZnO starts if Zn ions are present in the deposition bath. For pre-deposition electrolysis, a Pt electrode was used as counter electrode. The increase of the current was observed at the beginning of the electrolysis, which indicates the activation of the substrate. The increased activity of the substrate and also the increased number of the active site in the substrate leads the increase of the current. The number of the seed of ZnO particles is increased by increasing the number of active site in the substrate.^１６４

Close attention should be also paid for the preparation process of working electrodes. The contact between the substrate and the sample holder must be established tightly. The same holds for the insulating tape. When the contact was not well established, intense noises might be observed in the current-time curve; it is caused by the unstable contact between the sample holder and the SnO2 substrate. When the insulating tape was not well attached, the current would increase as time goes; it is caused by the increase of the substrate’s surface area as result of that the insulating tape peeled off. Those factors disturb the reproducibility of the film deposition. It should be noted that the oxygen bubbling is carried out in the deposition bath during the pre-deposition electrolysis process. To activate the substrate, it seems to be necessary to contain a high concentration of oxygen in the deposition bath since the evidence of the activation can be seen only when the oxygen is supplied into the deposition bath sufficiently. And the magnitude of the current depends strongly on the concentration of oxygen in the deposition bath. For the

deposition of EY / ZnO hybrid thin films, on the other hand, the gas tube of the oxygen should be out from the deposition bath to avoid the gas oxygen in the bath. (The detail is written later.) But for the pre-deposition electrolysis process, this does not matter.

Therefore the gas tube should be in the bath during the activation to keep a high concentration of oxygen in the bath.

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-2.5 -2.0 -1.5 -1.0 -0.5 0.0

Current / mA cm-2

Time / s

Fig. 5.3; The current-time curve during the deposition of EY / ZnO hybrid thin film. The film was prepared at -1.0 V vs. SCE for 20 minutes in the solution containing 5 mM ZnCl2, 0.1 M KCl and 50 µM EY with allowing to flow oxygen into the deposition cell. Zn rod was used as counter electrode. A rotating disk electrode was set at 500 rpm. The flow rate of oxygen and nitrogen was 200 and 400 ml / min.

respectively.

After the activation of the substrate, the process is stepped to the deposition of EY / ZnO hybrid thin film. The high concentrated ZnCl2 and EY solutions are added to the deposition bath. The concentration of Zn²⁺ and EY in the bath is adjusted to 5 mM and 50 µM respectively. Pt counter electrode is replaced with a Zn rod electrode. And the oxygen tube is removed from the deposition bath and set close to the water surface. During these processes, the SnO2 substrate stays in the deposition bath to keep the activated condition. Following those procedures, the deposition of EY / ZnO hybrid thin film can be started.

The position of the oxygen is one of the most important points for the deposition. To obtain porous ZnO (EY as SDA) films, it is known that there should be no gas oxygen under these conditions.^１６４ A porous structure of ZnO is obtained when EY molecules are reduced condition.^５６ And if EY molecules are reduced state, a transparent film can be obtained after the deposition since the EY molecules are reduced state (colorless). Such reduced dyes are slowly oxidized by exposing the film in air atmosphere and the film finally get the color approximately in 24 hours.^６０ However, Oxygen bubbles in the solution can oxidize the EY molecules which are on the surface of ZnO even during the deposition. In this case,

the film has the reddish color of EY even just after the deposition. When EY molecules are oxidized during the deposition due to the gas oxygen, the film shows the features as it was seen in the films prepared at more positive deposition potentials than the redox potential of EY even if the deposition potential is more negative than the redox potential of EY. In such films, the EY molecules are enclosed in the film and those dyes can not be removed by dipping into aqueous KOH (pH: 10.5). Therefore, the oxygen tube must be out from the deposition bath, but should be placed as close as possible to the water surface to keep the concentration.

The current-time curve during the electrochemical deposition of EY / ZnO hybrid thin films is shown in Fig. 5.3. The current of 1.4 mA cm^-2 was detected during the deposition.

Higher current of 1.7 mA cm^-2 was seen in the beginning of the deposition. This relatively higher current was caused by the higher concentration of oxygen at the electrode surface.

The concentration of the oxygen at the electrode surface can be expected to be higher in the beginning of the deposition. To utilize the activated substrate and to increase the number of ZnO seeds in the beginning of the deposition, it is required to start the deposition at the highest achievable concentration of oxygen in the bath. Once ZnO particles are created on the substrate, continuous growth can be expected. The decrease of the current can be seen as the result of that the time passed after removing the oxygen gas tube from the bath and some unavoidable intermixing with N2 for purging the electrode.

Finally, the current becomes a stable value, it indicates the stable supply of the dissolved oxygen to the surface of the substrate.

The reproducibility of the film preparation was optimized in this way. But nevertheless, further improvement especially for obtaining a stable and high concentration of oxygen in the deposition bath will lead to increased reproducibility.