Introduction

TiO₂ is a transition metal oxide which is chemically inert and stable. TiO₂ has three kinds of crystal structures, namely anatase, rutile and brookite [1]. Anatase type of TiO₂ is more photoactive than rutile because the surface area of anatase is greater than the rutile. So that the active site per unit of anatase is greater than rutile. Brookite structure is the most unstable and most difficult to prepare so it is rarely used in the process of photocatalyst [2].

The preparation of semiconductor thin film is one way to ease the semiconductor applications as a solar cell and photocatalyst in the degradation of harmful chemical compounds [3,4]. Photocatalyst technology is a combination of integrated photochemical and catalytic processes to be able to carry out a chemical transformation reactions. The transformation reaction takes place on the surface of a semiconductor catalyst materials induced by light [5].

Development in the preparation of TiO₂ semiconductor electrochemically is a newer method by combining the photocatalyst with an electrochemical process, known as photoelectrocatalysis [6-8]. Nurdin and Maulidiyah reported in developing photoelectrochemical cell systems, the most important part is the preparation of the nano-sized TiO₂ film [9]. Zhang et al. also reported the synthesis of mesoporous TiO₂ using tetrabutilortotitanat as precursor as well as polyethylen glycol (PEG) as template [10]. The synthesis result of mesoporous TiO₂ reported is having great surface area and photoelectrocatalytic activities. Many other studies have been conducted with various types of dopants, but dopant with PEG is more interesting than the other dopants, because PEG can improve the performance of the catalyst, and is able to increase the porosity, reduce the size of crystals and reduce the possibility of film cracking during the calcination process [11].

The photocatalytic process when the semiconductor absorbs light energy equal to or greater than the energy of its band gap, there will be a charge separation or photoexcitation in semiconductors. Electrons (e^–) will be excited into the conduction band leaving positive hole (h⁺) in the valence band. The positive hole has a high affinity for oxygen in H₂O molecules and form •OH radicals. Hydroxyl radical (•OH) is a highly reactive species that attacks organic molecules and can degrade into CO₂ and H₂O and halide ions if organic molecules containing halogen [7,12]. Therefore, the positive and negative holes on the activated titanium can then be used for various applications. One is for self-cleaning materials in a wide range using TiO₂ [9].

One of important parameters from the contact angle is the influence of the surface structure, as reported by Xiong et al. the descend of water contact angle on the surface structure of TiO₂ is caused by the formation of electron-hole, where the hole reacts with oxygen to form the surface of the empty oxygen and the electrons react with metal ions (Ti⁴⁺) to form ions (Ti³⁺) electron traps surface [13]. TiO₂ affects the structure of a surface contact angle and photoelectrocatalytic activity. Several templates of TiO₂ is friendly used for immobilization of TiO₂, polyethylene glycol (PEG) that is often used to form pores for the synthesis of porous TiO₂ films formed by hydrothermal method [14,15].

This research will develop a preparation method of the TiO₂ catalyst by adding PEG dopant immobilized on Ti plate. Thus, it is expected to improve the performance of the catalyst, and is able to increase the porosity, reduce the size of the crystals and reduce the possibility of film cracking during the calcination process.

Research Methods

Synthesis of TiO₂ Sol

TiO₂ sol was prepared by mixing 35 mL of ethanol, 2.4 mL of diethanolamine (DEA), and 7.5 mL of titanium tetra isopropoxide (TTIP). The mixture was stirred at room temperature for 1.5 hours. The mixture was added ethanol: water (4.5 mL: 0.5 mL) and 0.5 mL of PEG 200 and stirred for 1.5 hours. With the same treatment and added 1 mL of PEG 400 and 2 mL of PEG 1000 and stirred for 1.5 hours.

Preparation of TiO₂ Film

TiO₂ sol was used to coat on a preparations of glass substrate with a dip-coating method, then calcined at 450°C for 2 hours.

Contact Angle Measurement

The contact angle of measured glass slide was placed at the back of the plate in a horizontal position. Then water was dripped using a syringe on the preparations surface. Further, the droplet images taken with a digital camera that light and magnification had been set up to obtain images of water droplets in focus. Furthermore, the image obtained was input into a computer to measure its contact angle with software protactor (MB-ruler). Then the contact angle was recorded as the surface contact angle before irradiated by UV. Finally, preparations were irradiated by 18 watt of UV for 1 to 4 hours in a UV reactor. After 1 hour irradiation, this process were repeated. Changes in the contact angle on the surface were compared with every hour irradiation.

Photocurrent Response

LSV was measured with Linear Sweep Voltammetry using 0.1 M NaNO₃ solution, where 0.85 g of NaNO₃ was dissolved in 100 ml flask. LSV testing was conducted on the potential -1 to 1 volt, with a scan rate of 1×10^-4 V/s, in which each of TiO₂ without PEG, TiO₂-PEG 200, TiO₂-PEG 400 and TiO₂-PEG 1000 was tested by illuminating the UV-LED light. The LSV results were used to observe the photoelectrocatalytic activity during the testing process [16].

Results and Discussion

Determination of Band Gap Energy

Measurement with DRS UV-Vis aimed to determine the character of absorption in the wavelength range both for UV and Visible (200-600 nm). In Figure 1, it can be observed that for TiO₂ with additional variations of PEG concentration, it has different reflectance spectrum profile. The reflectance spectrum profile clearly showed that the TiO₂-PEG has an area of absorption in the visible light (λ> 400nm).

Figure 1: The spectrum of DRS UV-Vis Click here to View figure

Table 1: Values of Band Gap

TiO₂-PEG has band gap energy values smaller than TiO₂ degussa and TiO₂ standard. With the decrease in the band gap values, then the light energy needed by photohole and photoelectron will be smaller, simply by using a visible light source [7,16]. Moreover, it can also be seen that the higher concentration of PEG, it proved to have a tendency of decreasing the band gap values although the decrease was not significant, as can be seen on the Tabel 1 that the optimum band gap value was obtained at TiO₂-PEG 1000 sol.

Measurement of Contact Angle (Hydrophilic Test of TiO₂ -PEG Thin Film)

Hydrophilic test on TiO₂ catalyst films was measured by contact angle meter. The contact angle measurements was performed by varying the exposure time of 20 Watt UV (0 to 4 hours) and varying the thin film of TiO₂ which was added to the PEG. Figure 2 shows the contact angle without a thin film of TiO₂ exhibits superhydrophobic wherein the contact angle is greater than 100°, so that the level of wetness is lower. Contact angle is relatively constant despite irradiated by UV light [17]. As for the contact angle measurements without PEG, PEG 200, PEG 400 and PEG 1000, were hydrophilic due to a decrease in the contact angle from 40° to 20°. These showed that the TiO₂ film irradiated by UV light could transform into hydrophilic properties. Similarly, the length of time variation of UV radiation, the longer the exposure time, the intensity of UV rays on the surface of TiO₂ was films getting bigger, so that the wetting was greater and contact angle of water would decrease.

Figure 2: Contact Angle Measurement Click here to View figure

Measurement of Photocurrent Response

Figure 3 showed the photocurrent increased linearly at low potential and then saturated at higher potential. Photocurrent generated would be increased by the increasing concentration of organic compounds, it showed the increasing rate of photohole capturing, i.e the reacting hole with organic compounds on the surface of TiO₂ film [18]. With the increasing rate of capturing photohole the charge recombination processes on the surface of TiO₂ was reduced and the photocurrent value would increase. According to Maulidiyah et al. there are two factors affect the value of the photocurrent on the photoelectrocatalytic process, namely the transfer of electrons in the semiconductor layer and the capturing process of photohole on TiO₂ or solution interface, these two factors are important in reducing the recombination of electron-hole pairs [16,19].

Figure 3: Linear Sweep Voltammetry (LSV) of TiO2 + PEG electrode with illumination, an electrolyte solution of 0.1 M NaNO3 Click here to View figure

The photocurrent was increased in Figure 3, the increase of the potential bias given at lower potential range when the organic compounds contained in the solution and showed the photocurrent saturation at a higher potential bias. Photocurrent saturation at a higher potential indicates the maximum rate of photohole capturing on the surface of TiO₂. LSV response in Figure 3 shows that the three TiO₂ working electrodes added to the PEG 200, PEG 400 and PEG 1000 were photocurrent measurements showed of PEG 200 generated of 2.3×10^-3 A, PEG 400 the photocurrent generated was not too far from PEG 200 ranging of 2.2×10^-3 A. While for PEG 1000 the surge in photocurrent generated was as high as 4.2×10^-3 A showed that the activity of TiO₂-PEG 1000 was better than the TiO₂-PEG 200 and TiO₂-PEG 400.

Conclusion

1. The results of hydrophilic tests on the TiO₂-PEG film had been reached the hydrophilic property.
2. The photocurrent measurements value from three TiO₂ working electrodes added the PEG 200, PEG 400 and PEG 1000 were 2.3×10^-3 A, 2.2×10^-3 A and 4.2×10^-3 A, respectively.

Acknowledgement

We acknowledged the financial support of the Ditlitabmas-Dikti Ministry of Research, Technology and Higher Education, the Republic of Indonesia. We are also acknowledge to Surahman H. (Universitas Indonesia) for his valuable contribution in obtaining instrument measurements.

References

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