Fabrication of Titanium Oxide Thin-Film Electrodes with Photocatalytic Activities and an Evaluation of Their Photoelectrochemical Properties ^†

Abstract

We synthesized hollow spherical titanium oxide particles, which are one of the structural features of fabricating a thin-film photoelectrode, using the particles and evaluated their properties. The XRD diffraction results confirmed the main phase peaks of the target rutile-type hollow spherical titanium oxide (HSTR) and bronze-type hollow spherical titanium oxide (TiO₂(B)). The calcium carbonate used in the core material was also removed. The photocatalytic reaction measurement result showed that the activity of TiO₂(B) in ultraviolet light of 365 nm was higher than that of TiO₂(B). As shown in the visible spectrum, the photo adsorption wavelength of HSTR was approxim ately 700 nm, whereas TiO₂(B) was generally absorbed around 400 nm. A relationship between an electric current peak and the square root of a scan potential speed suggested a reversible reaction system in light irradiation.

1. Introduction

Energy is indispensable for our lives; however, the biggest problems in energy are the depletion of energy resources and environmental problems. Energy production based on fossil fuels causes air pollution and global warming, and the limited number of reservoirs threatens the stability of future energy supplies. To solve this problem, the conventional energy production method needs to be changed. Thus, renewable energy has been attracting attention. Although solar cells are most popular in the production of renewable energy using sunlight as an energy source, they have problems such as manufacturing costs and the need for a large area of land to increase the amount of generated power.

Dye-sensitized solar cells (DSSCs) [1] and perovskite solar cells [2] have been extensively studied to solve the above-mentioned problems. As DSSCs generally adopt halides as electrolytes and perovskite solar cells employ lead halides as constituent elements, the fabrication of devices does not emit toxic materials that harm the environment and the human body. As shown in Figure 1a, a solar cell generates electric power that needs to be converted and stored in the battery through a DC–DC converter. The quantity of energy conversions increases the energy loss in the process of solar energy to electrical energy and electrical energy to chemical energy. In addition, since the power generation part and the storage part are separated, the whole system becomes large, which causes high operation costs.

To solve the above-mentioned problems, we constructed a photoactive electrode of solar cells without non-toxicity using titanium oxides. Titanium oxides undergo photoactivation, known as the Honda–Fujishima effect [3]. This is a photoelectronic reaction in which titanium oxide particles are irradiated with ultraviolet light and electrons and holes are excited, promoting a redox reaction. In addition, titanium oxides are used as an electrode material for lithium-ion batteries; therefore, it is also possible to occlude lithium ions. If the excited electrons produced from photoirradiation are consumed to store lithium ions into titanium oxide particles on a single transparent electrode, a conventional solar system can be replaced with an integrated single system of a solar cell and a lithium-ion battery (Figure 1b). By making the primary particle shape a hollow sphere, the specific surface area increases to promote the reaction near the particle interface and broaden the active wavelength without adding other elements. Therefore, titanium oxides can be used as an integrated electrode material to generate and store electricity, simplify the entire system, and decrease the loss in energy conversion. Furthermore, they have little effect on the environment and human health as they are used in cosmetics.

Purpose

Figure 2 shows that titanium oxide has various crystal structures and that the bandgap changes depending on the crystal structures [4]. In this study, we analyzed a photocatalytic effect on the absorption wavelength produced by the differential crystal structure only by controlling the primary particle shape without adding other elements. Furthermore, we constructed the integrated battery system by fabricating the photoactive thin-film electrode and evaluating its characteristics.

2. Material and Method

2.1. Sample Synthesis

We synthesized titanium oxides with hollow spherical primary particles [5]. First, we fabricated calcium carbonate-coated titanium alkoxide (TCC). Starting from titanium alkoxide (TTIP), it was hydrolyzed with distilled water and coated on calcium carbonate (CaCO₃) in ethanol. Next, amorphous titanium was synthesized as a precursor. CaCO₃ particles in the core material were removed by stirring TCC at 300 rpm in a nitric acid aqueous solution. Finally, hollow spherical titanium oxide (HSTiO₂) was fabricated. Hollow spherical rutile-type titanium oxide (HSTR) was obtained by calcining spherical amorphous titanium at 900 °C for 48 h under an air atmosphere, and hollow-spherical, anatase-type titanium oxide (HSTA) was obtained by calcining spherical amorphous titanium at 500 °C for 1 h in an air atmosphere (Figure 3).

2.2. Sample Analysis

The synthesized samples were analyzed using powder X-ray diffraction (XRD, Rigaku (Akishima, Japan), SmartLab-SRD), a scanning electron microscope (SEM, Jeol (Akishima, Japan), JSM-IT100), and energy-dispersive X-ray spectroscopy (EDX). The optical properties of the samples were evaluated using ultraviolet–visible spectroscopy (UV-Vis., Jasco (Easton, MD, USA), V-650). In the structural analysis, an XRD apparatus was set to a Cu Kα ray, an accelerating voltage current of 40 kV-20 mA, and an angle range of 2θ = 10–90°. A scanning electron microscope with built-in EDX was used for shape observation and elemental analysis. The acceleration voltage was set to 15 kV, the shape observation was to a probe current (P.C.) of 40 mA, and the elemental analysis was set to P.C. 60 mA. The optical properties were evaluated using UV-Vis. with irradiation wavelengths at 200–900 nm.

2.3. Fabrication of Thin-Film Electrodes

An electrode was fabricated using the synthesized sample in this study. A mixed solution of polyvinylidene fluoride (PVdF) and HSTR was coated on an indium tin oxide substrate (ITO-PET) to a thickness of 200 μm using a bar coater.

Table 1. Mixing ratio of PVdF and HSTR.

PVdF [g]	HSTR [g]	wt. Ratio
0.05	non	1:0
0.05	0.05	1:1
0.05	0.02	1:0.4
0.05	0.01	1:0.2

shows a mixing ratio in weight of PVdF and HSTR.

2.4. Photoelectrochemical Properties

Using the prepared electrode as the working electrode, a photocatalytic reaction measurement and photoelectrochemical measurement were performed. Distilled water and absolute ethanol were used as solvents for photocatalytic reaction measurements, and a carbon rod was used as the counter electrode. The generated amount of hydrogen gas was estimated using the Faraday electrolysis formula (Equation (1)) by measuring the electrical current value while 365 nm light was irradiated for 60 min.

$$\begin{gathered} m=\frac{It}{F}·\frac{M}{n}\left[\mathrm{g}\right] \\ (\mathrm{C}\mathrm{u}\mathrm{r}\mathrm{r}\mathrm{e}\mathrm{n}\mathrm{t}:I\left[\mathrm{A}\right],\mathrm{T}\mathrm{i}\mathrm{m}\mathrm{e};t\left[\mathrm{s}\right],\mathrm{F}\mathrm{a}\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{a}\mathrm{y}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{t};F\left(9.6485\times {10}^4\mathrm{C}/\mathrm{m}\mathrm{o}\mathrm{l}\right), \\ \mathrm{F}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{u}\mathrm{l}\mathrm{a}\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{t}\mathrm{s}M\left[\mathrm{g}/\mathrm{m}\mathrm{o}\mathrm{l}\right],\mathrm{N}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r}\mathrm{o}\mathrm{f}\mathrm{r}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{n}\mathrm{s};n[-]) \end{gathered}$$

(1)

In photoelectrochemical measurements, a Pt wire electrode was used as the counter electrode, an Ag/AgCl electrode was used as the reference electrode, and a 1M Na₂SO₄ aqueous solution was used as the electrolyte. The conditions were in a potential range between −0.4 and +0.4 V vs. Ag/AgCl, a sweep rate between 5 and 50 mV/s, and an irradiation wavelength of 800 nm. Measurements were performed in UV-Vis. (Jasco, V-630BIO) using a halogen lamp as the light source.

3. Results and Discussion

3.1. Sample Analysis

The results of the sample analysis of HSTR are shown in Figure 4. Based on the XRD analysis result, we produced the rutile-type structure. The SEM observation result showed spherical particles about 0.2 μm in diameter. The EDX elemental mapping result indicated that the residual amount of calcium was negligible. TiO₂(B) analysis results are shown in Figure 5. XRD patterns in Figure 5a matched the crystal peak pattern of a previous report [6] for the titanium oxide bronze, indicating successful synthesis. However, as shown in Figure 5b, the desired hollow spherical shape was maintained, and the crystals grew into the needle-like shape that is characteristic of the bronze type.

The results of the UV-visible measurements (Figure 6) presented a red shift of a photo adsorption wavelength in the rutile type due to a shaping effect on the hollow spheres, whereas the wavelength of rutile-type TiO₂ is generally observed at approximately 400 nm. Since bronze crystals grew in a needle-like (layered) shape, the peak was the same as the conventional peak.

3.2. Photoelectrochemical Properties

Figure 7 shows the results of a photocatalytic reaction measurement using HSTR, which maintained the desired hollow and spherical shape. It was confirmed that the catalytic reaction was carried out through light irradiation.

Photoelectrochemical measurements using cyclic voltammetry revealed a current peak caused by redox reactions in the cyclic voltammogram (Figure 8a) when irradiated with light at a specific wavelength. In the absence of light irradiation, no peaks associated with redox reactions appeared during reductive and oxidative scans, and only peaks attributed to a series resistance appeared. Two current peaks were observed on the reduction side. However, Figure 8b showed that both peaks showed good linearity, suggesting that the reaction progressed at the electrode interface in two stages on the reduction side.

4. Conclusions

The primary particles of TiO₂(B) could not maintain hollow spheres and grew needle-like shapes. In addition, the light absorption wavelength corresponded to that of a previous report. On the other hand, the absorption wavelength of HSTR shifted from the previous report forward to the infrared wavelength side. This shift suggested that the shape affects the primary particles. Oxidation and reduction peaks were confirmed in a cyclic voltammogram, and current peak values changed linearly against the square root of the sweep speed. Therefore, the fabricated photoelectrodes had a reversible redox reaction and promoted the redox reaction in light irradiation.