Preparation of Polymer Optical Fiber and Application to Water Purification
Authors: Takahashi Goki , Nagata Morio , Nagakawa Haruki 
Published: December 2020

The photocatalytic reaction uses light energy to advance various chemical reactions, so they are attracting attention as a clean process. The range of photocatalysis applications is wide, and it has been put into practical use and researched in many fields such as water splitting for energy production, self-cleaning, and environmental purification. We focused on the field of water purification using photocatalysts. Hydroxyl radicals generated by electrons excited on the photocatalyst by light irradiation have strong oxidation. It removes harmful compounds such as organic compounds and bacteria. When decomposing harmful substances using a photocatalyst in water purification, the surface of the material coated with the photocatalyst is irradiated with light.
However, photocatalysis requires the introduction of light, which limits its use in dark places such as suspended water, water pipes, and water tanks. One way to address these issues is to provide the photocatalyst with light from the coating material itself. Optical fiber composites have been devised as one such method. Devices reported to date have consisted of silica fibers coated with a photocatalyst. However, silica fiber has limitations such as weak bending, small core diameter, and material cost. To overcome these shortcomings, we combined plastic optical fibers and photocatalysts. Plastic optical fiber（POF）was selected because it is more bendable than silica optical fiber, has a larger core diameter, is lower cost, and is easier to process.
Next, we examined the light leakage method. In previous studies, removal of the fiber cladding by heat or physical damage was common. However, it is not a preferable method for increasing the number of manufacturing processes and for uniform light leakage. This time, during the fiber manufacturing process, an optical fiber that leaks light was added by adding a light scattering agent during core manufacturing.
The light Leakage-type plastic optical fiber（POF）were made using previously reported methods. The preform was made by polymerizing methyl methacrylate (179 g, MMA) into a tubular shape with closed ends. (inner diameter / outer diameter 14.7 / 22.0 mm, length 600 mm).3.87 mg of titanium dioxide (TiO2) powder (P25), which is a material with a high refractive index, was added as a scattering agent when manufacturing the core. Then, the obtained preform was heat-drawn to produce L-POF having a diameter of 1000 μm.
The photocatalyst layer was formed by three kinds of coating methods. In the dip coating method, the photocatalyst layer was formed by immersing the fiber in a suspension of TiO2 dispersed in ethanol. In the paste coating method, commercially available TiO2 paste Was applied to the fiber. In the two-layer coating method, an adhesive solvent (NRC-300A,) was dip coated, and then an adhesive solvent (NRC-300C) in which TiO2 was similarly dispersed was dip coated.
The fabricated fiber was evaluated by near-field pattern measurement. This is to measure the light intensity at the fiber end face by observing the laser light incident from the end face of the optical fiber with a CCD camera installed at the other end face. The surface of each coated fiber was observed with a scanning electron microscope (SEM, S4800). The photocatalytic performance was evaluated by a methylene blue decomposition experiment.
In this study, we succeeded in producing L-POF by adding a light scattering agent to the core of POF. In addition, a TiO2 photocatalyst layer was formed on the surface, enabling decomposition of organic substances in the dark. Light incident from the end face of L-POF leaks into the clad layer due to the scattering agent inside the core and excites the photocatalyst. Electron-hole carriers excited on the photocatalyst generate reactive oxygen species that decompose methylene blue. Since this method uses POF, which is inexpensive and easy to process, it can realize various photocatalytic reactions not only in the purification of contaminated water but also in dark places. By changing the incident light and coating the photocatalyst, devices suitable for various reactions can be made. From a practical point of view, by connecting a POF and photocatalytic L-POF, light could be transported to target areas for photocatalytic application with reduced light loss.

Figure 1