Bio-inspired mineralization of nanostructured TiO2 on PET and FTO films with high surface area and high photocatalytic activity

Nanostructured TiO2 coatings were successfully formed on polyethylene terephthalate (PET) films and fluorine-doped tin oxide (FTO) films in aqueous solutions. They contained an assembly of nanoneedles that grow perpendicular to the films. The surface area of the coatings on PET films reached around 284 times that of a bare PET film. Micro-, nano-, or subnanosized surface roughness and inside pores contributed to the high nitrogen adsorption. The coatings on FTO films showed an acetaldehyde removal rate of 2.80 μmol/h; this value is similar to those of commercial products certified by the Photocatalysis Industry Association of Japan. The rate increased greatly to 10.16 μmol/h upon annealing in air at 500 °C for 4 h; this value exceeded those of commercial products. Further, the coatings showed a NOx removal rate of 1.04 μmol/h; this value is similar to those of commercial products. The rate decreased to 0.42 μmol/h upon annealing. NOx removal was affected by the photocatalyst’s surface area rather than its crystallinity.

Scientific RepoRtS | (2020) 10:13499 | https://doi.org/10.1038/s41598-020-70525-w www.nature.com/scientificreports/ solar simulator illumination. Evaluations of real water samples showed excellent anti-interference and recovery capabilities. Metal oxide nanomaterials and a biosensor fabricated using them were reviewed 7 . Nanomaterial deposition on conductive electrodes is a crucial step for achieving high performance, and a simple, stable, and reproducible method is considered essential for depositing nanomaterials for fabricating a biosensor. Many devices have reaction sites on the metal oxide surface, and therefore, they need to have a large surface area. In addition, studies are developing new devices by using the characteristic surface structure of metal oxides. Controlling the size, shape, and crystallinity of the metal oxide is known to greatly affect the device characteristics, and new metal oxide materials are being actively developed. There is also a strong need to control the shape and even the exposed crystal plane after the metal oxide material is crystallized. Especially, nanostructured TiO 2 films with high surface area and high photocatalytic activity are strongly required for photocatalysts and related devices. Performance of the nanostructured TiO 2 films is strongly affected by the size, shape, crystallinity, and the exposed crystal plane. The development of high performance nanostructured TiO 2 films is a powerful demonstration of precise control of the size, shape, crystallinity, and the exposed crystal plane.
Metal oxide nano-/microstructures have also been synthesized in animals and plants. The size, shape, crystallinity, and surface structure of metal oxides in aqueous solutions were controlled at room temperature and atmospheric pressure. They are called "Bio-mineralization" and have created a new academic field, "Bio-inspired mineralization". In the bio-inspired mineralization, we learn from nature and aim to develop novel materials that are not in nature.
This study investigates the bio-inspired mineralization of nanostructured TiO 2 . Nanostructured TiO 2 was formed on polymer or inorganic films in aqueous solutions. Further, its surface area and photocatalytic properties were investigated.
The morphology of the nanostructured TiO 2 film on the PET film was observed using a field-emission scanning electron microscope (FE-SEM; JSM-6335F, JEOL Ltd.) at 20 kV. Nitrogen adsorption-desorption isotherms were obtained using Autosorb-1 (Quantachrome Instruments). Nanostructured TiO 2 films on PET films were outgassed at 110 °C under 10 -2 mmHg for more than 6 h prior to measurement. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method using adsorption isotherms. The pore size distribution was calculated by the Barrett-Joyner-Halenda (BJH) method using desorption isotherms. The photocatalytic properties of the nanostructured TiO 2 on FTO films were evaluated at the Kanagawa Academy of Science and Technology (KAST), Japan, based on Japanese Industrial Standards (JIS).

Results and discussion
Bio-inspired mineralization of nanostructured TiO 2 . Ten sheets of PET films (50 mm long × 50 mm wide × 0.1 mm thick) were pasted on glass plates with polytetrafluoroethylene tapes. Ammonium hexafluorotitanate (10.31 g) and boric acid (9.321 g) were dissolved in 1,000 mL of distilled hot water 13 . The concentrations of ammonium hexafluorotitanate and boric acid were 50 and 150 mM, respectively. PET films were exposed to vacuum-ultraviolet light for 20 min in air using a low-pressure mercury lamp (PL16-110, SEN Lights Co.). They were immersed in the solutions at 50 °C for 24 h. The titanium oxide-formed surface of the PET films faced obliquely downward. Therefore, even if homogeneously nucleated titanium oxide particles formed in the solution settle, they are less likely to be deposited on the PET film. Further, the PET film is less likely to be bent or deformed in the solution because it is attached to the glass plate. Moreover, titanium oxide is formed only on the PET film surface because the back surface of the film is in close contact with the glass plate. Thereafter, the glass plates were removed from the solution. The PET films were peeled off from the glass plates, washed with running water, and dried under a strong air flow. Nanostructured TiO 2 with high surface area. PET films were uniformly covered with nanostructured TiO 2 (Fig. 1a). The coatings had an uneven surface structure (Fig. 1b). The needle-like surface structures had size of ~ 5-10 nm (Fig. 1c). Each needle had nanosized surface asperities.
The mass of the PET film was measured before and after immersion in the aqueous solution. The mass of nanostructured TiO 2 was calculated from the difference in mass before and after immersion. The composite was cut to obtain rectangular pieces with dimensions of ~ 3 mm × 10 mm. All of these pieces were filled in a glass sample holder for gas adsorption measurements. The gas adsorption amount of the composite can be measured with almost no influence of the PET film, and the measured gas adsorption amount of the composite can be regarded as the gas adsorption amount of nanostructured TiO 2 .
Nanostructured TiO 2 showed nitrogen adsorption-desorption isotherms (Fig. 2a). The pore size distribution of nanostructured TiO 2 was calculated from the nitrogen desorption isotherm by the BJH method (Fig. 2b). Nanostructured TiO 2 had inside pores and/or interparticle spaces of ~ 2-100 nm. The result did not indicate whether nanostructured TiO 2 has pores of 1 nm or less because such pores cannot be estimated by the BJH method. The BET specific surface area was calculated to be 503.6 m 2 /g (Fig. 2c). The total surface area of nanostructured TiO 2 was calculated by multiplying this value by the mass of nanostructured TiO 2 . The ratio of the surface area to the substrate projected area was calculated to around 284 times that of a bare PET film by dividing the total surface area of nanostructured TiO 2 by the projected area of substrates (25,000 mm 2 , 50 mm × 50 mm × 10 www.nature.com/scientificreports/ sheets). The total surface area is not affected by the error of the weight of the nanostructured TiO 2 in this calculation method, and therefore, it can be determined accurately. To the best of the authors' knowledge, the TiO 2 film surface area was the highest ever reported. Nanostructured TiO 2 in this study was considered to be different www.nature.com/scientificreports/ from general nanostructured TiO 2 reported elsewhere. Micro-, nano-, and subnanosized surface roughness and inside pores contributed to increased nitrogen adsorption. Surface crystal defects such as kinks, terraces, and secondary nuclei were also considered to contribute to increased nitrogen adsorption.
Nanostructured TiO 2 with high photocatalytic activity. FTO films were exposed to vacuum-ultraviolet light for 20 min. They were immersed in the solutions containing ammonium hexafluorotitanate (50 mM) and boric acid (150 mM) at 50 °C for 24 h. The photocatalytic properties of the nanostructured TiO 2 on FTO films were evaluated based on JIS R 1701-2: 2008 (acetaldehyde removal performance) and JIS R 1701-1: 2010 (nitrogen oxide (NO x ) removal performance). Acetaldehyde and NO x serve as evaluation indices for indoor and outdoor photocatalytic activity, respectively. Materials with high photocatalytic activity for acetaldehyde and NO x can be used in building interiors and exteriors, respectively. The photocatalytic characteristics were evaluated using two FTO films (50 mm × 26 mm × 1.1 mm in thickness) (AGC Fabritech Co., Ltd., TOC substrate (DU film)). The FTO layer (0.1 mm in thickness) was formed on a glass substrate (0.1 mm in thickness). The size of the test piece was around half of the JIS-specified size (50 mm × 100 mm). The measured values excluding regeneration efficiency for NO x removal characteristics were thus doubled.
In general, light irradiation on a photocatalytic material such as TiO 2 generates electrons and holes on the surface (Fig. 3). Oxygen and water in the air react with electrons and holes, respectively. These reactions produce hydroxy (OH) radicals and superoxide ions on the titanium dioxide surface. OH radicals have strong oxidizing power and remove electrons from acetaldehyde molecules. Acetaldehyde molecules lose electrons and break bonds. They were converted to CO 2 and/or H 2 O that were released to the atmosphere.
Acetaldehyde was removed from the sample at the rate of 2.80 μmol/h, and its removal ratio was 20.6% (Table 1). This rate was similar to that of commercial products certified by the Photocatalysis Industry Association of Japan 14 (Fig. 4). Further, acetaldehyde was converted to CO 2 at the rate of 4.56 μmol/h, and its conversion ratio was 16.8%.
The amount of NO x removed was 1.04 μmol (Table 1). This value was similar to that of commercial products (Fig. 5). The NO x adsorption and desorption amounts were 0.08 and 0.8 μmol, respectively. The amount of nitric oxide removed was 3.0 μmol. The amount of nitrogen dioxide generated was 1.24 μmol.
The regeneration efficiency upon washing with water was 1,400% (no conversion). The first and second elution amounts of NO x were 12.50 and 2.42 μmol, respectively. The regeneration efficiency exceeded 100%, suggesting that NO x was generated from the sample during the test and was added to the NO x elution amount. This result indicated that nanostructured TiO 2 contained nitrogen inside and/or on its surface. Ammonium hexafluorotitanate is one of the sources of nitrogen.  Notably, the acetaldehyde removal rate increased greatly from 2.80 to 10.16 μmol/h upon annealing, and the removal ratio was 74.8% (Table 1). The rate exceeded that of commercial products (Fig. 4). This was one of the advantages of nanostructured TiO 2 . Further, acetaldehyde was converted to CO 2 at the rate of 16.84 μmol/h, and its conversion ratio was 60.2%.
The NO x removal rate decreased from 1.04 to 0.42 μmol/h upon annealing (Table 1, Fig. 5). NOx removal was affected by the photocatalyst's surface area rather than its crystallinity. The NO x adsorption and desorption amounts were 0.08 and 0.7 μmol, respectively. The amount of nitric oxide removed was 2.7 μmol. The amount of