Titania

Ammonium hexafluorotitanate ((NH₄)₂TiF₆) and titanium butoxide (Ti(OBu)₄, Sigma–Aldrich Co., St. Louis, MO, USA, 97%) were used as the raw materials for the synthesis of titania nanosheets [16 (link)]. (NH₄)₂TiF₆ was dissolved in hydrochloric acid and then Ti(OBu)₄ was added dropwise, and the resulting mixture was stirred for 2 h. Subsequently, the hydrothermal reaction of the precursor solution was conducted at 180 °C for 6 h. The product was ultrasonically washed with distilled water and methanol, and then freeze-dried using a freeze dryer (FDS-1000, TOKYO RIKAKIKAI CO., LTD., Tokyo, Japan). A series of titania nanosheet samples were prepared by varying the F/Ti ratio in the precursor mixture as 0.3, 0.5, 0.8, 1.0, 1.5, and 2.0 (the resulting samples are denoted as NS0.3, NS0.5, NS0.8, NS1.0, NS1.5, and NS2.0, respectively). The obtained powders were characterized by X-ray diffraction (XRD; D8 Advance, Bruker AXS GmbH, Karlsruhe, Germany), UV-visible (UV-vis) spectrophotometry (Jasco V-550, JASCO International Co., LTD., Tokyo, Japan), and transmission electron microscopy (TEM; H-7100, Hitachi High-Technologies Corporation, Tokyo, Japan). Furthermore, we analyzed the lattice parameters by the whole powder pattern decomposition (WPPD) method using analysis software (DIFFRAC.TOPAS version 4.2, Bruker AXS GmBH). Based on the length and thickness of titania measured from the TEM images, the ratio of the {001} facet exposed on the surface of titania was calculated using the following equations: $$S001=2{\left(L-\frac{d}{tan\theta }\right)}^2$$
$$S101=\frac{2d}{sin\theta }\left(2L-\frac{d}{tan\theta }\right)$$
$$P001=\frac{S_{001}}{S_{001}+S_{101}}$$
where L is the average length of the titania nanosheet and d is the average thickness. S001 represents all {001} facets exposed in the single crystal and S101 represents the {101} facets. P001 is the percentage of the exposed {001} facet. θ = 68.3° is the theoretical value of the angle between the {001} and {101} facets of anatase [27 (link)].
The forbidden bandgap energy (Eg) was calculated using the Tauc plot. The optical bandgap was estimated using the following equation: $${\left(h\alpha \nu \right)}^{\frac{1}{n}}=k\left(h\nu -Eg\right)$$
where h is the Planck constant, ν is the frequency, α is the absorption coefficient, k is the proportionality constant, Eg is the optical bandgap, and n depends on the type of transition in the semiconductor material, with n = 1/2 for direct allowable transitions. By verifying the linearity of the plot using this equation, the x-intercept of the line through the inflection point of the graph was estimated as the optical bandgap. Assuming that scattering is constant over the wavelength range used, the Kubelka–Munk function was used instead of the absorption coefficient of titania. The Kubelka–Munk transform of the following equation was applied to calculate the absorption coefficient by substituting the diffuse reflectance measurements [28 ]: $$K=\frac{{\left(1-R\right)}^2}{2R}$$
Tauc plots were drawn based on the values obtained.

Titania nanotube array (TNA) films were fabricated using a modified anodization technique stated elsewhere [39 (link),40 (link)]. In short, titanium sheets were cut into small pieces of 2 × 4 cm² and then polished with sandpaper (400 grit) and a wet polishing cloth with an alumina micro polish slurry to create a flat, defect-free surface and remove the oxide layer on the surface. The polished Ti film was then ultrasonically cleaned in acetone and ethanol for 10 min each, then completely dried under a N₂ gas stream. Next, the cleaned Ti film was electrochemically anodized in an electrolyte solution containing 4:1 (v/v) of DI: EG, 0.26 M of NaF, and 0.94 M H₃PO₄. In the electrochemical cell, the Ti film was used as the anode and paired with a stainless-steel sheet as the cathode, and the distance between the two electrodes was less than 3 cm. After the electrochemical cell was set up, a direct current (DC) voltage of +30 V and 0.04 A was applied to the cell for 4 h and continuous stirring of the electrolyte was ensured during the anodization period to provide proper mass transport of charged ion particles in solution. The as-anodized Ti nanotube array film (anoTNA) was washed thoroughly with deionized water and ethanol, dried in ambient air, and annealed at 400 °C for 2 h to crystallize the anatase phase, and the resulting film was labeled as annTNA.

Phosphoric acid (H₃PO₄), acetone, ethanol, sodium fluoride (NaF), ethylene glycol (EG), methyl orange dye (MO), and H₂SO₄ (98%) were purchased from Sigma Aldrich (St. Louis, MO, USA) in the highest purity grade available and used as received. Titanium sheets (99.7% purity, Yunjie Metal Co., Baoji, China), stainless steel bars (Yunjie Metal Co., China), and polishing cloth (Struers LLC, Cleveland, OH, USA) were used to fabricate titania nanotube arrays. The H₂O₂ used in the experiments was pharmaceutical grade with 6% solution in water (Meliorate Health, Mumbai, India). The water used in all experiments was deionized ultrapure water (DI), attained from a Milli-Q direct water purification system (Fischer Scientific, Waltham, MA, USA) with a resistance of 18.2 MΩ at 25 °C.