TiO2 Nanotubes Transformation Into 4nm Anatase Nanoparticles: Anodizing Industrial Recycled Titanium for Photocatalytic Water Remediation

Introduction

Today, many materials with different properties have been investigated; this includes nanomaterials, which have the potential to influence modern society in many aspects. These kinds of materials are highly interesting, thus both nanoscience and nanotechnology have become attractive and exciting fields, because nano-systems may not behave like their bulk counterparts. The era of dealing with tiny objects has been gaining momentum in the past few years due to industrial progress, the scientific ability to fabricate, model, and manipulate things with a small number of atoms, and the almost daily discovery of novel size-induced phenomena.

The origin of the size-induced properties in nanomaterials depends on the surface phenomena (extrinsic contribution) and quantum confinement effects (intrinsic contribution). The surface to volume ratio increases rapidly when particle size decreases (Souza Filho & Fagan, 2011). There are many techniques for synthesizing nanoparticles, but study presents the investigation into obtaining them through the synthesis of TiO₂ nanotubes through an electrochemical method (Momeni et al., 2019; Yu et al., 2020). The analysis covers different aspects, such as morphology layer surfaces, shape, and size, chemical composition, crystalline size, catalytic and photocatalytic activity, and morphology before and after catalytic and photocatalytic tests. These analyses detail finding and observing a complete transformation of the structure to 4nm anatase nanoparticles.

There are many studies that have been done on nanomaterials, but those directly related to photocatalysis are primarily associated with TiO₂ or ZnO (Chen & Mao, 2007). TiO₂ is used for many applications, such as sunscreens, antibacterials, chemical sensors, pollutant filters, toner photoconductors, and in optoelectronics (Chen, Wang, Wei, & Zhu 2012; Cui, Gao, Qi, Liu, & Sun 2012; Liang, Luo, Tsang, Zheng, Cheng, & Li, 2012). The main use of TiO₂ is as a white pigment that is put into many products, such as white dispersion paints. Today, it is possible to find several industries throughout the world that are producing different kinds of nano-structured titanium dioxide on a large scale (Khvan, Kim, Hong, & Lee, 2011). The TiO₂ semiconductor material shows a vast number of interesting properties, which are maximized when these belong to the nanostructure. However, one of the emerging and intensively explored properties of this nanostructured oxide is its photocatalytic activity, primarily for the treatment of environmental pollution (Ortega-Diaz et al., 2020).

The photocatalytic phenomena of TiO₂ occur due to the electron-hole formation by absorbing photons with energy equal or higher than the bandgap of this semiconductor plus the potential of the surface. Photocatalysis occurs even the adverse presence of a large number of defects in the crystalline structure, such as oxygen vacancies, interstitial titanium atoms from the donor states, titanium vacancies, and interstitial oxygen atoms from the acceptor states. The electronic condition created in this structural arrangement produces a bandgap, an intrinsic characteristic of semiconductor materials, by the existence of a forbidden gap between the valence and the conduction band. The photocatalytic activity of TiO₂ occurs only when photons with energies greater than its bandgap energy can result in excitation of the valence band electrons, which jump to the conduction band, and then can promote a reaction. The absorption of photons with lower energy or longer wavelengths than the bandgap energy usually causes energy dissipation in the form of heat. The illumination of the photocatalytic surface with sufficient energy leads to the formation of a positive hole in the valence band and an electron in the conduction band (Nakata & Fujishima, 2012).