**2. Synthesis of titania nanoparticles**

An exponential growth of research activities has been noticed in nanoscience and nanotechnology in the past decades [17, 18]. When the size of the material becomes smaller and smaller, down to the nanometer scale, new physical and chemical properties are obtained. Properties also change by the changing in the morphologies of the shrinking nanomaterials. One of the best features of these materials is the swimming of electrons and holes in semiconductor nanomaterials, which are first organized using the well-known quantum confinement, and the moving features owned to phonons and photons are greatly related to the size and geometry of the materials [19]. The specific surface area and surface-to-volume ratio increase dramatically as the size of a material decreases [20, 21]. Continuously, breakthroughs have been made in the preparation, modification, and applications of TiO<sup>2</sup> nanomaterials. Here, we focus on recent progress in the synthesis, properties, modifications, and applications of TiO<sup>2</sup> nanomaterials. The syntheses of TiO2 nanomaterials, including nanoparticles, nanorods (NRs), nanowires (NWs), and nanotubes (NTs), are primarily categorized with the preparation techniques. For detailed instructions on each synthesis, the readers are referred to the corresponding literature related to the common major four preparation methods as well as the obtained shapes of titanium dioxide nanopowders.

#### **2.1. Synthetic methods for titanium dioxide nanostructures**
