## 1 Introduction  
### Titania Properties and Uses  
Titanium dioxide or titania is certainly the most studied semiconducting oxide due to its well-established photo-activity. Well-known features that render it so attractive are its low cost, negligible toxicity, high stability, easy handling, and resistance to chemical corrosion. This semiconductor has the known ability to absorb light in the ultraviolet (UV) wavelength range and generate excited charges, electrons and holes, which separate in the conduction and valence band, respectively, and are at the core of its photo-activity. This key property of \(\text{TiO}_{2}\) has been widely investigated firstly in catalysis and later in medicine, especially to attain antimicrobial coatings for medical implants [1]. The wide applications of titania for air and water remediation, cultural heritage preservation, and self-healing, as well as microbial inactivation and the mitigation of SARS-CoV-2 spreading onto surfaces, have been recently reviewed [2] and are thus not discussed in detail in this review.  
Despite the many advantages offered by titania, this material also suffers from some drawbacks, such as a rapid recombination of the photo-excited charge carriers and the relatively wide band gap (in the range of 3.2 eV) which implies a poor utilization of solar light due to the small percentage of UV radiation of sunlight reaching the Earth's surface. Remedies that have been developed to address these issues include the use of suitable supports [3], doping with other elements [4], embedding in composites [5], and the development of suitable nanostructures [6], also following green protocols [7]. These approaches have indeed allowed the practical widespread use of TiO\({}_{2}\)--for instance, for biomass conversion processes [8]--thanks to the improved hydrothermal stability of the catalyst [9]. In particular, the combination of titania with noble metals and carbon nanostructures has allowed the development of high-performing gas sensors [10] and photocatalysts [11]. Dye-sensitizers combined with titania allow metal-free photocatalysts to be attained for hydrogen production using visible light [12]. Hydrogen-peroxide production is another area in which titania is highly promising [13], as well as solar cells [14].  
The morphology and crystallinity of the titania phase are important parameters in the determination of the resulting photo-electronic properties of the final material [15]. However, only a few studies compare the photo-activity of the three phases of titania (rutile, brookite, and anatase; Figure 1) [16], while the majority of works focus on the generally best-performing anatase phase [17]. Titania can be produced by several methods, with sol-gel [18], hydrothermal [19], and solvothermal [20] processes being the most popular, and in some cases also microwave-assisted methods [21]. However, other approaches are also often used, such as anodic oxidation [22] and atomic layer deposition [23], and others are under continuous development; for instance, the use of laser beams [24], molecular layer deposition [25], and air-plasma spraying [26].  
Figure 1: Crystal structures of titania anatase, rutile, and brookite polymorphs. Reprinted from [21].