# Photocatalytic Lithography
## 2 Titanium Dioxide: A Gold Standard for Photocatalysis  
Titanium is the fourth most abundant metal on Earth, exceeded only by aluminum, iron, and magnesium. It was discovered in 1791 by [PERSON], who recognized the presence of a new element in the mineral ilmenite (iron titanate, FeTiO\({}_{3}\)). The element was rediscovered several years later in rutile (TiO\({}_{2}\)) by [PERSON], who named it \"titanium\" after the Titans of Greek mythology [4]. Titanium dioxide can crystallize in three different major structures: rutile, anatase, and brookite. Ratile is the most common and stable TiO\({}_{2}\) polymorph. Anatase and brookite are metastable, but the small differences in the Gibbs free energy between the three phases suggest that they are almost as stable as rutile at normal pressures and temperature. The structures of rutile, anatase, and brookite can be discussed in terms of (TiO\({}_{6}\))\({}^{8-}\) octahedra. The three crystal structures differ by the distortion of each octahedra and by the assembly patterns of the octahedral chains--anatase can be regarded to be built up from octahedra that are connected by their vertices, while rutile are connected by the edges and brookite by both vertices and edges [5]. Since its large-scale production and consequent widespread availability in the early twentieth century, titanium dioxide has been extensively used as a non-toxic pigment in paints and in personal care products. In 1972, [PERSON] and [PERSON] discovered the phenomenon of photocatalytic splitting of water on a TiO\({}_{2}\) electrode under ultraviolet (UV) irradiation [6], an event that marked the beginning of a new era in heterogeneous photocatalysis [7]. Apart from its photoactivity in the near-UV region, titanium dioxide has outstanding features in terms of non-toxicity, low cost, chemical and biological inertness, and photostability. Titanium oxide has been successfully employed for a huge variety of applications, ranging from everyday products (e.g., coatings, cosmetics, paints) to energy conversion (e.g., water splitting, dye sensitized solar cells), air and water remediation from pollutants, electronics, and sensing. The importance and variety of such applications have spurred enormous interest in the fundamental knowledge, the fabrication and the characterization of TiO\({}_{2}\)-based nanomaterials, leading to an unprecedented level of understanding for a metal oxide [8]. Many of the technological applications for TiO\({}_{2}\) are directly related to its photoactivity in the near-UV region (Figure 1). The initial process in photocatalysis is the generation of electron-hole pairs in the semiconductor, caused by the light-induced electron promotion from the valence band (VB) to the conduction band (CB). Unlike metals, semiconductors do not possess a continuum of inter-band energetic levels that can assist the e\({}^{-}\)-h\({}^{+}\) recombination. Thus, the e\({}^{-}\)-h\({}^{+}\) pair has a lifetime long enough to allow the transfer of the photoexcited electron or hole to a reagent adsorbed on the photocatalyst surface. This process is called \"direct photocatalysis\" if the semiconductor is eventually found chemically intact and the charge transfer to the adsorbed species is continuous and exothermic [5]. Since the charge transfer happens at the surface, electrons and holes must migrate to the semiconductor surface, where electrons can reduce an acceptor species and holes can combine with an electron released by the oxidation of a donor species. In ambient conditions, i.e., in slightly-humid air, oxygen O\({}_{2}\) is the acceptor, giving rise to the superoxide radical O\({}_{2}\)\({}^{\bullet-}\) and subsequently to hydrogen peroxide H\({}_{2}\)O\({}_{2}\). On the other hand, holes often combine with water and other hydroxyl species adsorbed on the surface, giving rise to hydroxyl radicals OH\({}^{\bullet}\), active intermediates in the oxidation of various substances [9; 10].