## 1 Introduction
### Ultra-violet Photofunctionalization on Titanium Surface
#### 1.3.1 Photocatalytic Degradation and Water Decomposition  
TiO\({}_{2}\) in any form, either in anatase, rutile, or brookite, exhibit excellent optical properties [26]. They possess powerful photocatalysts for various significant reactions due to their chemical stability and high reactivity. The original water decomposition reaction photocatalyst by light was proposed by [PERSON] and [PERSON] [65]. They suggested that the water molecules decomposed into oxygen and hydrogen with TiO\({}_{2}\) as a cathodic catalyst and in the presence of UV light, following the overall equations below:  
\[\text{(oxidation reaction)}\;\text{TiO}_{2}+\text{h}\
u\to\text{e}^{-}+ \text{h}^{+} \tag{1}\] \[\text{(reduction reaction)}\;\text{2H}_{2}\text{O}+4\text{h}^{+} \to\text{O}_{2}+4\text{H}^{+}\] (2) \[2\text{H}^{+}+2\text{e}^{-}\to\text{H}_{2}\] (3) \[\text{The overall reaction is}\;\text{2H}_{2}\text{O}+4\text{h}\
u\to \text{O}_{2}+2\text{H}_{2} \tag{4}\]  
TiO\({}_{2}\) surface has been reported to gradually increase in the water-contact angle induced by longer storage period, however, UV irradiation repeatedly regenerated surface amphiphilicity [22; 27; 28] from the water decomposition reaction. This is a photochemical reaction catalyzed by TiO\({}_{2}\) upon exposure to UV light. Some molecules such as oxygen and water molecules were adsorbed on or desorbed from the titanium surfaces under UV light consisting of wavelengths shorter than its band gap, approximately 415 nm [66] or with an energy above the band gap energy [67]. The photo-induced superhydrophilicity of TiO\({}_{2}\) surfaces was initially explained by an increase in the amount of hydroxyl groups formed through UV light irradiation [27]. During UV light exposure, photoexcited electrons are captured by oxygen molecules, creating holes and forming electron-deficient transition species, which later deprotonate water molecules to form two hydroxyl groups, each coordinated to different titanium cations [33]. This phenomenon creates surface oxygen vacancies at the bridging sites, thereby resulting in the conversion of relevant Ti\({}^{4+}\) sites to Ti\({}^{3+}\) sites [42]. The latter are favorable for dissociative water adsorption [57]. In the dark, the hydroxyl groups gradually desorb from the surface in the form of H\({}_{2}\)O\({}_{2}\) or H\({}_{2}\)O + O\({}_{2}\).  
Photocatalytic decomposition of organic contaminants is a process different to that of photoinduced hydrophilic conversion. When TiO\({}_{2}\) is irradiated with UV light (\(\lambda<380\) nm: energy greater than the band gap of TiO\({}_{2}\) = 3.2 eV), an electron-hole pair is generated. In turn, adsorbed molecules such as oxygen and water molecules are reduced rapidly and oxidized to produce reactive oxygen species such as superoxide ions (\({}^{\bullet}\)O\({}_{2}\)\({}^{-}\)) and hydroxyl radicals (\({}^{\bullet}\)OH) [32]. These oxygen radicals react with the inorganic or organic surface impurities, thereby leading to their decomposition and removing the hydrocarbon compounds from the titanium surface [67]. The overall reaction is schematically presented in Figure 2. Greater carbon contamination was observed on non-UV-treated surfaces compared with that on UV-treated surfaces, with carbon content reducing upon UV treatment [25; 42; 68]. The removal of carbon increased the wettability and altered the surface charge from electronegative to electropositive.