# Nanomaterials in Photocatalysis: An In-Depth Analysis of Their Role in Enhancing Indoor Air Quality
## 4 Photocatalytic Oxidation
### Mechanism  
Photocatalytic oxidation reactions are activated when the semiconductor surface is irradiated with photons with energy equal to or greater than the semiconductor band gap. The semiconductor absorbs the photons, promoting the excitation of electrons from the valence band to the conduction band of the semiconductor. Consequently, photogenerated electrons are obtained in the conduction band, while positively charged holes are formed in the valence band. These electron-hole pairs are generated within femtoseconds, setting the stage for subsequent chemical reactions [78]. In the second step, the photogenerated electrons are transferred to the semiconductor surface. Electrons and holes can directly react with the pollutants through oxidation/reduction reactions, or they can reduce electron acceptors or oxidize electron donors, \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Substance** & **Found in** & **Effects** & **References** \\ \hline Methylene chloride & Paint strippers, adhesives, and aerosol products & Dizziness, headaches, nausea, central nervous system depression and cancer & [51, 52, 53] \\ \hline Tetrachloroethylene & Solvent in dry cleaning and degreasing & Dizziness, headaches, nausea respiratory problems, kidney damage and cancer & [54, 55, 56] \\ \hline Toluene & Solvent in coatings and paints & Nausea, headache, vertigo, detrimental effects on the nervous system, kidney and liver & [57, 58, 59] \\ \hline Xylene & solvent used in the printing, rubber, and leather industries, paints, varnishes, and cleaning agents & Headaches, dizziness, and respiratory issues upon exposure. May affect the central nervous system, liver and kidney damage & [60, 61, 62] \\ \hline
1,3-Butadiene & used in the production of synthetic rubber and plastics & respiratory irritation and headaches, increased risk of cancer & [63, 64, 65] \\ \hline Glycol Ethers & solvents commonly found in paints, cleaning agents, and coatings & respiratory irritation and reproductive toxicity & [66, 67, 68] \\ \hline Limonene and other \({}^{\ast}\)green\({}^{\ast}\) products & fragrances, cleaning products, solvents & Respiratory irritation, allergic reactions & [69, 70, 71] \\ \hline NO\({}_{\text{x}}\) & combustion of fossil fuels & Respiratory irritation, decreased lung function, increase susceptibility to infections, chronic respiratory diseases, cardiovascular problems & [72, 73, 74] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._respectively. Specifically, photogenerated electrons and holes can interact with nearby H\({}_{2}\)O and O\({}_{2}\) molecules to form reactive oxygen species (ROS) such as hydroxyl radicals and superoxide ions. The reduction of O\({}_{2}\) leads to the formation of the superoxide ion (O\({}_{2}\).\({}^{-}\)), while the hydroxyl radical (-OH) is obtained by the ionization of H\({}_{2}\)O. Pollutant molecules are then adsorbed onto the active sites on the surface of the photocatalyst, where they can react with these ROS. Pollutants are primarily oxidized by the hydroxyl radicals derived from the oxidation of adsorbed H\({}_{2}\)O or adsorbed hydroxide ions (OH\({}^{-}\)). The main products of photocatalytic oxidation are CO\({}_{2}\) and H\({}_{2}\)O, but it is crucial to limit the formation of minor by-products, as these can also be hazardous compounds or reduce the efficiency of photocatalysis. By-products can occupy the active sites of the photocatalyst, thereby limiting access to the target pollutants [79]. The general photocatalytic oxidation reaction for the degradation of a pollutant can be expressed as described in Table 3 and as follows:  
\[\text{Pollutant}+\text{O}_{2}\rightarrow\text{CO}_{2}+\text{H}_{2}\text{O}+ \text{By}\_{\text{products}}\]  
The electron-hole pairs can react with each other through recombination on the surface or within the bulk of the semiconductor.