# Nanomaterials in Photocatalysis: An In-Depth Analysis of Their Role in Enhancing Indoor Air Quality
## 11 Transition Metal Dichalcogenides  
Transition metal dichalcogenides (TMDs) have been studied since the mid-20 th century, primarily for their unique electronic, optical, and mechanical properties. These materials are represented by the chemical formula MX\({}_{2}\), where \"M\" is a transition metal (e.g., Mo, W, Ti) and \"X\" is a chalcogen (S, Se, or Te). TMDs consist of layered structures where transition metal atoms are sandwiched between two layers of chalcogen atoms, forming weakly bonded van der Waals layers, which can be exfoliated into two-dimensional (2D) sheets. Early works focused on bulk materials like molybdenum disulfide (MoS\({}_{2}\)) and tungsten disulfide (WS\({}_{2}\)), particularly for their potential as solid lubricants due to their layered, easily sheared structures. Interest in TMDs expanded during the 1970s and 1980s, driven by their potential as semiconductors for photovoltaic and optoelectronic applications. Researchers discovered that certain TMDs, such as MoS\({}_{2}\) and WSe\({}_{2}\), exhibit direct or indirect bandgaps depending on their thickness, making them attractive for light-harvesting applications. Their catalytic properties, particularly in hydrogen evolution reactions, also gained attention during this period. The isolation of graphene in 2004 reignited interest in layered materials, including TMDs. Researchers began investigating their 2D forms, discovering that monolayer TMDs exhibit unique properties distinct from their bulk counterparts. For example, monolayer MoS\({}_{2}\) transitions from an indirect bandgap (bulk) to a direct bandgap (\(\sim\)1.8 eV), making it highly efficient for optoelectronic devices. TMDs have shown great promise in various photocatalytic applications (Table 6). One of the most notable is in hydrogen production, where materials like MoS\({}_{2}\) and WS\({}_{2}\) have demonstrated significant capabilities for photocatalytic water splitting. They can also efficiently degrade organic pollutants, such as dyes and volatile organic compounds (VOCs), through photocatalytic oxidation processes. Additionally, TMDs are being explored for carbon dioxide reduction, where they can convert CO\({}_{2}\) into valuable hydrocarbons, highlighting their potential in addressing both energy and environmental challenges. The layered structure of TMDs, especially in their exfoliated 2D form, provides an extremely high surface area, increasing the number of active sites available for photocat \begin{table}
\begin{tabular}{c c c c} \hline
**TMD Material** & **Structure** & **Applications** & **References** \\ \hline MoS\({}_{2}\) & Layered structure with a hexagonal crystal lattice & CO\({}_{2}\) reduction, degradation of pollutants & [252; 253; 254; 255] \\ WS\({}_{2}\) & Layered structure similar to MoS\({}_{2}\) & CO\({}_{2}\) reduction, degradation of pollutants & [256; 257; 258] \\ TiS\({}_{2}\) & Layered structure with a distorted & Photocatalytic degradation of pollutants & [259; 260] \\  & octahedral coordination & & \\ MoSe\({}_{2}\) & Layered structure with hexagonal symmetry & CO\({}_{2}\) reduction, degradation of pollutants & [261; 262; 263] \\ WSe\({}_{2}\) & Layered structure with hexagonal lattice & CO\({}_{2}\) reduction, degradation of pollutants & [264; 265] \\ ReS\({}_{2}\) & Layered structure with hexagonal symmetry & CO\({}_{2}\) reduction, degradation of pollutants & [266] \\ ZrS\({}_{2}\) & Layered structure with hexagonal symmetry & Organic pollutant degradation & [267; 268; 269] \\  & Layered structure with a distorted & Dye degradation & [270; 271] \\  & octahedral coordination & & \\ SnS\({}_{2}\) & Layered structure with hexagonal symmetry & Photocatalytic degradation of pollutants & [271; 272] \\ \hline \end{tabular}
\end{table} 
Table 6: Common TMDs materials in photocatalysis, structures, and examples of their applications. alytic reactions. This enhances their ability to interact with and degrade air pollutants like volatile organic compounds (VOCs) and nitrogen oxides (NO\({}_{\rm x}\)).