# Nanomaterials in Photocatalysis: An In-Depth Analysis of Their Role in Enhancing Indoor Air Quality
## 13 By-Products and Degradation in Photocatalysis  
While the potential formation of by-products during photocatalytic reactions is acknowledged in the current study, a more detailed discussion is warranted. By-products, including secondary chemical intermediates and reaction products, can be generated depending on the nature of the substrate, catalyst, and reaction conditions. These by-products may affect the overall efficiency of the process, reduce selectivity, or introduce environmental and health risks [79]. In this context, potential by-products such as reactive oxygen species (ROS) [273], organic degradation products, and metal leaching from catalysts should be considered. ROS, such as hydroxyl radicals (-OH) and superoxide anions (O\({}_{2}\)\({}^{-}\)), are often formed during photocatalysis and can lead to secondary oxidation products that may be toxic or difficult to remove. Organic substrates undergoing incomplete oxidation can produce intermediate compounds like aldehydes, ketones, or carboxylic acids, which could pose environmental hazards or interfere with the desired photocatalytic process. Other examples of by-products include the formation of oxygenated species such as hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) from humid air, which, if not properly handled, can cause further degradation or undesired reactions [274]. In some photocatalytic processes, the photoreduction of substrates may lead to the production of toxic substances such as heavy metal ions or halogenated compounds when halide-containing substrates are used. To mitigate the formation of undesired by-products, several strategies can be employed. First, optimizing reaction conditions such as light intensity and temperature can help control reaction pathways and reduce the formation of secondary products. Additionally, the use of more stable and selective photocatalysts [275,276], such as those with improved electronic properties or better surface characteristics, can help minimize side reactions. The degradation of photocatalytic nanomaterials is another critical factor to consider, as it directly impacts the stability, reusability, and environmental implications of these systems [277]. Prolonged photocatalytic reactions or harsh operational conditions, such as exposure to intense light, high temperatures, or extreme pH, can lead to the breakdown or structural modification of nanomaterials [278]. This degradation often results in reduced photocatalytic efficiency over time and the release of nanoparticles or dissolved ions into the surrounding environment. One of the most common risks associated with nanomaterial degradation is the leaching of metal ions from catalysts. For example, photocatalysts containing metals like titanium (Ti), zinc (Zn), or silver (Ag) may release these ions when exposed to reactive oxygen species (ROS), acidic or basic conditions, or prolonged irradiation [279]. The leached ions can have toxic effects on aquatic ecosystems, as they may accumulate in organisms or interfere with biochemical pathways. In particular, silver ions (Ag\({}^{+}\)) and zinc ions (Zn\({}^{2+}\)) are known to exhibit cytotoxic effects, even at low concentrations, potentially posing risks to both environmental and human health. In addition to metal leaching, the structural degradation of nanomaterials can lead to the generation of nanoscale fragments, including smaller nanoparticles or amorphous residues [280; 281]. These fragments may exhibit different physicochemical properties compared to the original material, such as higher mobility or increased reactivity. For example, reduced particle size can enhance the penetration of nanoparticles into biological tissues, raising concerns about bioaccumulation and cytotoxicity. Studies have indicated that certain degraded nanomaterials can trigger oxidative stress, inflammation, or genotoxicity in living organisms, underscoring the need for a thorough assessment of their long-term environmental and biological impacts.