**4. The influence of gamma radiation on titanium dioxide photocatalysis**

Titanium dioxide (TiO2), a widely-employed photocatalyst, boasts reactive properties, commendable stability, cost-effectiveness, and biocompatibility. TiO2 nanoparticles, of exceptional interest as photocatalysts, exhibit sterling photocatalytic activity since their nanometric size and high surface area promote such reactivity. Photocatalytic activity of TiO2 materials has been linked to a number of variables, gamma radiation among them. The effect of gamma rays on the photocatalytic activity of TiO2 nanoparticles has been investigated using a multitude of samples.

A research study by Bouregui et al. [16] examined the impact of gamma radiation on the photocatalytic performance of controlled atmosphere TiO2 nanoparticles. The nanoparticles underwent gamma irradiation at varying doses for investigation into their optical, structural, and photocatalytic attributes. According to the results, gamma radiation exerted favorable effects on the optic and structural properties of the TiO2 nanoparticles. Further, there was a rise in nanoparticle crystallinity while their bandgap energy decreased due to increased gamma radiation dosages. The photocatalytic activity of the nanoparticles also improved with gamma radiation dosages by enhancing their surface area and creating oxygen vacancies. Such vacancies served as recombination centers for electron-hole pairs, elevating the charge separation efficacy and photocatalytic activity of the nanoparticles.

The impact of gamma radiation on the photocatalytic features of TiO2 thin films was investigated in other studies conducted by Semalti et al. [22] and Haldar et al. [23]. Semalti et al. found that photocatalytic activity was amplified by gamma radiation, particularly at higher doses, due to the accumulation of oxygen vacancies and the development of Ti3+ ions, which boosted carrier separation. In contrast, Haldar et al. found that TiO2 nanoparticle photocatalytic activity decreased with gamma radiation exposure due to defects and impurities formed in the crystal lattice, leading to reduced efficiency of charge separation and decreased photocatalytic activity.

A report published by Sajjadi et al. [24] shared that TiO2 nanoparticles' photocatalytic activity reduced after gamma irradiation. Their research showed that as gamma radiation dose increased, the degradation rate of methylene blue dye under UV light irradiation also reduced. This is due to the reduction of oxygen vacancies and hydroxyl radicals. Xue et al. [25], on the other hand, discovered a rise in photocatalytic activity of TiO2 nanoparticles at higher doses of gamma radiation. However, they associated the decrease in activity at low doses to the formation of recombination centers for electron-hole pairs and oxygen vacancies, but the high concentration of Ti3+ ions enhanced the carriers' separation.

We may reconcile these two studies by considering that oxygen vacancies and Ti3+ ions may impact TiO2 nanoparticle photocatalysis. Low gamma radiation may increase oxygen vacancies, limiting photocatalytic activity, but at higher doses Ti3+ ions may dominate oxygen vacancies, boosting photocatalytic activity.

Ti3+ ions may impact the photocatalytic activity of TiO2 nanoparticles [26], which can also be influenced by nanoparticle size, shape, contaminants, dopants, and irradiation conditions. Gamma radiation is known to affect the electronic structure and surface area of TiO2 crystals, potentially altering the generation and recombination

of electron-hole pairs vital to photocatalytic activity. The degree and type of defects formed by gamma radiation are dependent on a range of variables such as radiation dose, nanoparticle type, and environmental factors.

TiO2 materials remains an encouraging option for environmental use cases like purifying water and controlling air pollution despite contradictory findings. Using TiO2-based photocatalysis in solar photocatalysis has proven to effectively decontaminate and disinfect water [27]. The utilization of sunlight activates the photocatalytic reaction in this technology, without requiring any external energy sources or chemicals. Moreover, nanotechnologies based on TiO2 have been created to purify and recycle water [28]. Their performance under visible light irradiation [29] is enhanced by combining TiO2 nanoparticles with other materials like graphene.

Photocatalytic performance of TiO2 materials can be enhanced by altering its surface properties. TiO2 nanoparticles that have high-energy {001} facets exhibit improved photocatalytic activity due to increased adsorption of reactant molecules and facilitated separation of charge carriers. This phenomenon has been previously established by researchers [30]. The photocatalytic activity of TiO2 nanoparticles can be enhanced by modifying their surface with metals or nonmetals, which improves their electron transfer and surface plasmon resonance properties.

Machine learning methods were utilized to examine the photocatalytic mechanism of semiconducting materials like TiO2. Photocatalytic processes involve various intricate steps such as adsorption-reaction mechanisms for reactants' generationseparation mechanisms for charged particles role played by defects impurities; all these procedures can be investigated with these techniques. To explore the photocatalytic mechanism of TiO2 nanoparticles under UV and visible light irradiation, Wu et al. [31] used machine learning algorithms. This study revealed that nanoparticles' photocatalytic activity was most significantly influenced by its surface structure and defects. Additionally, other important factors include its adsorption energy of reactant molecules and its energy barrier for electron transfer.

To summarize, the influence of gamma radiation on the photocatalytic operation of TiO2 nanoparticles is still being questioned. Although some studies observed an improvement in photocatalytic performance, others noted a decrease. The inconsistent findings could be ascribed to the different experimental parameters and TiO2 nanoparticles employed in each analysis. Additional investigation is necessary to fully comprehend the fundamental mechanisms and optimize the conditions for employing gamma-irradiated TiO2 nanoparticles in photocatalytic applications. TiO2 remains a hopeful photocatalyst for environmental implementations, and its photocatalytic efficiency may be boosted by combining it with other substances or altering its surface attributes. Learning algorithms may provide useful understandings into the intricate techniques implicated in photocatalysis and lead the way in designing more dependable photocatalysts.
