Surface modification of titanium dioxide (TiO2) is a critical strategy to enhance its photocatalytic performance without altering its bulk composition. These techniques focus on engineering the surface properties to optimize light absorption, charge carrier dynamics, and interfacial reactions. Key methods include acid/base treatment, plasma modification, and surface fluorination, each of which modifies surface hydroxyl groups, acidity, and adsorption behavior, ultimately influencing photocatalytic efficiency.

Acid and base treatments are straightforward yet effective approaches to tailor the surface chemistry of TiO2. Treatment with acids such as hydrochloric acid (HCl) or sulfuric acid (H2SO4) introduces protons onto the TiO2 surface, increasing the density of Brønsted acid sites. These sites enhance the adsorption of negatively charged species, which is beneficial for degrading anionic pollutants. Acid treatment also removes surface contaminants and creates defects that act as trapping centers for photogenerated electrons, reducing recombination rates. Conversely, base treatment with sodium hydroxide (NaOH) or ammonia (NH3) increases the concentration of surface hydroxyl groups, creating Lewis basic sites. These sites improve the adsorption of cationic pollutants and facilitate the generation of reactive oxygen species (ROS) such as hydroxyl radicals (•OH), which are crucial for oxidative degradation. The balance between acidic and basic sites is essential for optimizing surface reactivity, as excessively acidic or basic surfaces may hinder photocatalytic activity by altering charge transfer pathways.

Plasma modification is another powerful technique to functionalize TiO2 surfaces without introducing bulk impurities. Plasma treatments using gases such as oxygen (O2), nitrogen (N2), or argon (Ar) can generate reactive species that interact with the TiO2 surface. Oxygen plasma, for example, increases the concentration of surface hydroxyl groups and creates oxygen vacancies. These vacancies serve as electron traps, prolonging the lifetime of photogenerated charge carriers. Nitrogen plasma introduces nitrogen-containing functional groups, which can enhance visible-light absorption by creating localized states within the bandgap. However, the primary advantage of plasma treatment lies in its ability to uniformly modify surface properties without damaging the crystal structure. The process also improves wettability, which enhances the interaction between the catalyst surface and aqueous reactants, promoting interfacial electron transfer.

Surface fluorination is a unique approach that significantly alters the surface properties of TiO2. Fluorine ions (F−) can replace surface hydroxyl groups, creating a fluorinated layer that modifies surface acidity and electronic structure. Fluorination increases the positive charge density on Ti sites due to the strong electronegativity of fluorine, which enhances the adsorption of organic molecules and facilitates hole transfer to adsorbed species. The fluorinated surface also suppresses the recombination of electron-hole pairs by trapping electrons in Ti3+ states and holes in surface fluorine complexes. Additionally, fluorination stabilizes surface Ti–OH groups, which are active sites for ROS generation. The resulting surface exhibits higher photocatalytic activity for both oxidation and reduction reactions due to improved charge separation and interfacial reactivity.

The modification of surface hydroxyl groups plays a central role in determining photocatalytic performance. Hydroxyl groups act as adsorption sites for reactants and participate in the formation of ROS. Acid/base treatments directly alter the density and nature of these groups, while plasma and fluorination indirectly influence their stability and reactivity. For instance, acid treatment reduces the number of surface hydroxyl groups but increases their acidity, favoring specific reaction pathways. Plasma treatment replenishes hydroxyl groups and introduces new defects, whereas fluorination replaces them with fluorine but maintains high surface reactivity through alternative mechanisms.

The impact of these modifications on charge separation and interfacial reactions is profound. Surface treatments create trapping sites for electrons and holes, preventing their recombination and increasing their availability for redox reactions. Acid-treated surfaces often exhibit enhanced electron trapping due to defect formation, while plasma-treated surfaces improve hole mobility through increased hydroxylation. Fluorination uniquely balances electron and hole trapping by introducing fluorine-related surface states. These changes directly influence the kinetics of photocatalytic reactions, such as pollutant degradation and water splitting, by optimizing the surface charge transfer processes.

Adsorption properties are also significantly affected by surface modifications. Acid treatment enhances the affinity for anionic species, while base treatment improves cationic adsorption. Plasma treatment increases overall surface hydrophilicity, promoting the adsorption of polar molecules. Fluorination creates a highly polarized surface that strongly interacts with organic pollutants, facilitating their degradation. The interplay between surface charge, hydrophilicity, and adsorption capacity determines the overall photocatalytic efficiency, as reactant adsorption is often the rate-limiting step in heterogeneous catalysis.

In summary, surface modification techniques such as acid/base treatment, plasma modification, and fluorination provide precise control over the physicochemical properties of TiO2 photocatalysts. By engineering surface hydroxyl groups, acidity, and adsorption characteristics, these methods enhance charge separation and interfacial reactivity without resorting to doping or hybrid material strategies. The choice of modification depends on the target application, as each technique offers distinct advantages in optimizing photocatalytic performance. Understanding these surface interactions is crucial for designing advanced TiO2-based photocatalysts for environmental and energy applications.