The photocatalytic activity of titanium dioxide (TiO2) is strongly influenced by its crystal phases and exposed facets. TiO2 primarily exists in three naturally occurring polymorphs: anatase, rutile, and brookite. Each phase exhibits distinct electronic and structural properties that dictate its photocatalytic performance. Additionally, the exposed crystal facets play a critical role in surface reactivity, charge carrier dynamics, and interfacial interactions. Understanding these factors is essential for designing efficient TiO2-based photocatalysts.

Anatase, rutile, and brookite differ in their crystal structures and bandgap energies. Anatase has a tetragonal structure with a bandgap of approximately 3.2 eV, while rutile also exhibits a tetragonal structure but with a slightly narrower bandgap of around 3.0 eV. Brookite, an orthorhombic phase, has a bandgap similar to anatase but is less studied due to its challenging synthesis. The conduction band minimum of anatase is more negative than that of rutile, facilitating stronger reduction potential for photocatalytic reactions. However, rutile generally shows higher charge carrier mobility but suffers from rapid electron-hole recombination. Brookite, though less common, has demonstrated intermediate properties between anatase and rutile in certain photocatalytic applications.

The photocatalytic efficiency of TiO2 is not solely determined by the individual phases but also by the interfaces between them. Phase junctions, particularly anatase-rutile heterophase systems, have been widely investigated for their synergistic effects. The most notable example is the commercially available P25 TiO2, which consists of approximately 80% anatase and 20% rutile. The heterojunction between these phases enhances charge separation due to the energy level alignment at the interface. Electrons from the anatase conduction band can transfer to rutile, while holes migrate in the opposite direction, reducing recombination losses. Brookite-anatase and brookite-rutile interfaces have also shown improved photocatalytic activity, though their mechanisms are less understood compared to anatase-rutile systems.

Exposed crystal facets significantly influence surface reactivity and photocatalytic behavior. TiO2 crystals are bounded by low-index facets such as {001}, {101}, and {010} for anatase; {110}, {101}, and {001} for rutile; and {210}, {001}, and {111} for brookite. The surface energy and atomic arrangement of these facets dictate their chemical activity. For anatase, the {001} facet is highly reactive due to its high surface energy and abundance of undercoordinated titanium atoms, making it favorable for oxidation reactions. In contrast, the {101} facet is more stable but less reactive. Rutile {110} is often considered the most active facet for photocatalytic processes, while brookite {210} has shown promising activity in some studies. Controlling facet exposure is crucial for optimizing surface-mediated reactions such as adsorption, dissociation, and charge transfer.

Synthesis strategies for phase-controlled TiO2 nanomaterials involve precise control over reaction conditions. Hydrothermal and solvothermal methods are commonly employed due to their ability to regulate phase formation through temperature, pH, and precursor selection. For instance, anatase is typically obtained at lower temperatures (below 600°C), while rutile becomes thermodynamically favorable at higher temperatures (above 600°C). Brookite formation is often achieved under acidic conditions with specific precursors like titanium trichloride. The use of structure-directing agents, such as fluoride ions, can selectively stabilize high-energy facets like anatase {001}. Sol-gel processing allows for phase tuning through calcination temperature and precursor chemistry, while chemical vapor deposition can produce phase-pure films with controlled facet orientation.

Heterophase TiO2 systems can be engineered through sequential or co-synthesis approaches. Sequential methods involve post-synthesis annealing to induce phase transformation, creating controlled interfaces between anatase and rutile. Co-precipitation or one-pot hydrothermal synthesis enables the direct formation of mixed-phase TiO2 with intimate contact between phases. The relative ratio of phases can be adjusted by varying reaction time, temperature, or precursor concentrations. For facet control, kinetic growth suppression techniques are employed to favor the exposure of high-energy facets. For example, the addition of hydrofluoric acid during hydrothermal synthesis promotes anatase {001} facet growth by stabilizing these surfaces.

The photocatalytic performance of TiO2 is evaluated based on charge carrier generation, separation, and surface reactions. Phase junctions enhance charge separation by creating internal electric fields at the interfaces, while facet engineering optimizes surface reaction kinetics. The combination of these strategies can lead to highly active photocatalysts for applications such as water splitting, pollutant degradation, and CO2 reduction. For instance, anatase-rutile composites with dominant {001} facets have demonstrated superior activity in methylene blue degradation compared to single-phase materials. Similarly, brookite-anatase hybrids have shown enhanced performance in hydrogen evolution due to improved charge transfer.

In summary, the photocatalytic activity of TiO2 is governed by its crystal phase composition and exposed facets. Anatase, rutile, and brookite each contribute unique electronic and structural properties, while phase junctions facilitate efficient charge separation. Facet-dependent reactivity further tailors surface chemistry for specific photocatalytic applications. Synthesis methods that enable precise control over phase and facet formation are critical for optimizing TiO2 photocatalysts. By leveraging these principles, researchers can design advanced TiO2 nanomaterials with enhanced photocatalytic efficiency for sustainable energy and environmental applications.