**1. Introduction**

Titanium dioxide (TiO2) is one of the most studied photocatalysts, especially considering its application in the photocatalytic degradation of micropollutants. TiO2 can induce specific redox reactions through photogenerated charge carriers in photocatalysis. Such process can be divided into subsequent steps, including (1) excitation of electrons in the TiO2 structure; (2) dissociation of the generated excitons to free electrons and holes; (3) migration of the charge carriers to the surface; and (4) transfer of the e or h<sup>+</sup> to substrate present at the surface. All of these steps are common for every photocatalytic reaction, and each deals with limitations that influence the overall process efficiency. However, steps (3) and (4) occur at the surface. Therefore, any change at the interface between the photocatalyst and a substrate can

induce significant changes in the involved elementary reactions. The importance of this interface and photocatalyst surface was realized very early in the photocatalytic studies, discussing problems like surface polarization with excess electrons, modification with noble metals, and surface complexation with bidentate benzene derivatives to improve the transfer of the charge carriers [1–3]. Simultaneously, the challenges of surface trapping and recombination of the generated charge carriers were also highlighted.

However, concerning these early studies, the exact geometry of the photocatalyst surface was not considered at this point, and ultrafine particles were studied without well-defined geometry. In the last years, significant progress has been made both in the preparation procedures of the TiO2 particles and in the application of computational methods to simulate the geometry and properties of such interfaces at the atomic scale. As a result, stabilization of a specific interface structure can be achieved during the photocatalyst preparation, leading to the formation of faceted particles terminated with specific, well-defined crystal planes. At present, the application of such single-crystalline particles can be considered a state-of-the-art approach for investigating the details of photocatalytic reactions. Moreover, when systematically studied, it allows for the optimization of the final structure of the photocatalyst and increases its activity in a specific reaction [4]. This is primarily a result of a preferred electronic structure of the exposed facet. Therefore, photogenerated electrons and holes may accumulate at different crystal facets leading to improved charge carriers' separation and more selective photocatalytic reactions. In this regard, this chapter concisely highlights recent state-of-the-art progress in (1) the synthesis of crystalfacet exposed anatase, rutile, and brookite, (2) crystal facet-dependent properties of TiO2, and (3) the correlation of surface properties with photocatalytic activity in photodegradation of emerging pollutants present in water, H2 generation, and CO2 reduction into valuable chemicals. Furthermore, by reviewing the research progress on crystal facet engineering of TiO2, we hope to provide directions for future selective semiconductor modification with electron-donor or electron-acceptor to improve the overall efficiency in photocatalytic reaction kinetics and mechanism.
