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Numerous studies have shown that catalysts with facets engineering exhibit greater catalytic performance. This exploits one or more unique properties of the well‐defined crystal facets to tune the overall catalytic activity and/or selectivity.

For metal catalysts, activity and selectivity are related to the surface atomic structure of the catalysts, where they can be tuned to enhance effective adsorption and/or promote favorable coordination of adsorbates. These processes are strongly influenced by the arrangement and coordination of surface atoms as well as by the corresponding surface density of states of the different facets. For example, there are two types of flat surfaces of Pt, namely, the Pt{111} facet (hexagonal surface) where each surface atom has six nearest neighbors, and the Pt{100} facet (square surface) where each surface atom has four nearest neighbors. The hexagonal surface is up to seven times more active than the square surface in the aromatization reaction of n‐heptane to toluene, but the square surface is seven times more active than the hexagonal surface in the alkane isomerization reaction of isobutane to n‐butane [62]. Also in electrocatalysis, the Pt{210} facet has high activity for electrooxidation of formic acid and electroreduction of CO₂ [63]; the Pt{410} facet exhibits high performance in NO decomposition [64]; the Pt{730} shows superior activity in electrooxidation of formic acid and methanol [34].

For the semiconductors in photo‐related catalytic processes, the performance of the faceted semiconductor crystals is affected by the synergetic combination of the intrinsic properties of the bulk and surfaces. A typical photocatalytic process includes the following steps: light absorption of the semiconductor catalyst, excited charges (electrons and holes) generation, excited electrons and holes recombination (bulk and surface), charge migration to the surface, charge trapping at the surface, and transfer to reactants. As such, an ideal semiconductor with good reactivity and selectivity is expected to have a suitable bandgap to absorb the light as much as possible, good separation of charge carriers to avoid the recombination at the bulk and surface, good adsorption of reactants at the surface, fast surface transfer of charge carriers from catalysts to reactants, and suitable CBM and VBM position to provide electron–hole pairs with sufficient redox potentials to catalyze the reactions. Each aspect is affected by one or more properties of the surface and bulk. And these effects are not isolated and may affect one another. Figure 2.6 shows how the properties synergistically affect the reactivity and selectivity of semiconductor catalysts and the significance of facets engineering.

Figure 2.6 Diagram of how the facets engineering affects the selectivity and reactivity of a semiconductor photocatalyst.