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As mentioned previously, the surface electric field of a metal oscillates when the light strikes the surface. The oscillating electric field causes a rippling wave pattern in the spatial distribution of electrons. According to Lenz's law, the wave created by the surface plasmon opposes the electromagnetic wave of the incident light. The oscillating electrons absorb the energy of light and reemit the energy as the reflected light, due to which metals have shiny and reflective surfaces. However, when the particle size becomes very small, the surface plasmon is confined to a very small surface (i.e. LSPR). When the electron cloud is excited at one of the resonance frequencies, light absorption will become stronger. This is how LSPR frequency affects the light absorption of metal nanoparticles. The plasmon frequency is determined by electron density, dielectric constant, and effective mass of an electron. The well‐defined facets of a crystal form different shapes with more symmetries compared with spherical particles [54]. The surface charges tend to accumulate at edges and corners, which further promote surface polarization, i.e. the charge separation between mobile electrons and immobile atoms. Surface polarization determines the frequency and intensity of LSPR as it provides the main restoring force for electron oscillation. Large surface polarization reduces the restoring force, resulting in a redshift of resonance peak, and multiple distinct symmetries may induce several light absorption peaks [55]. Therefore, the same metal nanoparticles with different size and shape may exhibit different colors, indicating diverse light absorption.

The light absorption of semiconductors is quite different from that in metals due to the electronic band structure in semiconductors. Between the VB and CB of semiconductors, no electron states exist in this energy range called the bandgap. In some semiconductors, the minimal energy state of the CB (conduction band minimum, CBM) and the maximal energy state of the VB (valence band maximum, VBM) are situated in the same crystal momentum in the Brillouin zone (direct gap); in other semiconductors, they are not (indirect gap). There is a slight difference in light absorption between these two types of bandgap structure. But it is not necessary to discuss in this chapter. In general, light absorption of a semiconductor is associated with its bandgap. Semiconductors only absorb photons with the energy equal to or greater than the bandgap. As a result of facets‐dependent anisotropic surface electronic properties that in turn influence the band positions, semiconductor crystals with different dominant facets show shifting in light absorption edges. In applications such as photoelectrochemical catalysis, faceted semiconductors can enhance the light harvesting of photoelectrodes [56–58].

The combination of plasmonic metals and semiconductors with facets engineering has the great potential to adjust the light harvesting for photoelectrodes. For example, the LSPR absorption of the faceted plasmonic metal nanoparticles, such as Au and Pd, can be tuned by embedding them in Cu₂O to form core–shell heterostructure [59–61].