Читать книгу Heterogeneous Catalysts - Группа авторов - Страница 25

For a metal, the surface electric field is oscillating when the light strikes the surface. Light, an electromagnetic wave, oscillates the electric field in a plane perpendicular magnetic field. The electric field's oscillatory patterns would cause a rippling wave pattern in the distribution of electrons, where the resonant oscillation of conduction electrons is called surface plasmon resonance (SPR). The SPR only exists in metals or other electrically conductive materials containing conduction electrons. When the size of the metal crystals shrinks to the nanoscale, which is smaller than the wavelength of the incident light, the surface plasmon is confined to a very small surface rather than the bulk material, known as localized surface plasmon resonance (LSPR). The LSPR frequency affects the light absorption and scattering of metal nanoparticles. How the LSPR frequency is affected by facets (shapes) of a nanoparticle is explained in a later section, but as a consequence, the color of metal nanoparticles will be changed, and it is sensitive to the shape of nanoparticles (with different facets exposed). Based on the Mie theory, it is possible to tune the LSPR spectra of Ag nanocrystals of different shapes, as shown in Figure 2.4 [51].

Figure 2.4 Calculated UV–visible extinction (black), absorption (red), and scattering spectra (blue) of Ag nanocrystals, illustrating the effect of shape on its spectral characteristics, including isotropic sphere (a), anisotropic cubes (b), tetrahedra (c), and octahedra (d), triangular plate (e) and circular disc (f).

Source: Wiley et al. 2006 [51]. Reproduced with permission of American Chemical Society.

(See online version for color figure).

For a faceted semiconductor, the surface electric field is a disparate situation. Besides the variation of the bandgap, the band edge position shifts as a function of different facets due to surface band bending. The different band edge positions provide varying redox potentials of the photogenerated electrons and holes, resulting in spatial separation of charge carriers and the built‐in electric field. In addition, selectively depositing a noble metal as cocatalysts on the surface facets can further enhance the strength of the built‐in electric field. When a single BiVO₄ crystal enclosed by {010} and {011} facets was characterized by spatially resolved surface photovoltage spectroscopy (SRSPS), {011} facets exhibited a much higher signal intensity of surface photovoltage than {010} facet [52]. This phenomenon indicated a significant difference in surface band bending between BiVO₄{011} and {010} facets. As a consequence, the different band bending will lead to the variation in the spatial distribution of the charge carriers and build an electric field between different facets. By changing the area ratio of (011)/(010) facets of BiVO₄ crystal, the surface built‐in electric field varied as well. Such an intrinsic difference in the surface photovoltage between different facets can be further enhanced by selectively depositing cocatalysts, such as MnO_x and Pt deposited on faceted BiVO₄ crystal, as shown in Figure 2.5 [53].

Figure 2.5 (a) Scanning electron microscopy (SEM) image of a BiVO₄ single crystal with Pt photodeposited on {010} facet and MnO_x photodeposited on {011} facet. (b) Spatial distribution of the surface photovoltage signals. Pink and green colors correspond to holes and electrons separated toward the external surface, respectively. Schematic band diagrams across the border between the {011} and {010} facets of (c) a bare single BiVO₄ photocatalyst particle and of (d) a single BiVO₄ photocatalyst particle with MnO_x cocatalyst selectively deposited at {011} facets (green line) and with MnO_x and Pt nanoparticles selectively deposited at {011} and {010} facets, respectively (dashed pink line).

Source: Zhu et al. 2017 [53]. Reproduced with permission of American Chemical Society. (See online version for color figure.)