As explained previously, another approach to capturing longer-wavelength light includes modifying the surface of TiO2 with narrow band gap semiconductors (like CdS, PbS, CdSe, Bi2S3) and molecular dyes as sensitizers. This approach, although theoretically possible for water photoelectrolysis, has practical problems, chief among them that most photo-sensitizers are too unstable for practical application. Instead of visible light sensitization, an emerging new method for water splitting involves the incorporation of cocatalysts onto the semiconductor support, such as Co-Pi and IrO2. These oxygen-evolving cocatalysts serve to reduce the kinetic limitations inherent in multi-electron redox reactions at semiconductor surfaces [31,41,42,73].
An emerging new approach for increasing the photocatalytic activity of semiconductors such as TiO2 and a-Fe2O3 involves the use of plasmonic nanomaterials, such as Au and Ag nanoparticles [38,43,44,94,95]. When exposed to visible light of the appropriate frequency, plasmonic nanoparticles exhibit collective oscillations of conduction electrons known as localized surface plasmons (LSPs). This coherent oscillation of the electron gas results in the creation of an enhanced EM field, spatially confined near the nanoparticle surface [96]. As the optical transition rate in semiconductors is proportional to the magnitude of the incident electric field (E), these strongly localized EM fields generated by LSPs can be used to increase the rate of charge-carrier formation in the semiconductor, and thus the rate of photocatalytic reactions [38,44,94,95,97,98]. This scheme effectively uses the unique optical properties of plasmonic nanomaterials to act as optical nanoantennas, which efficiently couple the incident EM radiation into the semiconductor [38,95,97,98]. Another possible advantage of using Au nanoparticles interfaced with semiconductors such as TiO2 involves the direct electron transfer from the Au NPs to the TiO2 CB. The excitation of LSPs generates a non-thermalized Fermi distribution with sufficient energy to overcome the Schottky barrier formed at the Au/TiO2 interface and tunnel to the conduction band of TiO2 [81,99-103]. Due to their large optical cross-sections, Au nanoparticles are excellent candidates for visible light sensitizers when interfaced directly with a wide band gap semiconductor such as TiO2. Moreover, the optical response of noble metal nanoparticles can be tuned across the visible to near-infrared spectral regime by varying the size, shape, and surrounding dielectric material. The ability to control the optical properties allows the Au/TiO2 composite photocatalyst to take advantage of a broad spectrum of solar energy. Based on this mechanism, Silva et al. applied Au nanoparticles supported on P25 titania (Au/TiO2) for water splitting under visible light (532 nm laser or polychromatic light X > 400 nm) [104]. They separately probed the water oxidation and reduction mechanisms by use of sacrificial reagents such as methanol and silver nitrate, and observed that the Au/TiO2 photocatalyst could photocatalytically split water into hydrogen (Figure 12.11a) and oxygen (Figure 12.11b). Since gold nanoparticles represent a chemically robust sensitizer with tunable optical properties, this approach may offer additional stability, flexibility, and longevity as compared to traditional molecular dyes.
