Metal Nanoparticle Photocatalysts: Synthesis, Characterization, and Application

Abstract

than photons spend travelling at light speed in vacuum. [4] The NP-light interaction can produce both a high concentration of energetic electrons and a strong electric field at the NPs' surfaces. The lifetime of the coherent electron oscillation due to LSPR effect is ≈5-100 fs, [5] and can dephase through three pathways (depicted in Figure 1): (1) elastic radiative reemission of photons, (2) nonradiative Landau damping, causing the excitation of energetic electrons and holes in the metal particles, and (3) the interaction of excited surface plasmons with unpopulated adsorbate acceptor states. [5] All three plasmon decay processes can excite adsorbates present at the surface of the NPs. Energy transfer by the first decay pathway usually requires UV photons to cause excitation of the internal electronic transition of an adsorbate, [6] and requires large-sized particles to make the process dominant (larger than 40 nm for Au NPs as an example [1] ). Moreover, the low efficiency of photoemission during the process excludes the radiative energy transfer from being a major contributor of plasmonic enhancement. [7] Pathways (2) and (3) are the nonradiative decay processes, which are dominant for small NPs. In process (2), plasmon decay by Landau damping, where the energy is converted to a single electron/ hole exciton, occurs ≈10 fs after the initial plasmon excitation. [8] The excited electrons can interact with other electrons through Coulombic inelastic scattering (electron-electron scattering), couple to phonon modes, thereby heating the metal lattice, and to the surroundings (electron-phonon energy relaxation). [9] When metal NPs adsorb chemicals, the addition of adsorbates to the surface of the plasmonic nanostructures can induce the third ultrafast (≈5 fs) dephasing pathway, which is called chemical interface damping (CID). [4] In the CID process, the decay of a resonant plasmon causes direct excitation of an electron to an unoccupied adsorbate acceptor state. As shown in Figure 2, the hot electrons generated by light excitation are injected into the lowest unoccupied molecular orbital (LUMO) of an adsorbed molecule directly, and the injection lowers the energy of the formed transient anions by weakening the chemical bonds of the adsorbate. [10] Chemical transformation can then occur if the deposited energy is enough to overcome the activation barrier for the dissociation of the adsorbed molecule. Considering the lifetime of vibrational levels of adsorbates on metals is around 1-10 ps, [9] subsequent electron injections can occur before the molecular vibration has fully dissipated.Metal nanoparticles (NPs) have emerged as a kind of new photocatalyst to drive various chemical reactions by visible-light irradiation. A distinct advantage of metal NP photocatalysts is that their light absorption is not limited to a certain wavelength but instead they are able to utilize a broad range of wavelengths, constituting a large fraction of the solar spectrum. Metal NPs like gold, silver, and copper NPs can strongly absorb vis...

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