Activation Energies of Plasmonic Catalysts

Abstract

The activation energy of a catalytic reaction serves not only as a metric of the efficacy of a catalyst but also as a potential indicator of mechanistic differences between the catalytic and noncatalytic reaction. However, activation energies are quite underutilized in the field of photocatalysis. We characterize in detail the effect of visible light excitation on the activation enthalpy of an electron transfer reaction photocatalyzed by plasmonic Au nanoparticles. We find that in the presence of visible light photoexcitation, the activation enthalpy of the Au nanoparticle-catalyzed electron transfer reaction is significantly reduced. The reduction in the activation enthalpy depends on the excitation wavelength, the incident laser power, and the strength of a hole scavenger. On the basis of these results, we argue that the activation enthalpy reduction is directly related to the photoelectrochemical potential built-up on the Au nanoparticle under steady-state light excitation, analogous to electrochemical activation. Under optimum light excitation conditions, a potential as high as 240 mV is measured. The findings constitute more precise insights into the mechanistic role and energetic contribution of plasmonic excitation to chemical reactions catalyzed by transition metal nanoparticles.

Surface Plasmonic‐Assisted Photocatalysis and Optoelectronic Devices with Noble Metal Nanocrystals: Design, Synthesis, and Applications

“…It is the diameter of the noble metal nanocrystals and not their length that causes a change in the Fermi level of the attached semiconductors. For example, when NRs of the same diameter but different lengths are deposited on TiO 2 photocatalysts, the apparent change in the Fermi level is the same . This indicates that the Fermi levels of the metal can be tuned to a suitable position by selecting and controlling the actual diameter of the noble metal NPs for enhanced H 2 activity.…”

“…Understandably, the intensity of light and the excitation wavelength have a significant contribution to the evolution of H 2 gas during the solar water splitting. As the excitation wavelength is increased, the steady‐state polarization is enhanced, and the Fermi levels of noble metals are shifted more upward to reach the CB of the attached semiconductors to improve charge separation for the enhanced H 2 evolution …”

“…As the excitation wavelength is increased, the steadystate polarization is enhanced, and the Fermi levels of noble metals are shifted more upward to reach the CB of the attached semiconductors to improve charge separation for the enhanced H 2 evolution. [57] The apparent increase or decrease in the Fermi level of the metal (E f *) is strongly affected by the size of metal particles. Since small particles are firmly attached to the surface of the semiconductors as compared to the large particles, therefore, they induce more negative shifts in the Fermi level, and consequently they are more suitable for photocatalytic activities.…”

“…For example, when NRs of the same diameter but different lengths are deposited on TiO 2 photocatalysts, the apparent change in the Fermi level is the same. [57] This indicates that the Fermi levels of the metal can be tuned to a suitable position by selecting and controlling the actual diameter of the noble metal NPs for enhanced H 2 activity. Several injected electrons from the noble metal nanocrystals also affect the Fermi levels of the attached semiconductors.…”

“…[77] This approach facilitates the creation of ingestion equilibrium of the hot charge carriers within the plasmonic nanocrystals, hence allowing the hot-electron extraction. [57,77] To realize this concept, Moskovits and co-workers [33b,78] have reported the binary application of cocatalyst that has been modified using plasmonic nanocrystals. They have fabricated vertically oriented gold NR arrays decorated with very small co-catalysts such as the TiO 2 NPs and cobalt/ borate as shown in Figure 9.…”