Photoelectrochemical (PEC) water splitting is a promising approach for producing hydrogen without greenhouse gas emissions. Despite decades of unceasing efforts, the efficiency of PEC devices based on earth-abundant semiconductors is still limited by their low light absorption, low charge mobility, high charge-carrier recombination, and reduced diffusion length. Plasmonics has recently emerged This article is protected by copyright. All rights reserved.2 as an effective approach for overcoming these limitations, although a full understanding of the involved physical mechanisms remains elusive. Here, the reported plasmonic effects are outlined such as resonant energy transfer, scattering, hot electron injection, guided modes and photonic effects, as well as the less investigated catalytic and thermal effects used in PEC water splitting. In each section, the fundamentals are reviewed and the most representative examples are discussed, illustrating possible future developments for achieving improved efficiency of plasmonic photoelectrodes.
Hydrogen storage in chemical compounds is a promising strategy to enable lightweight, high-density, and safe hydrogen technologies. However, the hydrogen release rate from these chemicals is limited by the intrinsic catalytic activity of metal catalysts, which can be enhanced by light irradiation. Here, nanohybrids including a core of plasmonic TiN and multiple Pt nanocrystal catalytic centers are assembled and show, under resonant conditions at 700 nm, hot electron-driven hydrogen evolution from ammonia borane at an apparent quantum yield of 120%. It is also demonstrated that solar irradiation enhances the activity of TiN–Pt nanohybrids by one order of magnitude through two synergistic mechanisms: hot electrons and collective-heating contributions. Using the microscopic calculation of the photo-induced temperature around a single nanocrystal, it is revealed that the collective plasmonic heating regime dominates the macroscopic temperature distribution in the system. The presented data show that plasmonic hot electrons and photothermal heating can be used in synergy to trigger hydrogen release from ammonia borane on demand, providing a general strategy for greatly enhancing the activity of metal catalysts in the dark.
Surface plasmons have shown increasingly widespread applications in the last decade, especially in the field of solar energy conversion, recently leading to the use of metal nanoparticles as plasmonic photocatalysts. The latter offers great potential in overcoming traditional catalysts by providing localized heating and unconventional reaction pathways leading to improved product selectivity. A complete understanding of the underlying mechanisms remains, however, elusive due to the close resemblance between thermal and non-thermal effects, both leading to enhanced reaction rates. In this tutorial, we will introduce the basic physics of surface plasmons and the interaction mechanisms with surrounding molecules. We will then discuss the main strategies to evaluate photothermal effects and the main signatures of hot electron-driven processes. These aspects will be covered in specific examples of plasmonic photocatalysis for energy-relevant chemical reactions in the case of colloidal suspensions and at the solid/gas interphase in solid pellets, which involve different thermal constraints and thus different experimental strategies to reveal the effects of localized heating and hot electrons.
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