Hot Carrier Generation and Extraction of Plasmonic Alloy Nanoparticles

Abstract

The conversion of light to electrical and chemical energy has the potential to provide meaningful advances to many aspects of daily life, including the production of energy, water purification, and optical sensing. Recently, plasmonic nanoparticles (PNPs) have been increasingly used in artificial photosynthesis (e.g., water splitting) devices in order to extend the visible light utilization of semiconductors to light energies below their band gap. These nanoparticles absorb light and produce hot electrons and holes that can drive artificial photosynthesis reactions. For n-type semiconductor photoanodes decorated with PNPs, hot charge carriers are separated by a process called hot electron injection (HEI), where hot electrons with sufficient energy are transferred to the conduction band of the semiconductor. An important parameter that affects the HEI efficiency is the nanoparticle composition, since the hot electron energy is sensitive to the electronic band structure of the metal. Alloy PNPs are of particular importance for semiconductor/PNPs composites, because by changing the alloy composition their absorption spectra can be tuned to accurately extend the light absorption of the semiconductor. This work experimentally compares the HEI efficiency from Ag, Au, and Ag/Au alloy nanoparticles to TiO2 photoanodes for the photoproduction of hydrogen. Alloy PNPs not only exhibit tunable absorption but can also improve the stability and electronic and catalytic properties of the pure metal PNPs. In this work, we find that the Ag/Au alloy PNPs extend the stability of Ag in water to larger applied potentials while, at the same time, increasing the interband threshold energy of Au. This increasing of the interband energy of Au suppresses the visible-light-induced interband excitations, favoring intraband excitations that result in higher hot electron energies and HEI efficiencies.

Efficient Nanoporous Hematite Photoanodes Prepared by Electron Beam Evaporation and Au Modification

“…Au NPs transfer the plasmonic energy to the nearby semiconductor through different plasmonic mechanisms including hot electron injection, plasmon induced resonant energy transfer and near field electromagnetic enhancement . Also, Au NP decoration results in reduction of charge recombination at the semiconductor/electrolyte interface, which improves the PEC performance of the semiconductor photoanodes ,,. Although many studies have made significant progresses in production of Au modified hematite photoanodes, modification of hematite films prepared by PVD methods have less been investigated in literatures and the previous studies did not proceed to a remarkable PEC improvement …”

“…In addition, the IPCE of these samples extends to sub‐bandgap region (λ>600 nm) where no inter‐band optical absorption in the bare hematite takes place. The latter can be attributed to hot‐electron injection from Au NPs into the conduction band of hematite ,,. Figure S6 in supplementary data shows an optical absorption edge of λ=600 nm, obtained by Tauc plot analysis for the porous bare hematite film.…”

“…Figure d shows the IPCE enhancement factor as calculated by dividing the IPCE values of the Au‐3/hematite photoanode to those of the bare hematite photoanode. If hot electron injection is the only mechanism which affects the photo‐response of hematite sample, the IPCE increment curve would only be depend on the surface plasmon resonance . Due to the negligible photoactivity of the bare hematite, the enhancement factor raises significantly at longer wavelengths beyond 650 nm.…”

“…The first one considers a co‐catalytic role for the Au NPs by assisting the charge transfer from the semiconductor surface to the electrolyte ,,. In this approach, the migration of hot electrons through the photoanode toward the counter electrode in the external circuit, and involvement of the so called “hot holes” left behind in the Au NPs, in the oxygen evolution reaction, has been considered as a co‐catalytic role for the Au NPs ,. On the other hand, another scenario for the facilitating surface charge transfer considers a surface passivation role for the Au NPs.…”

“…[71][72][73] Also, Au NP decoration results in reduction of charge recombination at the semiconductor/electrolyte interface, which improves the PEC performance of the semiconductor photoanodes. [55,59,74] Although many studies have made significant progresses in production of Au modified hematite photoanodes, modification of hematite films prepared by PVD methods have less been investigated in literatures and the previous studies did not proceed to a remarkable PEC improvement. [65] In this work, nanostructured porous hematite photoanodes are prepared using an oblique angle deposition (OAD).…”