Harvesting Hot Holes in Plasmon-Coupled Ultrathin Photoanodes for High-Performance Photoelectrochemical Water Splitting

Abstract

The harvesting of hot carriers produced by plasmon decay to generate electricity or drive a chemical reaction enables the reduction of the thermalization losses associated with supra-band gap photons in semiconductor photoelectrochemical (PEC) cells. Through the broadband harvesting of light, hot-carrier PEC devices also produce a sensitizing effect in heterojunctions with wide-band gap metal oxide semiconductors possessing good photostability and catalytic activity but poor absorption of visible wavelength photons. There are several reports of hot electrons in Au injected over the Schottky barrier into crystalline TiO2 and subsequently utilized to drive a chemical reaction but very few reports of hot hole harvesting. In this work, we demonstrate the efficient harvesting of hot holes in Au nanoparticles (Au NPs) covered with a thin layer of amorphous TiO2 (a-TiO2). Under AM1.5G 1 sun illumination, photoanodes consisting of a single layer of ∼50 nm diameter Au NPs coated with a 10 nm shell of a-TiO2 (Au@a-TiO2) generated 2.5 mA cm–2 of photocurrent in 1 M KOH under 0.6 V external bias, rising to 3.7 mA cm–2 in the presence of a hole scavenger (methanol). The quantum yield for hot-carrier-mediated photocurrent generation was estimated to be close to unity for high-energy photons (λ < 420 nm). Au@a-TiO2 photoelectrodes produced a small positive photocurrent of 0.1 mA cm–2 even at a bias of −0.6 V indicating extraction of hot holes even at a strong negative bias. These results together with density functional theory modeling and scanning Kelvin probe force microscope data indicate fast injection of hot holes from Au NPs into a-TiO2 and light harvesting performed near-exclusively by Au NPs. For comparison, Au NPs coated with a 10 nm shell of Al2O3 (Au@Al2O3) generated 0.02 mA cm–2 of photocurrent in 1 M KOH under 0.6 V external bias. These results underscore the critical role played by a-TiO2 in the extraction of holes in Au@a-TiO2 photoanodes, which is not replicated by an ordinary dielectric shell. It is also demonstrated here that an ultrathin photoanode (<100 nm in maximum thickness) can efficiently drive sunlight-driven water splitting.

Single Atom Catalysts for Selective Methane Oxidation to Oxygenates

“…For instance, Vahidzadeh et al reported that AuNPs embedded in a thin film of amorphous TiO 2 can inject hole holes in TiO 2 film which promotes PEC water oxidation. 392 Decoration of plasmonic nanoparticles on TiO 2 can not only improve the photocatalytic performance but can also control the product selectivity under different light irradiation. Zeng et al found that periodically modulated TiO 2 nanotubes (PMTiNTs) photonic crystal modified with AuNPs demonstrates CH 4 as the main product under solar simulated light while formaldehyde was a dominating product when the catalyst was irradiated with solar simulated/and or UV and 550 nm light.…”

Plasmon Mediated Electron Transfer and Temperature Dependent Electron‐Phonon Scattering in Gold Nanoparticles Embedded in Dielectric Films

“…[10] Nevertheless, if metal NPs are in intimate contact with an adjacent semiconductor, the excited carrier can be injected into semiconductor before the relaxation. [11][12][13] Plasmonic energy conversion from photoexcited metal NPs is a promising alternative path compared to conventional electron-hole separation in semiconductor materials. [14] Plasmon driven hot charge carrier transfer to semiconductors (SC) has been discussed in several excellent reviews.…”

“…Apart from this scattering process, LSPR can be damped in several ways such as inter band damping, intra‐band (Landau), electron‐surface scattering and chemical interface damping mechanism (CID) [10] . Nevertheless, if metal NPs are in intimate contact with an adjacent semiconductor, the excited carrier can be injected into semiconductor before the relaxation [11–13] . Plasmonic energy conversion from photo‐excited metal NPs is a promising alternative path compared to conventional electron‐hole separation in semiconductor materials [14] .…”

Hot hole transfer from Ag nanoparticles to multiferroic YMn2O5 nanowires enables superior photocatalytic activity

“…The harvesting of hot holes in such plasmon-semiconductor heterojunctions is relatively rare [33][34][35] in comparison to the large number of reports on successful hot electron harvesting in junctions between n-type semiconductors and coinage metals. One major engineering problem is the difficulty in achieving high performance Schottky barriers between p-type semiconductors and high work function coinage metals.…”