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

Ehsan Vahidzadeh; Sheng Zeng; Kazi M Alam; Pawan Kumar; Saralyn Riddell; Narendra Chaulagain; Sergey Gusarov; Alexander E Kobryn; Karthik Shankar

doi:10.1021/acsami.1c10698

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

ACS Appl Mater Interfaces. 2021 Sep 15;13(36):42741-42752. doi: 10.1021/acsami.1c10698. Epub 2021 Sep 3.

Authors

Ehsan Vahidzadeh¹, Sheng Zeng¹, Kazi M Alam^{1

2}, Pawan Kumar¹, Saralyn Riddell¹, Narendra Chaulagain¹, Sergey Gusarov², Alexander E Kobryn², Karthik Shankar¹

Affiliations

¹ Department of Electrical and Computer Engineering, University of Alberta, 9211-116 Street, Edmonton, Alberta T6G 1H9, Canada.
² Nanotechnology Research Centre, National Research Council Canada, 11421 Saskatchewan Drive, Edmonton, Alberta T6G 2M9, Canada.

PMID: 34476945
DOI: 10.1021/acsami.1c10698

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.

Keywords: FDTD electromagnetic simulations; artificial photosynthesis; interband damping; plasmonic photocatalysis; spontaneous dewetting.