Hot Charge Carriers in Quantum Dots: Generation, Relaxation, Extraction, and Applications

Many parameters influence the photocatalytic production of H 2. Identifying and quantifying them is necessary for correct comparison of photocatalysts and right understanding of involved mechanism. In this work, we studied the photocatalytic dehydrogenation of isopropanol with titania-supported ruthenium disulfide. We studied the influence of seven parameters on the photocatalytic activity: the temperature, the composition of the reactive mixture, the mass of the photocatalyst, the flux and the energy of the incident photons, the co-catalyst loading and the nature of the support. Their influence was studied, not only on the rate of hydrogen production (or photon yield) but also on the apparent activation energy and on the preexponential factor, deduced from an Arrhenius law. The photon yield as a function of the co-catalyst loading show an optimum of activity. A 6.9 % photon yield was obtained at 0.84 wt% and = 3.65. The rate-determining step for the photocatalytic dehydrogenation of the isopropanol with RuS 2 /TiO 2 is, at optimal conditions, the electron transfer from TiO 2 to RuS 2. The latter is not favored because of the band diagrams of RuS 2 and TiO 2. The electron transfer can be optimized working with incident photons having a higher energy, thanks to a hot carrier effect observed in RuS 2 /TiO 2 .

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“…so-called action spectra), the , and the , . It is not the first time that hot charge carriers were evidenced in photocatalysts [91,92].…”

Section: Electron Transfer and Influence Of The Incident Photons Energymentioning

confidence: 99%

Photocatalytic production of H2 is a multi-criteria optimization problem: Case study of RuS2/TiO2

et al. 2021

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“…The longest-lived hot electrons in semiconductor QDs exhibit lifetimes near 1 ns in core-shell QDs, 165,168 however they typically relax back to the band edge within picoseconds, depending on the QD surface chemistry. 78,[169][170][171] One advantage of employing QDs in this context is their ability to bind organic molecules as ligands (up to several hundred per CdS QD). Pre-association between substrates and the QD surface can help bypass the kinetic obstacles associated with highly reducing but short-lived excited states.…”

Section: Discussionmentioning

confidence: 99%

“…The ps lifetimes of hot electrons in QDs, while insufficient for efficient collisional electron transfer, are sufficiently long for electron transfer to surface-bound or nearby species. 78,80,172 While many of the substrates used in this study do not contain functional groups commonly used as strongly-binding QD ligands, many classes of weakly coordinating molecules, including solvent, may transiently associate with the QD surface through dispersion, electrostatic, or other noncovalent interactions. 48,105,130 In principle, this would allow the observed reductions to take place through ultrafast charge transfer to the adsorbed substrate competitive with hot electron cooling.…”

Section: Discussionmentioning

confidence: 99%

“…While short lived, 68,77 these highly reducing "hot" electrons can be transferred to nearby or adsorbed species, and have been employed to improve aqueous hydrogen evolution and CO 2 reduction. 73,[78][79][80] Auger processes are well studied phenomena in nanomaterials, however they have not been employed to drive synthetic organic transformations requiring powerful reducing agents. We show here that CdS QDs can be employed as powerful photoreductants for organic synthesis through a two-photon mechanism involving Auger recombination to generate hot electrons.…”

Section: Introductionmentioning

confidence: 99%

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CdS Quantum Dots as Potent Photoreductants for Organic Chemistry Enabled by Auger Recombination

Widness

Enny

McFarlane-Connelly

et al. 2022

Preprint

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Strong reducing agents (< -2.0 V vs SCE) enable a wide array of useful organic chemistry, but suffer from a variety of limitations. Stoichiometric metallic reductants such as alkali metals and SmI2 are commonly employed for these reactions, however considerations including expense, ease of use, safety, and waste generation limit the practicality of these methods. Recent approaches utilizing energy from multiple photons or electron-primed photoredox catalysis have accessed reduction potentials equivalent to Li0 and shown how this enables selective transformations of aryl chlorides via aryl radicals. However, in some cases low stability of catalytic intermediates can limit turnover numbers. Herein we report the ability of CdS nanocrystal quantum dots (QDs) to function as strong photoreductants and present evidence that a highly reducing electron is generated from two consecutive photoexcitations of CdS QDs with intermediate reductive quenching. Mechanistic experiments suggest that Auger recombination, a photophysical phenomenon known to occur in photoexcited anionic QDs, generates transient thermally excited electrons to enable the observed reductions. Using blue LEDs and sacrificial amine reductants, aryl chlorides and phosphate esters with reduction potentials up to -3.4 V vs. SCE are photo-reductively cleaved to afford hydrodefunctionalized or functionalized products. In contrast to small molecule catalysts, the QDs are stable under these conditions and turnover numbers up to 47500 have been achieved. These conditions can also effect other challenging reductions, such as tosylate protecting group removal from amines, debenzylation of alcohols, and reductive ring-opening of cyclopropanecarboxylic acid derivatives.

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“…We will introduce the two main concepts of consideration when designing a HCPV device, namely 1) the maintenance and 2) the extraction of a hot-carrier population, both of which will be discussed in detail in the later sections. In no way is this attempting to be an exhaustive review of HCPVs, and for further reading on that topic the reader is referred to excellent, existing review articles, 1,3,[29][30][31][32] most recently by König et al 15 .…”

Section: Hot-carrier Photovoltaicsmentioning

confidence: 99%

Hot-carrier optoelectronic devices based on semiconductor nanowires

Fast

Aeberhard

Bremner

et al. 2021

Applied Physics Reviews

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In optoelectronic devices such as solar cells and photodetectors, a portion of electron-hole pairs are generated as so-called hot carriers with an excess kinetic energy that is typically lost as heat. The long standing aim to harvest this excess energy to enhance device performance has proven to be very challenging, largely due to the extremely short-lived nature of hot carriers. Efforts thus focus on increasing the hot carrier relaxation time, and on tailoring heterostructures that allow for hot-carrier extraction on short time-and length-scales. Recently, semiconductor nanowires have emerged as a promising system to achieve these aims, because they offer unique opportunities for heterostructure engineering as well as for potentially modified phononic properties that can lead to increased relaxation times. In this review we assess the current state of theory and experiments relating to hot-carrier dynamics in nanowires, with a focus on hot-carrier photovoltaics. To provide a foundation, we begin with a brief overview of the fundamental processes involved in hot-carrier relaxation, and how these can be tailored and characterized in nanowires. We then analyze the advantages offered by nanowires as a system for hot-carrier devices and review the status of proof-of-principle experiments related to hot-carrier photovoltaics. To help interpret existing experiments on photocurrent extraction in nanowires we provide modeling based on non-equilibrium Green's functions. Finally, we identify open research questions that need to be answered in order to fully evaluate the potential nanowires offer towards achieving more efficient, hotcarrier based, optoelectronic devices.

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Hot Charge Carriers in Quantum Dots: Generation, Relaxation, Extraction, and Applications

Cited by 13 publications

References 110 publications

Photocatalytic production of H2 is a multi-criteria optimization problem: Case study of RuS2/TiO2

Photocatalytic production of H2 is a multi-criteria optimization problem: Case study of RuS2/TiO2

CdS Quantum Dots as Potent Photoreductants for Organic Chemistry Enabled by Auger Recombination

Hot-carrier optoelectronic devices based on semiconductor nanowires

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