Core/shell nanoparticles as efficient reducing agents

(Nanowerk Spotlight) Semiconductor nanoparticles, also known as quantum dots (QDs), have been studied for their size-tunable electronic and optical properties for over 30 years. Applications have been demonstrated for biological sensing and imaging, as well as for electronic devices such as LEDs and low-cost solar cells.
Recent endeavors have examined the efficacy of QDs as redox agents. This allows them to reduce H+ to generate solar fuels from sunlight and to effect other organic transformations such as CO2 reduction (Read more: ACS Energy Letters, "Designing the Surfaces of Semiconductor Quantum Dots for Colloidal Photocatalysis").
Unfortunately, core QDs are unstable because they tend to precipitate in solution in a matter of hours which results in surface damage that renders the nanoparticles useless. Concerning the use of QDs as fluorescent sensors or for LEDs, this well-known problem can be solved by overcoating the core with an inorganic shell.
Such core/shell dots are more robust due to their literal insulation from the environment. However, this insulation minimizes the redox activity of the semiconductor.
Atomistic (A) and course-grained (B) models of CdZnS and CdZnS/ZnS, respectively, reveal QD hole states that are closely paired to a surface-bound reduced organic substrate
Atomistic (A) and course-grained (B) models of CdZnS and CdZnS/ZnS, respectively, reveal QD hole states that are closely paired to a surface-bound reduced organic substrate. The result is a high Stern-Volmer quenching constant (C) for the core/shell QD compared to the core material alone. (Image courtesy of the researchers) (click on image to enlarge)
As recent report from a team of researchers from the University of Illinois at Chicago (UIC) and US Army researchers from Ft. Belvoir has turned this paradigm upside down, as they demonstrate core/shell nanoparticles can be engineered to display enhanced redox activity towards organic substrates.
As published in the journal Nanoscale ("Charge carrier pairing can impart efficient reduction efficiency to core/shell quantum dots: applications for chemical sensing"), CdZnS quantum dots were being developed as fluorescent sensors for high energy (i.e. explosive) chemicals such as RDX and TNT.
To their surprise, the material was found to be more effective towards TNT reduction when passivated by a ZnS shell, by a factor of 300%. This observation prompted further experimental and theoretical investigations.
They found that the inorganic passivation prevents trapping of the semiconductor’s electron and hole by surface defects, allowing the quantum dot to remain in the excited state for a longer period of time.
This in turn provided more opportunity for a substrate to diffusively interact with the semiconductor, upon which it is reduced.
One mystery remained – while the enhanced excited state lifetime was shown to enhance substrate reduction by the passivated QD, the inorganic shell should have nonetheless put a stop to redox activity.
This problem was studied theoretically; using the supercomputer facilities at UIC the group found that the near-degeneracy of the valence states in core CdZnS and the ZnS shell allows the hole to easily cross over from the core into the shell.
This means that the electron on the surface-bound substrate and the hole in the shell can remain in close contact, which minimizes the energy of the state.
The researchers posit that this close interaction must stabilize the reduced substrate, which counters the effect of the shell’s electronic barrier.
The unexpected demonstration of enhanced redox activity of core/shell CdZnS/ZnS quantum dots will impact on several scientific fronts. Many new analytical platforms are being developed that rely on QD reduction and oxidation as signal transduction mechanisms.
Furthermore, QDs are being sought after as suppliers of electron and hole charge carriers for biological and chemical processes; many of these are relevant to alternative energy and chemical syntheses.
These activities may benefit from the use of CdZnS/ZnS QDs, or from a design that allows for charge carriers to remain in close contact upon reduction or oxidation of a preferred organic substrate.
Provided by the University of Illinois at Chicago as a Nanowerk exclusive

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