Electron Transfer Between Colloidal ZnO Nanocrystals

Abstract

Colloidal ZnO nanocrystals, capped with dodecylamine and dissolved in toluene, can be charged photochemically to give stable solutions in which electrons are present in the conduction bands of the nanocrystals. These conduction band electrons are readily monitored by EPR spectroscopy, with g* values that correlate with the nanocrystal sizes. Mixing a solution of charged small nanocrystals with a solution of uncharged large nanocrystals, e-CB:ZnO–S + ZnO–L, causes changes in the EPR spectrum indicative of quantitative electron transfer from small to large nanocrystals. EPR spectra of the reverse reaction, e-CB:ZnO–L + ZnO–S, show that electrons do not transfer from large to small nanocrystals. Stopped-flow kinetic studies monitoring the change in the UV band edge absorption show that reactions of 50 μM nanocrystals are complete within the 5 ms mixing time of the instrument. Similar results are obtained for the reaction of charged nanocrystals with methyl viologen (MV2+). These and related results indicate that the electron transfer reactions of these colloidal nanocrystals are quantitative and very rapid, despite the presence of ~1.5 nm long dodecylamine capping ligands. These soluble ZnO nanocrystals are thus well-defined redox reagents suitable for electron transfer studies involving semiconductor nanostructures.

Dispersible Plasmonic Doped Metal Oxide Nanocrystal Sensors that Optically Track Redox Reactions in Aqueous Media with Single‐Electron Sensitivity

“…To that end, our results are complementary to recent work by Gamelin and Mayer to advance the understanding of lightdriven redox chemistry of ZnO nanocrystals in organic media as it relates to their infrared plasmonic properties, [20][21][22][23][24][25][26] as well as work by Manna and Tassone, [27][28][29][30][31][32] Talapin and Feldmann, [ 33 ] and Jain and Alivisatos [ 34 ] investigating chemically induced NIR plasmon shifts in substrate-bound Cu 2− x E nanostructures, where E = S, Se, or Te. Our use here of plasmonic-doped metal oxides is expected to overcome the experimental challenges in using noble metal NCs to detect optically one or a few redox events per nanocrystal, owing to the signifi cantly higher carrier concentrations typifi ed by Au and Ag nanostructures.…”

Redox Potentials of Colloidal n-Type ZnO Nanocrystals: Effects of Confinement, Electron Density, and Fermi-Level Pinning by Aldehyde Hydrogenation

“…As detailed previously, photodoping leads to bleaching of the ZnO bandedge absorption (inset) and growth of a comparably intense IR intra-band absorption feature, both indicating a growing population of delocalized CB electrons (e − CB ). 1–3,5,6,14–16,23–26 The IR absorbance can be analyzed to provide the average number of electrons per ZnO nanocrystal (〈 n 〉) at each level of photodoping. In the present experiments, new absorption features at 390 and 500 nm are also observed upon photodoping, attributable to the LMCT and d−d transitions of cobaltocene (Cp 2 Co), respectively.…”

New materials for tunable plasmonic colloidal nanocrystals

“…112 Indeed, because of quantum confinement, the conduction band of small NCs is higher in energy than that of larger NCs. 112 Therefore, when two NCs are brought in contact, extra charges (electrons) can flow from smaller to larger NCs, but not vice versa.…”