A General Kinetic-Optical Model for Solid-State Reactions Involving the Nano Kirkendall Effect. The Case of Copper Nanoparticle Oxidation

Oxidation of copper nanoparticles (NPs) and other solid-state reactions have been shown recently to involve the nanoscale Kirkendall effect (NKE). The process, resulting in the formation of internal voids, has been rarely considered in most of the simple reaction kinetic models currently available. Here we present a general solid-state reaction kinetic model based on the assumption of steady-state diffusion profiles for rationalizing the evolution of the optical behavior observed in plasmonic Cu NPs undergoing solid-state oxidation to Cu₂O. While the model is applied here to an oxidation process, it is applicable in principle to any system showing Kirkendall voiding. An analytical expression of the rate law for spherical NPs is derived, and implications of the model are discussed. By combining the general kinetic model with Mie scattering solutions under the quasi-static approximation, the extinction cross section of Cu NPs as a function of the Cu-to-Cu₂O conversion fraction is modeled, for NPs of different initial sizes and for different degrees of shrinkage upon formation of a Cu₂O shell layer. The experimental observation of an initial increase in the surface plasmon (SP) band extinction intensity, followed by a decrease, is qualitatively reproduced using the combined model. This characteristic behavior of Cu nanoplasmonic systems is not expected theoretically in sufficiently small NPs; however, NPs of > ∼6 nm in diameter are expected to exhibit this type of behavior. Our results suggest that the NKE may be important in describing the optical behavior of plasmonic NPs subject to solid-state oxidation

Refractive Index Sensing Using Visible Electromagnetic Resonances of Supported Cu₂O Particles

Plasmonic metal nanostructures, in colloidal or surface-supported forms, have been extensively studied in the context of metamaterials design and applications, in particular as refractometric sensing platforms. Recently, high refractive index (high-<i>n</i>) dielectric subwavelength structures have been experimentally shown to support strong Mie scattering resonances, predicted to exhibit analogous refractive index sensing capabilities. Here we present the first experimental demonstration of the use of supported high-<i>n</i> dielectric nano/microparticle ensembles as refractive index sensing platforms, using cuprous oxide as a model high-<i>n</i> material. Single-crystalline Cu₂O particles were deposited on transparent substrates using a chemical deposition scheme, showing well-defined electric and magnetic dipolar resonances (EDR and MDR, respectively) in the visible range, which change in intensity and wavelength upon changing the medium refractive index (<i>n</i>_m). The significant modulation of the MDR intensity when <i>n</i>_m is modified appears to be the most valuable empirical sensing parameter. The Mie scattering properties of Cu₂O particles, particularly the spectral dependence of the MDR on <i>n</i>_m, are theoretically modeled to support the experimental observations. MDR extinction changes (i.e., refractive index sensitivity) per particle are >100 times higher compared to localized surface plasmon resonance (LSPR) changes in supported Au nanoislands, encouraging the evaluation of Cu₂O and other high-<i>n</i> dielectric particles and sensing modes in order to improve the sensitivity in optical (bio)­sensing applications

Chemical Deposition of Cu₂O Nanocrystals with Precise Morphology Control

Copper(I) oxide nanoparticles (NPs) are emerging as a technologically important material, with applications ranging from antibacterial and fungicidal agents to photocatalysis. It is well established that the activity of Cu₂O NPs is dependent on their crystalline morphology. Here we describe direct preparation of Cu₂O nanocrystals (NCs) on various substrates by chemical deposition (CD), without the need of additives, achieving precise control over the NC morphology. The substrates are preactivated by gold seeding and treated with deposition solutions comprising copper sulfate, formaldehyde, NaOH, and citrate as a complexant. Production of NC deposits ranging from complete cubes to complete octahedra is demonstrated, as well as a full set of intermediate morphologies, <i>i</i>.<i>e</i>., truncated octahedra, cuboctahedra, and truncated cubes. The NC morphology is defined by the NaOH and complexant concentrations in the deposition solution, attributed to competitive adsorption of citrate and hydroxide anions on the Cu₂O {100} and {111} crystal faces and selective stabilization of these faces. A sequential deposition scheme, <i>i.e.</i>, Cu₂O deposition on pregrown Cu₂O NCs of a different morphology, is also presented. The full range of morphologies can be produced by controlling the deposition times in the two solutions, promoting the cubic and octahedral crystal habits. Growth rates in the ⟨100⟩ and ⟨111⟩ directions for the two solutions are estimated. The Cu₂O NCs are characterized by SEM, TEM, GI-XRD, and UV–vis spectroscopy. It is concluded that CD furnishes a simple, effective, generally applicable, and scalable route to the synthesis of morphologically controlled Cu₂O NCs on a variety of conductive and nonconductive surfaces