Helium droplet assisted synthesis of plasmonic Ag@ZnO core@shell nanoparticles

Alexander Schiffmann,Harald Fitzek,Wolfgang E Ernst,Daniel Knez,Ferdinand Hofer,Thomas Jauk,Florian Lackner

doi:10.1007/s12274-020-2961-z

Abstract

Plasmonic Ag@ZnO core@shell nanoparticles are formed by synthesis inside helium droplets with subsequent deposition and controlled oxidation. The particle size and shape can be controlled from spherical sub-10 nm particles to larger elongated structures. An advantage of the method is the complete absence of solvents, precursors, and other chemical agents. The obtained particle morphology and elemental composition have been analyzed by scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS). The results reveal that the produced particles form a closed and homogeneous ZnO layer around a 2–3 nm Ag core with a uniform thickness of (1.33 ± 0.15) nm and (1.63 ± 0.31) nm for spherical and wire-like particles, respectively. The results are supported by ultraviolet photoelectron spectroscopy (UPS), which indicates a fully oxidized shell layer for the particles studied by STEM. The plasmonic properties of the produced spherical Ag@ZnO core@shell particles are investigated by two-photon photoelectron (2PPE) spectroscopy. Upon excitation of the localized surface plasmon resonance in Ag at around 3 eV, plasmonic enhancement leads to the liberation of electrons with high kinetic energy. This is observed for both Ag and Ag@ZnO particles, showing that even if a Ag cluster is covered by the ZnO layer, a plasmonic enhancement can be observed by photoelectron spectroscopy.

Highlights

Zinc oxide has become one of the most popular materials for applications in the fields of photocatalysis and optoelectronics due to its intriguing properties
For scanning transmission electron microscopy (STEM) analysis, an amorphous carbon TEM grid was decorated with two different types of nanoparticles in order to give an example of the capabilities of the helium nanodroplet synthesis approach in terms of possible particle shapes
These two selected types encompass small, spherically shaped Ag@ZnO core@shell nanoparticles deposited for 40 min, as well as wire-like Ag@ZnO core@shell nanoparticles deposited for 3 min. Note that both types of nanoparticles were deposited on a single TEM grid, both species can be seen in the images presented in the following

Summary

Introduction

Zinc oxide has become one of the most popular materials for applications in the fields of photocatalysis and optoelectronics due to its intriguing properties. ZnO is a non toxic n-type semiconductor with a wide direct band gap of 3.37 eV, very similar to TiO2 but with a higher absorption efficiency under solar irradiation and a large exciton binding energy of about 60 meV [1]. As a photocatalyst it shows high potential in degradation processes of organic pollutants via the generation of reactive oxygen species and exhibits antibacterial properties, especially when scaled down to the nano regime [2]. A modification with Ag can significantly enhance the photocatalytic activity of ZnO [8] because the electron-hole pair concentration is increased by the strong local fields induced by the excitation of the localized surface plasmon in Ag [9]

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Conclusion