Gaining Control over Radiolytic Synthesis of Uniform Sub-3-nanometer Palladium Nanoparticles: Use of Aromatic Liquids in the Electron Microscope

Abstract

Synthesizing nanomaterials of uniform shape and size is of critical importance to access and manipulate the novel structure-property relationships arising at the nanoscale, such as catalytic activity. In this work, we synthesize Pd nanoparticles with well-controlled size in the sub-3 nm range using scanning transmission electron microscopy (STEM) in combination with an in situ liquid stage. We use an aromatic hydrocarbon (toluene) as a solvent that is very resistant to high-energy electron irradiation, which creates a net reducing environment without the need for additives to scavenge oxidizing radicals. The primary reducing species is molecular hydrogen, which is a widely used reductant in the synthesis of supported metal catalysts. We propose a mechanism of particle formation based on the effect of tri-n-octylphosphine (TOP) on size stabilization, relatively low production of radicals, and autocatalytic reduction of Pd(II) compounds. We combine in situ STEM results with insights from in situ small-angle X-ray scattering (SAXS) from alcohol-based synthesis, having similar reduction potential, in a customized microfluidic device as well as ex situ bulk experiments. This has allowed us to develop a fundamental growth model for the synthesis of size-stabilized Pd nanoparticles and demonstrate the utility of correlating different in situ and ex situ characterization techniques to understand, and ultimately control, metal nanostructure synthesis.

The formation of cerium(iii) hydroxide nanoparticles by a radiation mediated increase in local pH

“…8,46 An alternative that we have explored in previous work is selecting a solvent that creates only reducing species with no need of scavenging additives. 20 In the present research, as seen in Fig. 4, the thermodynamic stability of the Ce(OH) 3 phase can be achieved only for pH > 10.4.…”

“…9 As such, quantitative control of reactions necessitates careful experimentation and interpretation, since the dose can introduce artefacts and be difficult to control. 7,13,18,39 The quasi-2D geometry of a liquid cell provides an ideal scenario for fast diffusion of radicals to the irradiated area 20,40 and the presence of membranes mainly dictates the location of particles growth, as repeatedly observed experimentally. These extreme conditions provide opportunities for new synthesis and new scientic insights.…”

“…The development of in situ liquid stages [14][15][16][17] for (scanning) transmission electron microscopes (S)TEM has enabled nanoparticle nucleation to be studied with immediate quantitative imaging of the nal products. [18][19][20] This in situ (S)TEM approach exploits the chemical reactivity of the radiolytic species formed by the interaction of high-energy imaging electrons with the precursor-containing solution (radiation chemistry), and can avoid large excesses of chemical reagents.…”

Ligand-free synthesis of gold nanoparticles incorporated within cylindrical block copolymer films

“…Mößmer et al 50 have shown that after incorporation of a gold salt in PS-b-P2VP micelles, the electron beam of the microscope can reduce the gold salt into small NPs located in the PVP core of the micelles. A similar case of palladium nanoparticles formation in organic media (toluene) has been studied by Abellan et al 51 and gives clues about the mechanism involved in such a reaction.…”

Investigation of Electron Beam Deposition Parameters Within a Scanning Electron Microscope

“…Conventionally, one seeks to avoid beam damage as it changes the nature of the sample. However, recent studies have sought to utilize this radiative damage by probing the phenomena related to beam-specimen interactions such as crystallization or amorphization [1][2][3][4].The advent of fluidic-cell electron microscopy extends these studies of beam-sample phenomena to liquid and gaseous environments not possible in traditional high-vacuum electron microscopy. The small (femtoliter) volumes and confined geometries required to achieve imaging for fluidic-cell studies in S/TEM restrict the possible concentration ranges and impact uniformity of reaction conditions in these experiments.…”