In the last decades, radiolytic synthesis routes have exploited the chemical effects of the absorption of high-energy radiation on precursor solutions, to form nanostructures by reproducing a selective reducing/oxidizing environment. Radiation chemical synthesis provides a powerful means to form nuclei which are homogeneously distributed in the whole volume and where the growth rate can be easily controlled.[1] The latter has recently been achieved using liquid cell electron microscopy, with examples of formation kinetics of particles in polar (water)[2] and non-polar (toluene)[3] solvents following a linear relation with applied dose rate. Fine control of particles size with conventional radiation sources typically requires increasing the production of radicals (thus applying “high doses”). On the contrary, the challenge in the scanning transmission electron microscope (STEM), where incident doses are inherently higher by orders of magnitude, is to reduce drastically such production. To illustrate this, we recently proposed the use of non-polar systems (toluene), which are not typically used for radiolytic synthesis outside the STEM, as a solvent that produces much lower amount of radiolytic ionic species upon electron irradiation, as compared to water or other polar solvents such as alcohols (Figure 1 shows an example of

Thus far, most experiments in the liquid cell involved the use of water as a solvent, which explains the large amount of work dedicated to understanding the effects of the electron beam in aqueous conditions. Radiation chemical yields in water are large due to the relatively low bond energies in water molecules, meaning highly reactive oxidizing and reducing radicals and species are created in about an equal amount.[4] Reproducing net reducing conditions for nanoparticle growth can be achieved with the addition of substances that convert primary radicals into free reducing radicals (using OH· scavengers, for instance). The use of organic solvents in the liquid cell allows for tuning the polarity of the medium, giving access to a broader range of synthesis conditions but producing more complex radiolytic products. Here I will discuss, revisiting a number of examples from the literature and presenting our most recent work, general methods for finding more suitable synthesis environments for controlled nanoparticles formation in the liquid cell. Much of the presentation will focus on the solvent radiolysis which is what predominantly dictates the species and yields involved in the chemical processes leading to nanostructure synthesis.[4]

References:

[1] J. Belloni, Catal. Today, 2006, 113, 141-156; S.-H. Choi et al., Colloids Surf., A, 2005, 256, 165-170; J. Belloni et al., New J. Chem., 1998, 22, 1239-1255; M. A. J. Rodgers and Farhataziz, Radiation Chemistry: Principles and Applications, VCH Publishers, New York, N.Y. , 1987.

[2] D. Alloyeau et al., Nano Lett., 2015, 15, 2574-2581; J. H. Park et al., Nano Lett., 2015, 15, 5314-5320.

[3] P. Abellan et al., Langmuir, 2016, 32, 1468-1477.

[4] SuperSTEM is the UK EPSRC National Facility for Aberration-Corrected STEM, supported by the Engineering and Physical Science Research Council (PA). Part of this work was supported by the Chemical Imaging Initiative, under the Laboratory-Directed Research and Development Program at Pacific Northwest National Laboratory (PNNL) and by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, and was performed in part using the facilities located in the William R. Wiley Environmental Molecular Sciences Laboratory, a DOE national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located and the PNNL. PNNL is operated by Battelle for DOE.

Figures:

We recently introduced the use of aromatic hydrocarbons (toluene) as solvents that are very resistant to high energy electron irradiation and achieved synthesis with tunable kinetics for sub-3nm particles by creating net reducing conditions (with no addition of scavengers for oxidizing radicals) and very low yields of reactive species for a given incident electron dose. Dihydrogen (H2) was proposed as the main primary species involved in the reduction process leading to Pd0 particle formation, which is a widely used reductant in the synthesis of supported metal catalysts. Image reprinted with permission from [4] Copyright 2016 American Chemical Society.

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