Radiation-Assisted Formation of Metal Nanoparticles in Molten Salts

Abstract

Knowledge of structural and thermal properties of molten salts is crucial for understanding and predicting their stability in many applications such as thermal energy storage and nuclear energy systems. Probing the behavior of metal contaminants in molten salts is presently limited to either foreign ionic species or metal nanocrystals added to the melt. To bridge the gap between these two end states and follow the nucleation and growth of metal species in molten salt environment in situ, we use synchrotron X-rays as both a source of solvated electrons for reducing Ni 2+ ions added to ZnCl 2 melt and as an atomic-level probe for detecting formation of zerovalent Ni nanoparticles. By combining extended X-ray absorption fine structure analysis with Xray absorption near edge structure modeling, we obtained the average size and structure of the nanoparticles and proposed a radiation-induced reduction mechanism of metal ions in molten salts.

Particle formation mechanisms supported by in situ synchrotron XAFS and SAXS studies: a review of metal, metal-oxide, semiconductor and selected other nanoparticle formation reactions

“…artifacts. Relevant here is that Frenkel and collaborators have shown 85 that a photon flux of B3 Â 10 13 photons per second results in an estimated B10 16 solvated electrons 94 even when that radiolysis involves the otherwise improbable, highly energetic removal of an electron from divalent zinc, Zn 2+ + hn -Zn 3+ + e À . Highly relevant here is that in particle formation systems exhibiting autocatalytic, exponential growth as is commonly seen, any trace nucleation events due to X-ray radiation-induced radiolysis will then be quickly magnified, autocatalytically and, hence, exponentially.…”

“…The Tandem CHCRR run simultaneously to the SAXS undercuts the synchrotron data some, but there was evidence of Ir(0) metal fouling found on the SAXS cell window, 41 presumably due to actually well-precedented-but little discussed and certainly under recognized-X-ray radiolysis during such synchrotron studies. [85][86][87][88][89][90][91][92][93] There is compelling precedent in the well-studied Ir(0) n nanoparticle system that even trace amounts of adventitious Ir(0) hugely increase the nucleation and subsequent growth rates, sometimes resulting in complete elimination of the induction period of the typically sigmoidal kinetics curve. 54,82 A fundamental contribution from as well as these particular synchrotron XANES and SAXS studies is, then, to raise a red flag concerning the involvement in X-ray-based methods that are more often than not assumed to be completely direct without…”

X-ray scattering reveals ion clustering of dilute chromium species in molten chloride medium

“…1 † ). X-ray absorption spectroscopy (XAS) is a powerful tool to investigate metal ion speciation, 15–18 but provides reliable information only about the first coordination sphere, and recent XAS experiments were unable to determine the local structure of Cr 3+ in a KCl–MgCl 2 molten salt mixture at the high temperatures (>973 K) 19 that are relevant to technological processes in CSP and MSR applications. The accurate identification and understanding of coordination environments and dynamic structural behaviour of chromium species could provide useful insights into the processes of corrosion and transport in molten salts.…”

Comparative analysis of XANES and EXAFS for local structural characterization of disordered metal oxides

“…EXAFS analysis commonly assumes Gaussian distribution of bond lengths, which is usually sufficient to understand the local symmetry of well defined materials, such as homogeneous, single-phase bulk solids . However, interatomic distance distribution can be more complicated in many materials, such as metallic nanoparticles, mesoporous materials and metals in molten salts (Boubnov et al, 2020;Prasai et al, 2015;Billinge & Levin, 2007;McGreevy & Pusztai, 1990;Dias et al, 2021), where the local atomic environment is characterized by strong asymmetry in the bond length distribution. Such an asymmetry has been shown to be a source of artifacts in conventional structural analysis methods by EXAFS (Yevick & Frenkel, 2010).…”