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

Abstract

Following a brief description of synchrotron X-ray absorption fine structure (XAFS) spectroscopy and small/wide angle X-ray scattering (SAXS/WAXS), the definition of and four primary criteria for attaining reliable, disproof-based chemical mechanisms of particle formation are given. A total of 74 papers using synchrotron techniques for mechanistic investigation are then analyzed in detail via the construction of four tables provided in the ESI that analyze each of the 74 papers. Six primary case studies are selected out of the 74 total papers for presentation in greater detail in the main text, specifically the illustrative case histories of: (i) palladium nanoparticles studied using SAXS, (ii) rhodium nanocubes examined using XAFS, (iii) iridium nanoparticles studied by XAFS and SAXS, (iv) gold nanoparticles studied by XAFS, SAXS, and XRD, (v) cadmium-selenide nanocrystals studied by XAFS, and (vi) zinc-oxide nanoparticles studied using SAXS/WAXS/UV-vis. Additionally, two shorter case studies are presented that address particle formation shapes: tungstite nanoplatelets and copper nanocrystals. Two summary tables are presented in the main text that present the current state of disproof-based, deliberately minimalistic particle formation mechanisms expressed in terms of generalized, pseudoelementary steps. Finally, a Conclusions section with nine takeaways and an Outlook section are also provided. The goal of the present review is to expedite the use of powerful synchrotron SAXS and XAFS studies to provide reliable, disproof-based chemical mechanisms and their associated quantitative differential equations that can then be employed to predict particle-size distributions via the recent development of Mechanism-Enabled Population Balance Modeling (ME-PBM).