Mechanisms of SnO₂ Nanoparticles Formation and Growth in Acid Ethanol Solution Derived from SAXS and Combined Raman–XAS Time-Resolved Studies

Although nanocrystalline SnO 2 is among the most intensely studied nanoscale semiconductor metal oxide, the actual mechanisms of nanoparticle formation and growth often remain unclear due to limited accessibility to in situ derived time-resolved information about the evolutions of precursor speciation and particle size. By overcoming such limitations, we report on the formation of SnO 2 nanoparticles by hydrolysis and condensation of precursors, and we give new insights on their nucleation and growth mechanisms based on in situ time-resolved SAXS and combined Raman/extended X-ray absorption fine structure (EXAFS) measurements. Raman and EXAFS spectroscopy indicate that the dissolution of tin tetrachloride pentahydrate in ethanol (precursor solution) leads to a mixture of tin chloride monomeric complexes [SnCl x (H 2 O) 6– x ] 4– x composed of 41% [SnCl 5 (H 2 O)] −, 41% [SnCl 4 (H 2 O) 2 ], and 18% [SnCl 3 (H 2 O) 3 ] + . The combination of X-ray absorption spectroscopy (XAS) and Raman speciation with independent small-angle X-ray scattering (SAXS) results allows us to propose a five-step mechanism of formation. The first three steps are observed under water addition and aging at room temperature, corresponding to the prenucleation of low nuclearity species, followed by a monomer–tin-oxo cluster aggregation growth and cluster–cluster growth, leading to the formation of double or triple chains structure further interconnected to form SnO 2 nanoparticles. During heating from 25 to 70 °C and aging at 70 °C, a densification process followed by an advanced nanocrystallite growth through the addition of mononuclear species to the surface of the nanoparticles have been identified. These well-time separated steps could be used as a versatile way to control the growth processes and fine-tuning of the size of SnO 2 nanocrystallites.