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

Although nanocrystalline SnO2 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 SnO2 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 [SnClx(H2O)(6-x)](4-x) composed of 41% [SnCl5(H2O)](-), 41% [SnCl4(H2O)(2)], and 18% [SnCl3(H2O)(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 SnO2 nanoparticles. During heating from 25 to 70 degrees C and aging at 70 degrees 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 SnO2 nanocrystallites.