Investigation of the structural and electrochemical properties of size-controlled SnO₂ nanoparticles

Four types of size-controlled SnO₂ nanoparticles were synthesized by a colloidal method to investigate the structural and electrochemical properties of various particle sizes. The samples were characterized by means of transmission electron microscopy (TEM), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and electrochemical measurements. TEM and XRD analyses showed that the samples were well dispersed with an average particle size of ∼20 nm (sample A), ∼11 nm (sample B), ∼5.6 nm (sample C), and ∼2.3 nm (sample D). XPS analyses of four size-controlled SnO₂ samples confirmed that the chemical state of the Sn was Sn ⁴⁺, as evidenced by the presence of SnO₂, not Sn ²⁺ related to the Sn(OH)₂. The Sn NEXAFS spectra of the four samples indicated that the samples had a tetragonal rutile structure and showed no noticeable change in the oxidation state of tin ions with decreased particle size, in good agreement with the XPS analysis. The electrochemical properties of lithium insertion in the first cycle were different and systematically varied with a plateau at about 0.8 V in the four types of samples. Furthermore, the optimum particle size for enhanced lithium insertion/extraction was found to be in the range ∼11 nm, among the four types of size-controlled SnO₂ nanoparticles. When the particle size decreased from 11 nm to 5.6 and 2.3 nm, performance inversely decreased due to the absence (or small amounts) of Li₂O phase.