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 Sn4+, as evidenced by the presence of SnO2, not Sn2+ 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 ele...