SnO2–Au hybrid nanoparticles as effective catalysts for oxygen electroreduction in alkaline media

SnO2 nanoparticles were synthesized by a facile electrochemical method based on anodic oxidation of a tin metal sheet. The resulting particles, with an average diameter of about 20 nm, were then loaded with varied amounts of HAuCl4, forming SnO2–Au(x) (x = 0, 0.095, 0.38, 0.76, 1.9, 3.0, and 3.8 at.%) hybrid nanoparticles after calcination at elevated temperatures (700 °C). X-ray diffraction (XRD) and transmission electron microscopic (TEM) measurements showed that the SnO2 particles exhibited high crystallinity with a rutile structure, and spherical Au nanoparticles were dispersed on the surface of the SnO2 support. Based on the TEM images and the width of the Au(2 0 0) XRD diffraction peak, the size of the Au nanoparticles was found to be between 15 and 35 nm in diameter and decrease with increasing loading of the original HAuCl4 precursor. The electrocatalytic activity of the resulting SnO2–Au(x) composite nanoparticles toward oxygen reduction reactions (ORR) was then evaluated by cyclic and rotating disk voltammetric measurements in alkaline solutions. It was found that the incorporation of gold nanoparticles led to apparent improvement of the catalytic activity of SnO2 nanoparticles. Moreover, the ORR electrocatalytic activity exhibited a strong dependence on the gold loading in the hybrid nanoparticles, and the most active catalyst was found with a gold loading of 1.9 at.%, based on the reduction current density and onset potential of ORR. Furthermore, at this gold loading, oxygen reduction was found to follow the efficient four-electron reaction pathway, whereas at other Au loadings, the number of electron transfer involved in oxygen reduction varied between 1 and 3. Additionally, Tafel analysis suggested that at low overpotentials, the first electron transfer might be the limiting step in oxygen reduction, whereas at high potentials, oxygen adsorption appeared to play the determining role at the SnO2–Au hybrid electrodes. These results indicate that the SnO2–Au composite nanoparticles might serve as effective catalysts for oxygen electroreduction in alkaline media.