Synthesis and Gas-Sensing Properties of Pd-Doped SnO₂ Nanocrystals. A Case Study of a General Methodology for Doping Metal Oxide Nanocrystals

Pd-modified SnO 2 nanocrystals, with a Pd/Sn nominal atomic ratio of 0.025, were prepared by injecting SnO 2 sols and a Pd precursor solution into tetradecene and dodecylamine at 160 °C. Two different doping procedures were investigated: in co-injection, a Pd acetylacetonate solution in chloroform was mixed with the SnO 2 sol before the injection; in sequential injection, the Pd solution was injected separately after the SnO 2 sol. The obtained suspensions were heated at the resulting 80 °C temperature, then the product was collected by centrifugation and dried at 80 °C. When using co-injection, in the dried products PdO and Pd nanoparticles were observed by high-resolution transmission electron microscopy. Only SnO 2 nanocrystals were observed in dried products prepared by sequential injection. After heat-treatment at 500 °C, no Pd species were observed for both doping procedures. Moreover, X-ray photoelectron spectroscopy showed that, in both the doping procedures, after heat-treatment Pd is distributed only into the SnO 2 nanocrystal structure. This conclusion was reinforced by the measurement of the electrical properties of Pd-doped nanocrystals, showing a remarkable increase of the electrical resistance if compared with pure SnO 2 nanocrystals. This result was interpreted as Pd insertion as a dopant inside the cassiterite lattice of tin dioxide. The addition of Pd resulted in a remarkable improvement of the gas-sensing properties, allowing the detection of carbon monoxide concentrations below 50 ppm and of very low concentrations (below 25 ppm) of other reducing gases such as ethanol and acetone.