Nanostructural study of NiTi-TiO2-C core-shell nanoparticles generated by spark discharge method

Nickel-titanium (NiTi) nanoparticles are ultrafine smart materials manifesting shape memory effect at very small scales. We have produced NiTi nanoparticles surrounded by an amorphous carbon shell using an innovative spark discharge system. The resulting nanoparticles were studied using various characterization methods to systematically study their size, morphology, size distribution, composition, structure, and thermal behavior. Field-emission scanning electron microscopy and dynamic light-scattering results indicated that the average size of the produced nanoparticles was about 13 nm. High-resolution transmission electron microscopy, energy-dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy showed that all of the particles had a titanium oxide coating covered with a similar to 2 nm-thick carbon layer. X-ray diffraction analysis demonstrated that the carbon and titanium oxide layers were amorphous and confirmed the formation of NiTi nanoparticles. Differential scanning calorimetry demonstrated an austenite to martensite phase transformation behavior in the produced nanoparticles and further indicated the formation of NiTi phases. EDS mapping showed an incomplete oxidation for nanoparticles suggesting that the amorphous carbon and titanium oxide layers synergistically act together as a combined protective layer. These results indicate that our novel spark discharge generator is an effective system for the synthesis of NiTi nanoparticles coated with a continuous film of carbon.