Raman spectroscopy investigation of magnesium oxide nanoparticles

We investigate Raman spectra (100 cm^-1 to 3900 cm^-1) of magnesium oxide nanoparticles with nominal sizes of 10 nm, 20 nm, 40 nm, 50 nm, and 300 nm. The crystal structure of MgO prohibits first-order modes and yet, there are numerous reports of relatively intense peaks throughout the literature. Raman signals at approximately 278 cm^-1 and 445 cm^-1 that were attributed to MgO nanoparticles by previous authors are shown to belong to layers of Mg(OH)₂ formed on the surface of MgO nanoparticles. Through an annealing process at 400 °C in an O₂ atmosphere, we observe that modes in the 3700 cm^-1 spectral region, which are a signature of OH groups, disappear together with modes at 278 cm^-1 and 445 cm^-1, thus establishing a necessary criterion to associate all of these peaks to the presence of OH groups on the surface.