Carbon Dots and Carbon Spheres produced by Hydrothermal Synthesis of Oligochitosan Solutions

Porous carbon spheres with sizes ranging from tens to thousands of nanometers and luminescent carbon nanodots, which size is smaller than 10 nm can be produced by hydrothermal synthesis of dilute polymer solutions. Using natural polymers as the source for carbon dots and carbon spheres is promising from the ecological point of view. Both types of carbon structures are interesting for applications, but the phase separation into carbon spheres and carbon nanodots during the hydrothermal synthesis has not been studied. In this work, the products of hydrothermal treatment of dilute solutions of the water soluble oligomer of natural polysaccharide chitosan were studied. The absorption and luminescence spectra of carbon nanodots, and the scanning electron microscopy of carbon spheres were investigated. With increasing the synthesis duration, the degree of carbonization of both components, as well as the size of carbon spheres increase. It is shown for the first time that the quantum yield and luminescence linewidth of carbon nanodots depend on the filling degree of the autoclave.