Porous Silica as a Nanotemplate for the Solid State and Liquid Phase Synthesis of Luminescent Carbon Dots

Samples of nanoporous silicon oxide (pore size of 40 nm and 6 nm with specific are of 90 m2/g and 233 m2/g respectively) were synthesised by sol-gel method and used as nanotemplate for solid stated and liquid phase synthesis of lluminescent carbon nanodots (nanoagregates). Solid state method was two step process of chemical treatment of porous silicas by toluene solution of phenylstrimethoxisilane followed by pyrolysis at temperature in range of 400-600 0C for 60-180 minutes. Three characteristic emission bands in UV and VIS spectral range (360-380 nm, 410-420 nm, 440-450 nm) and their combinations were found in annealed por-SiO2:C depending on annealing time and temperature. The nature of light emission of pyrolytic carbon moieties in SiO2:C nanopowder was analysed and discussed in frame of the concepts of polycyclic carbon clusters and their aggregates based on the analogy with polycyclic organic molecules and luminescent carbon nanodots. It is suggested to assign the characteristic emission bands at about 380 nm and 420 nm to polycyclic carbon fluorophores with 2- and 3-ring structure respectively. The other low energy characteristic emission bands at wavelength 440-450 nm and higher are assigned to amorphous nanoaggregates. It is found that PL intensity of por-SiO2:C nanocomposite is sensitive to the absorption of the air. Response of the PL on absorption/desorption of air moieties was studied and discussed. Hydrophylic carbon nanodots were synthesized by liquid phase thermolysis of sucrose in dimethyl sulfoxide solution at 200 0C that resulted in formation of luminescent colloid. Porous silica were infiltrated by this colloid and then dried. Luminescent moieties absorbed in porous silicas were then extracted in water resulting in aqueous solution that exhibited intense visible photoluminescence with maximum intensity at 430-440 nm and well pronounced excitation band at 360-400 nm. This emission is suggested to be assigned to hydrophilic amorphous carbon nanodots (nanoagregates). Examination of Z-parameter of the colloids gave a value of the size of carbonnanodots below 1 nm. Spectral parameters (maximum position and spectral width) of the emission and excitation of photoluminescence were found to be dependent on the size of the pores in porous silica template that can be assigned to the effect of the size and size distribution of carbon nanodots (nanoagregates). As a summary, it was shown that nanoporous silica with high specific surface area can be used as solid state “container” for both solid state and liquid phase synthesis of carbon nanoemitters with emission spectrum variable from the near UV to the visible.