Evolution from UV emission of phenyl groups to visible emission of pyrolytic nanocarbons dispersed in fumed silica: Alternative insight into photoluminescence of carbon nanodots

SiO2:C nanopowder, luminescent in ultraviolet and visible spectral ranges, was synthesised using fumed silica as morphological matrix (specific surface of 295 m2/g) and phenyltrimethoxysilane (PhTMS) as carbon precursor. Fumed silica was impregnated by toluene solution of PhTMS followed by drying and annealing at 600 °C. Two series of SiO2:C nanocomposites with different carbon concentration were synthesised and studied. The main idea of the study was to explore the correlation of structural reconstruction and evolution of photoluminescent properties of carbon moieties. Chemically modified fumed silica exhibited ultraviolet (UV) photoluminescence band at about 330 nm originated from phenyl groups. After calcinations at 600 °C two overlapped emission bands were found at about 380 nm and 420 nm in “low-carbon” SiO2:C, while “high-carbon” series exhibited broad but homogeneous emission band peaked at about 440–450 nm. One more characteristic emission band at 495 nm in SiO2:C appeared and developed under intense ultraviolet radiation. Evolution of light emission from ultraviolet to visible was analysed in correlation with structural reconstruction of carbon examined by FTIR spectroscopy, X-ray photoelectron spectroscopy, high resolution scanning transmission electron microscopy, and electron energy loss spectroscopy. The nature of light emission of pyrolytic carbon moieties in SiO2:C nanopowder was 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.