Green production of tunable multicolor nanoparticles with diamond structure from long-flame coal by a one-step mild oxidation

The complex compositions and structural features of low-rank coal obstruct its wide use in materials applications directly. Graphene nanocarbon hydrosol with uniform particle size and containing nanodiamond structures were prepared from long-flame coal by a simple H2O2 oxidation, achieving the highest extraction rate of 30.86%, and the depolymerization degree was 80.21% when the temperature was 80 °C. The average diameter of the carbon nanoparticles was 1.86 ± 0.62 nm with 2–3 graphite flake layers, and these graphite flake layers were derived from the stable crystalline domains in the raw coal. Carbon nanoparticles exhibited good excitation wavelength-dependent and concentration-dependent fluorescence, which can be tunable from blue to green fluorescence by adjusting the concentration (0.19–1.5 g/L) and excitation wavelength (340–460 nm), the reason may be due to the combined effect of the relative intensity changes of various types of oxygenous groups, low-lying effects resulting from n(OH)→π*(CO) interactions between the hydroxyl groups and carbonyl groups, and the aggregation of particle. The post-reaction residue has a similar structural composition to that of humic acid and could be used for developing functional materials and agricultural restoration. Therefore, preparing carbon nanoparticles and humic acid by mild oxidation of hydrogen peroxide for the high-value utilization of long-flame coal has good application prospects.