Graphitic carbon nitride (g-C3N4) optical sensors for environmental monitoring

In this work, highly fluorescent graphitic carbon nitride (g-C 3 N 4 ) nanostructures fabricated via the thermal polymerization of two nitrogen rich precursors, namely urea (g-C 3 N 4 -U) and melamine (g-C 3 N 4 -M), are explored as optical sensors for environmental monitoring. The morphological, structural and optical properties of the obtained materials were fully investigated and were found to be heavily influenced by the source reagent. The photoluminescence (PL) emission of these nanostructures was studied under different environmental conditions, including variations of the atmospheric pressure (0.1 to 1000 mbar) and temperature, upon excitation with a UV pulsed laser source. Following exposure to an oxygen-containing atmosphere, both g-C 3 N 4 -U and g-C 3 N 4 -M exhibited distinct sensing behavior expressed by the reversible bi-parametric changes in their room temperature (RT) PL emission, namely intensity and peak wavelength. Urea-derived g-C 3 N 4 was found to be more sensitive compared to the melamine-produced material, with a maximum relative change in the PL emission intensity ( %ΔI ) of 30 %, and a maximum wavelength shift of 4 nm. These results highlight, for the first time, the importance of the judicious choice of the g-C 3 N 4 precursor for gas sensing applications. Moreover, temperature-dependent photoluminescence measurements carried out with the urea-derived g-C 3 N 4 revealed that the PL emission of the nanostructures exhibited a profound temperature dependence. The emission intensity decreased when increasing the temperature over a wide temperature range (RT to 600 K), while its spectral profile became broader and the emission maximum was shifted to higher energies. Our experimental findings denote a strong influence of the PL features of the urea-derived g-C 3 N 4 on the environmental conditions, highlighting its potential use as a sensitive probe in environmental monitoring.