Panasiuk Y. Spectral-luminescent and photocatalytic properties of graphene-like carbon nitride and its nanocomposites with zinc oxide

A thermal treatment of bulk graphitic carbon nitride (GCN) in aqueous solutions of tetraethylammonium hydroxide was shown for the first time to yield colloidal solutions retaining the aggregative stability up to the GCN concentration of 50 g per L. Diluted colloids were found to contain mostly single-layer carbon nitride (SLCN) sheets with a thickness of 0.33–0.34 nm and a lateral size of around 30–50 nm, as well as a small fraction of a-few-layer particles with a thickness of 3–5 nm and a size of ~100 nm. A dependence of the photoluminescence (PL) band positions and intensity of colloidal carbon nitride on the excitation wavelength was established. In particular, the excitation with the light corresponding to the longer-wavelength absorption band of colloidal SLCN results in the broadband PL with a maximum at 460–470 nm and a quantum yield of 45–50% originating from SLCN. When the colloid is excited at ?ex = 260 nm a low-intensity PL band peaked at 390 nm with a PL quantum yield of 5% is observed assigned to the adducts of the SLCN with the products of apartial GCN hydrolysis. An antibate character of the dependences between the PL intensity of bulk GCN and colloidal SLCN on the synthesis temperature (TC) of the corresponding bulk materials was found. In particular, the PL intensity of GCN decreases with an increase in TC, while for SLCN produced from corresponding bulk samples the PL intensity grows drastically with an increase in TC. The binary ZnO/SLCN nanostructure were produced by introducing the colloidal SLCN into the mixtures where ZnO nanoparticles (NPs) are growing and their optical and PL properties studied. The average size of zinc oxide “core” NPs in such heterostructures can be tuned by adding the SLCN sheets at different moments of the NP growth. The ZnO NPs decorate both sides of the SLCN sheets. The coupling of SLCN with ZnO NPs was found to result in the PL quenching and a reduction of the radiative life time of both components indicating the spatial separation of the photogenerated charge carriers between the SLCN and ZnO NPs. The mutual position of conduction band levels in the ZnO/SLCN heterostructures changes due to the spatial exciton confinement in zinc oxide nanoparticles. As a result, in the case of ZnO NPs larger than 4 nm only the photoinduced charge transfer from SLCN to ZnO NPs can occur rendering the NPs charged. For smaller ZnO NPs, oppositely, favourable conditions for the electron transfer from ZnO NPs to SLCN arise making impossible the photoinduced charging of the ZnO NP “cores”. It was found that a treatment of bulk GCN with the concentrated HNO3 imparts it with the photocatalytic activity in hydrogen evoluton from aqueous solutions of various electron donors. The activation effect originates from a partial elimination of nitrogen atoms from the heptazine building blocks resulting in the formation of structural defects and carboxylate groups acting as traps of the photogenerated charge carriers and active sites of the photocatalytic process. The highest activation effect can be achieved in optimized conditions, that is in the case of the GCN synthesized at 500 °С and treated with nitric acid for 2 h.