Metal-free carbon dot-microporous graphitic carbon heterojunctions as photocatalysts for CO2 reduction

A series of photocatalysts containing various loadings of carbon dots (CD) on microporous graphitic carbons (mpC) was prepared by pyrolysis at 900 degrees C under inert atmosphere of alpha-cyclodextrin containing preformed CDs. CDs were obtained by microwave hydrothermal conversion of citric acid and urea, and they were purified by centrifugation and partition between aqueous and organic solvents. The CD/mpC composites exhibit the expected morphology for mpC in TEM with particles of about 20 nm and microporous channels of 0.6 nm. XRD and Raman spectra of CD/mpC were also coincident with those of pristine mpC, but XPS and elemental analysis detected the presence of pyridinic N from the CDs. The optimal CD/mpC sample exhibits higher photocatalytic activity for the simultaneous H-2 evolution and CO2 reduction to CH4 than the independent components or than a mechanical CD and mpC mixture reaching a productivity rate of 29 and 5 mu mol g(catalyst)(-1) h(-1) for H-2 and CH4, respectively. CD/mpC photocatalysts were stable for four consecutive runs.