Tailoring energy band gap and microwave absorbing features of graphite-like carbon nitride (g-C3N4)

•Tuning optical and microwave absorbing performance of the g-C3N4 by tailoring its morphology.•Fabricating g-C3N4 aerogel based on a novel sintering method.•Modifying morphology and establishing defect onto the g-C3N4 nanostructures using citric acid as a novel precursor.•Using sonochemistry, hydrothermal, and combustion methods as innovative complementary processes to modify the morphology.•Employing polyacrylonitrile as a novel absorbing medium and investigating filler ratio on the microwave characteristics.