The porous g-C3N4 with nitrogen defects and cyano groups (NC-g-C3N4) is prepared via an alkali-assisted heat treatment of urea. Alkali can break hydrogen bonds, which accelerate thermal polymerization of urea and formation of nitrogen defects/cyano groups. The presence of nitrogen defects extends the absorption of visible light to longer wavelengths region. The cyano groups can trap g-C3N4's photoinduced electrons and therefore suppress charge recombination. The formation of porous structure increases the surface area and exposes more active sites. As a consequence, compared to pure g-C3N4, NC-g-C3N4 shows boosted visible photocatalytic nitrogen fixation activity (1.59 mmol h(-1) g(-1)) without co-catalysts. (C) 2019 Elsevier Inc. All rights reserved.