Homogeneous ultrathin g-C3N4 nanosheets/Al2O3 heterojunctions are synthesized using melamine and Al(NO3)(3) via in-situ reaction and the following thermal polymerization approach. The in-situ reaction between melamine and Al(NO3)(3) results in the existence of HNO3-acidified melamine and Al(OH)(3) aggregates via the hydrolysis of Al(NO3)(3). After thermal polymerization, the aggregates are converted to g-C3N4/Al2O3 composites. The thermal polymerization of acidified melamine and the support effect of aluminum hydroxide for g-C3N4 during the calcination process lead to highly dispersed amrophous Al2O3 on ultrathin g-C3N4 nanosheets, which is beneficial for the separation of photogenerated electron-hole pairs in the heterojunction. The degradation rate for Rhodamine B (RhB) over the most activie sample is 16.6 times than that of pristine g-C3N4 under visible light irradiation, which can be attributed to the high specific surface area, highly dispersion of amorphous Al2O3 on ultrathin g-C3N4 nanosheet, and the effective electrons transfer from g-C3N4 to the amorphous Al2O3. (C) 2016 Elsevier B.V. All rights reserved.