Enhancement of up-conversion emissions in ZnO: Er3+-Yb3+ after Gd2O3 surface modification

► ZnO: Er3+-Yb3+/Gd2O3 core/shell nanoparticles are synthesized via a seeded. ► ZnO: Er3+-Yb3+ acts as seed and Gd2O3 as shell. ► Visible up-conversion (UC) emissions are observed in the seed sample. ► Gd2O3 shell surface modification enhances the UC emission evidently. ► The intensity of UC emission increase with the increase of Gd2O3 deposition times.