ZnO:Er3+ and ZnO:Er3+-Yb3+ nanoparticles (NPs) are fabricated by a sol-gel method, afterwards parts of which are separated and surface modified in Mo(NO3)(3) solution. Analyses on phase and structure based on X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) techniques indicate that Er3+ and Yb3+ are incorporated into the ZnO lattice successfully and after Mo treatment, a thin layer of MoO3 forms on the NPs surface, forming core/shell structures. Raman scattering spectra reveal the existence of ZnMoO4 in the shell part. Visible up-conversion (UC) is observed in all the samples, with Mo treated and untreated ZnO:Er3+ emitting dominant but relatively weak red light, corresponding to F-4(9/2)-I-4(15/2) transition of Er3+. In Yb3+-codoping systems, the integral UC intensity is enhanced obviously though red emission still dominates the UC spectra before surface modification. In the Mo treated system, ZnO:Er3+-Yb3+/MoO3, green emission is increased while the red is suppressed in comparison to ZnO:Er3+-Yb3+, with the intensity of green to red ratio (GRR) changing from 0.25 to 8. A novel phenomenon is discovered that the green emissions in our samples involve three-photon processes. (C) 2011 Elsevier Inc. All rights reserved.