This work presents a critical review on the studies of defect chemistry of oxide nanoparticles for creating new functionalities pertinent to energy applications including dilute-magnetic semiconductors,giant-dielectrics,or white light generation.Emphasis is placed on the relationships between the internal structure and defective surfaces of oxide nanoparticles and their synergy in tailoring the materials properties.This review is arranged in a sequence:(1) structural fundamentals of bulk oxides,using TiO2 as a model simple oxide to highlight the importance of polymorphs in tuning the electronic structures;(2) structural features of simple oxide nanoparticles distinct from the bulk,which show that nanoparticles can be considered as a special solid under the compression as originated from the surface defect dipole-dipole interactions;and(3) new functions achieved through extending the defect chemistry concept to the assembled architectures or multi-component oxide nanoparticles,in which defect surfaces enable the localized electrons or intermediate levels to produce giant dielectric performance or tunable light generation.It is concluded that understandings of defect chemistry provide diverse possibilities to manipulate electrons in oxide nanoparticles for functionalities in energy-relevant applications. This work presents a critical review on the studies of defect chemistry of oxide nanoparticles for creating new functionalities pertinent to energy applications including dilute-magnetic semiconductors, giant-dielectrics, or white light generation. Emphasis is placed on the relationships between the internal structure and defective surfaces of oxide nanoparticles and their synergy in tailoring the materials properties. This review is arranged in a sequence: (1) structural fundamentals of bulk oxides, using TiO2 as a model simple oxide to highlight the importance of polymorphs in tuning the electronic structures; ( 2) structural features of simple oxide nanoparticles distinct from the bulk, which show that nanoparticles can be considered as a special solid under the compression as originated from the surface defect dipole-dipole interactions; and (3) new functions achieved through extending the defect chemistry concept to the assembled architectures or multi-component oxide nanoparticles, in which defect surfaces enable the localized electrons or intermediate levels to produce giant dielectric performance or tunable light generation. It is concluded that understandings of defect chemistry provide diverse possibilities to manipulate electrons in oxide nanoparticles for functionalities in energy-relevant applications.