Defect–Dopant Coupling in TiO2 Nanotubes: Local Defect Chemistry, Electronic Reconstruction, and Carrier Dynamics

The electronic properties of TiO2 nanotubes are strongly governed by the local atomic configurations of native defects and incorporated dopants. Establishing this structure–property relationship is difficult because experimental measurements generally reflect ensemble-averaged behavior, while atomic-scale energetic and electronic properties are often investigated using idealized models of nanotubes, anatase, or rutile TiO2. This review examines how defect location, charge state, dopant position, local chemical environment, and lattice relaxation influence defect stability, dopant–defect association, and electronic reconstruction. Experimental findings and calculations specific to TiO2 nanotubes are discussed together with studies examining individual defect configurations in anatase and rutile TiO2. The discussion distinguishes defect formation from dopant–defect association and considers how these processes modify localized electronic states, charge compensation, and polaronic behavior. Previous studies show that coupled dopant–defect configurations can generate localized electronic states, shift or passivate pre-existing defect states, and redistribute charge across neighboring lattice sites. These configuration-induced electronic changes directly affect carrier trapping, polaron migration, and nonradiative recombination pathways. Quantitative relationships between specific atomic configurations and finite-temperature carrier kinetics are still not well established for TiO2 nanotubes.