In this study, ZnO nanowires (NWs) have been synthesized through a simple chemical method and their optical properties were investigated using UV–vis, photoluminescence (PL) and Raman spectroscopy. PL studies revealed the red shift in the near band edge (NBE) emission peak with increasing diameter from 170 nm to 560 nm. By taking the sample with diameter d = 170 nm as a reference, the value of this red shift was found to be varied from 55 meV to 149 meV with the increase in diameter. Furthermore, this red shift shows the direct relation with the width of extra Zinc interstitial (Zni) or metal clusters related defect depletion layer at the boundary walls of nanowires. In this paper, the explanation for this red shift in PL is established by band bending at the edges of NWs caused by the depletion layer of excess Zn related defects. The confinement of the electrons at the surface of NWs was also found to be responsible for this red shift.