In this work, an easy and low cost aqueous solution growth method was used to directly grow various-length ZnO nanorod arrays as the extended-gate sensing electrode of the pH sensors. To reduce the Fermi level pinning effect caused by the surface states and the dangling bonds resided on the sidewall surface of the ZnO nanorod arrays, the passivation was carried out using the photoelectrochemical (PEC) method. To investigate the function of the length of the ZnO nanorods, various-length ZnO nanorods were grown in the pH sensors. The experimental results revealed that the sensitivity of the pH sensors with passivated ZnO nanorod arrays was better than that of the pH sensors with unpassivated ZnO nanorods, when pH value varied from 4 to 12. Furthermore, the pH sensors with passivated ZnO nanorod arrays exhibited more linear sensing response range for the length of ZnO nanorods from 0.25 to 1.25 mu m compared to the pH sensors with unpassivated ZnO nanorod arrays. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3610399] All rights reserved.