The eﬀects of niobium on the structure and electronic properties of passive ﬁlms formed on zirconium alloys in pH 8.5 buﬀer solution were examined by photo-electrochemical analysis. For Zr–xNb alloys (x = 0, 0.45, 1.5, 2.5 wt%), the photocurrent began
to increase at an incident energy of 3.5–3.7 eV and exhibited the ﬁrst peak at 4.3 eV and the second peak at 5.7 eV. From the (Iph hv)
1/2 vs. hv plot, indirect band gap energies of E1g ¼ 3:01–3:47 eV and E2g ¼ 4:44–4:91 eV were obtained. With increasing Nb content, the relative photocurrent intensity of the ﬁrst peak increased signiﬁcantly. Compared with the photocurrent spectrum of the thermal oxide of Zr–2.5Nb, it was revealed that the ﬁrst peak in the photocurrent spectrum for the passive ﬁlm formed on
Zr–Nb alloy was generated by two types of electron transitions; one caused by hydrous ZrO2 and the other created by Nb. Two
electron transition sources were overlapped over the same range of incident photon energy. In the photocurrent spectrum for passive ﬁlm formed on Zr–2.5Nb alloy in which Nb is dissolved into the matrix by quenching, the relative photocurrent intensity of the ﬁrst peak increased, implying that dissolved Nb acts as another electron transition source.