To further investigate the composition and structure of TiO2, we carried out time-offlight secondary ion mass spectrometry (TOF-SIMS) and X-ray diffraction (XRD) analyses. In the TOF-SIMS measurement, the elemental composition of C, N, and Ti within the TiO2 layer on both BiVO4/TiO2 and BiVO4/PEI/TiO2 photoelectrodes was confirmed. Although all elements are present in both samples, the C and N signals are much higher in the BiVO4/PEI/TiO2 (Fig. and Supplementary Fig. ). Especially, BiVO4/PEI/TiO2 exhibited a significantly higher intensity of C and N near the TiO2 surface (Supplementary Fig. ), which is consistent with the XPS analysis. XRD analysis exhibited negligible changes in the phases of BiVO4 and an amorphous TiO2 with no diffraction peaks regardless of the presence of PEI (Fig. ). These results reveal that the PEI interfacial layer does not affect the amorphous nature of the TiO2 but does contribute to an increase in the amount of embedded C and N, altering the intrinsic properties of the a-TiO2.