Next, we determined the elemental composition of the photoanodes before and after deposition of TiO2 and PEI layer by X-ray photoelectron spectroscopy (XPS) analysis (Fig. ). The peaks of Bi (4f and 4d), V (2p), and O (1s) were observed in the bare BiVO4. The Bi and V peaks disappeared upon TiO2 deposition, and additional peaks of Ti (2p), N (1s), and C (1s) appeared, commonly originating from the TiO2 precursor tetrakis(dimethyl-amido)titanium (TDMAT). The disappearance of the Bi, V, and O peaks is attributed to the formation of a uniform and thick TiO2 protective layer on top of the BiVO4 surface. BiVO4/PEI/TiO2 also indicated a similar trend with BiVO4/TiO2 but showed additional C peaks of C-O/C-NH2 and C-NHR (285.4 and 287.5 eV) and stronger N peaks. Especially, the high intensity of the N peak is presumed to originate from PEI and suggests the incorporation of PEI within the amorphous TiO2 layer (Supplementary Fig. ).