Photoelectrochemical Water Splitting and H 2 Generation Enhancement Using an Effective Surface Modification of W-Doped TiO 2 Nanotubes (WT) with Co-Deposition of Transition Metal Ions

W-doped TiO 2 nanotube arrays (WT) were fabricated by in situ electrochemical anodization of titanium substrate. The results of the influence of different photo-deposited transition ions (Cr x Fe 1−x , 0 ≤ x ≤ 1) on the surface of WT on photoelectrochemical (PEC) water splitting and H 2 generation are presented. The crystallinities, structural, elemental, and absorption analysis were conducted by XRD, SEM, RAMAN, EDX, and UV–Vis absorption spectroscopy, which demonstrated anatase as the main crystalline phase of TiO 2 , and the existence of Cr x Fe 1−x (nano)particles/film deposited on the surface of WT. The SEM images revealed that the deposition rate and morphology are highly related to the ratio of Cr and Fe ions. Under visible light illumination, the entire photoelectrodes showed a very good response to light with stable photocurrent density. PEC measurements revealed that the mixture of transition ions with a certain ratio of ions (Cr 0.8 Fe 0.2 –T) led to enhanced photocurrent density more than that of other modifiers due to decreasing charge recombination as well as improving the charge transfer. Moreover, PEC water splitting was conducted in an alkaline solution and the Cr 0.8 Fe 0.2 –T photoelectrode generated 0.85 mL cm −2 h −1 H 2 , which is over two times that of pristine WT.