Mn/Fe co-doped TiO2 nanotube arrays for photoelectrochemical water splitting in neutral medium

Graphical representation of photoelectrochemical cell with Mn/Fe TNT@Ti-500 working electrode. [Display omitted] •Fabrication of stable Mn/Fe-TNTs on Ti substrate by electrodeposition.•Creating shallow trap states by doping Mn and Fe in TNTs to trap photoexcited electrons.•Band gap of Mn/Fe-TNTs annealed at 500 °C is 2.1 eV.•Mn/Fe-TNT@Ti-500 shows overall photocurrent density of 331 μA/cm2 @ − 0.4 VRHE.•Mn/Fe-TNT@Ti-500 is photostable for 14400 s @ 0.2 VRHE. Manganese and iron co-doped TiO2 nanotube arrays (TNTs) are grown on Ti foil by in situ electro-anodization method and employed for overall photoelectrochemical water splitting. The band gap of TiO2 is reduced from 3.3 eV to 3.1 eV with co-doping of Mn and Fe in Mn/Fe-TNT@Ti sample. Annealing Mn/Fe-TNT@Ti at 500 °C leads to the formation of trap states associated with the Fe3+ and Mn2+ ions present in the TiO2 lattice. The anatase structure of TNTs in Mn/Fe-TNT@Ti-500 sample is confirmed by XRD and Raman spectroscopy. An anodic shift in the onset potential from 0.2 VRHE to 0.08 VRHE is observed in Mn/Fe-TNT@Ti-500 sample compared to Mn/FeTNT@Ti sample. The overall photocurrent density at −0.4 VRHE is higher for Mn/Fe-TNT@Ti-500 sample (331μA/cm2) than Mn/Fe-TNT@Ti sample (251μA/cm2). The oxygen evolution activity shows similar trend with Mn/Fe-TNT@Ti-500 displaying a photocurrent density of 13.6 μA/cm2 at 1.23 VRHE. The anodic photocurrent density of Mn/Fe-TNT@Ti-500 sample is six times higher than that of the Mn/Fe-TNT@Ti at 0.8 VRHE. The incident photon to current efficiency (IPCE) measurements show that the photon absorption and conversion efficiency of the Mn/Fe-TNT@Ti-500 sample are significantly higher than that of the Mn/Fe-TNT@Ti sample. The results of this study complement the DFT calculations carried out with the generalized gradient approximation (GGA) technique. This work demonstrates possibility of designing composite materials for efficient photoreduction studies using n-type titanium nanotubes as precursors.