There remains a pressing challenge in the efficient utilization of visible light in the photoelectrochemical applications of water splitting. Here, we design and fabricate pseudobrookite Fe 2 TiO 5 ultrathin layers grown on vertically aligned TiO 2 nanotube arrays that can enhance the conduction and utilization of photogenerated charge carriers. Our photoanodes are characterized by low onset potentials of B0.2 V, high photon-to-current efficiencies of 40-50% under 400-600 nm irradiation and total energy conversion efficiencies of B2.7%. The high performance of Fe 2 TiO 5 nanotube arrays can be attributed to the anisotropic charge carrier transportation and elongated charge carrier diffusion length (compared with those of conventional TiO 2 or Fe 2 O 3 photoanodes) based on electrochemical impedance analysis and first-principles calculations. The Fe 2 TiO 5 nanotube arrays may open up more opportunities in the design of efficient and low-cost photoanodes working in visible light for photoelectrochemical applications.

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A metal organic framework (MOF), synthesized from cobalt salt, melamine (mela), and 1,4-dicarboxybezene (BDC), was used as precursor to prepare Co/CoN/N-CNT/C electrocatalyst via heat treatment at different temperature (700-900 °C) under nitrogen atmosphere. Crystallites size and microstrain in the 800 °C heat-treated sample (MOFs-800) were the lowest, whereas the stacking fault value was the highest among the rest of the homemade samples, as attested to by the Williamson-Hall analysis, hence assessing that the structural or/and surface modification of Co nanoparticles (NPs), found in MOFs-800, was different from that in other samples. CNTs in MOFs-800, interacting with Co NPs, were formed on the surface of the support, keeping the hexagonal shape of the initial MOF. Among the three homemade samples, the MOF-800 sample, with the best electrocatalytic performance toward oxygen reduction reaction (ORR) in 0.1 M KOH solution, showed the highest density of CNTs skin on the support, the lowest I/I ratio, and the largest N atomic content in form of pyridinic-N, CoN, pyrrolic-N, graphitic-N, and oxidized-N species. Based on the binding energy shift toward lower energies, a strong interaction between the active site and the support was identified for MOFs-800 sample. The number of electron transfer was 3.8 on MOFs-800, close to the value of 4.0 determined on the Pt/C benchmark, thus implying a fast and efficient multielectron reduction of molecular oxygen on CoN active sites. In addition, the chronoamperometric response within 24 000 s showed a more stable current density at 0.69 V/RHE on MOFs-800 as compared with that of Pt/C.