ORIGINAL PAPER Facile one-step process for synthesis of vertically aligned cobalt oxide doped TiO 2 nanotube arrays for solar energy conversion Heba Ali 1 & N. Ismail 1 & Mohamed Mekewi 2 & A. C. Hengazy 3 Received: 15 February 2015 /Revised: 3 June 2015 /Accepted: 4 June 2015 # Springer-Verlag Berlin Heidelberg 2015 Abstract Vertically aligned cobalt oxide doped TiO 2 nano- tube arrays (Co.-oxide TNTAs) were synthesized via one pot- anodic oxidation of pure titanium substrate in the presence of ammonium fluoride and cobalt salt. After subsequent anneal- ing in air the produced arrays were in the tubular structure and doped with Co.-oxide. The designed TNTAs and Co.-oxide TNTAs were tested as photoanode electrodes in a photoelectrochemical cell. Energy dispersive X-ray (EDX) spectroscopy confirms the incorporation of Co. in the doped TNTAs. The Tauc plots estimated from UV–Vis diffuse reflec- tance spectra displayed that the insertion of the optimum amount of Co.-oxide leads to a decrease in the band gap of TiO 2 from 3.2 to 2.9 eV. The influences of various cobalt salt concentrations in the electrolyte solution (5, 10, 15, 20 and 25 mM) on the morphology were studied. The evaluation of the photocurrent and photoconversion efficiency was per- formed for all the fabricated electrodes. Morphological studies illustrated that the addition of cobalt salt with small concen- tration has no an obvious effect on the ordered tubular struc- ture of TNTAs, whereas, at higher concentrations the tubular structure was partially collapsed. Co.-oxide enhanced the photoconversion efficiency of TNTA electrode by 30 % at optimum concentration under 110 mW/cm 2 solar simulator illumination. Keywords TiO 2 nanotube array . Potentiostatic anodization . Solar water splitting . Optical properties Introduction Many studies have been developed using various types of powdered photocatalysts for photoelectrochemical water split- ting [1–4]. This process has a major drawback, which is the production of H 2 and O 2 as a mixture. Furthermore, it con- sumes energy and decreases the overall water splitting effi- ciency [5, 6]. On the other hand, the utilization of a thin film as a photoanode offers a separate evolution of hydrogen at the cathode and oxygen at the anode. A solar water splitting system using a photocatalyst as an anode and Pt as a cathode to produce O 2 and H 2 is considered to be an important challenging task for the development of renewable energy production technology. Irradiation with en- ergy equal to or higher than that of the photocatalyst band gap leads to generation of electrons and holes in the conduction band and valence band, respectively [7–9]. Holes oxidize H 2 O to produce O 2 at the photoanode and electrons reduce H + to H 2 at Pt electrode. These reactions are described as follows [8, 10]: At anode : H 2 O þ 2h þ → 1 2 O 2 þ2H þ At cathode : 2H þ þ2e ‐ → H 2 TiO 2 is the most extensively studied photoanode material for water splitting since 1972 [11]. However, it has a high energy gap 3.0 eV for rutile and 3.2 eV for anatase [12–14] and it is excited only by UV light, which is about 4–5 % of the overall solar spectrum. In addition to its high rate of electron– hole recombination [15, 16]. Therefore, several approaches * Heba Ali Heba_future@yahoo.com 1 Physical Chemistry Department, National Research Centre, Cairo, Dokki, Egypt 2 Chemistry Department, Faculty of Science, Ain Shams University, Cairo, Abbassia, Egypt 3 Central Metallurgical R&D Institute (CMRDI), P.O. Box 87, Helwan, Tebbin, Egypt J Solid State Electrochem DOI 10.1007/s10008-015-2919-3