Enhancement in hydrogen generation using bamboo like TiO 2 nanotubes fabricated by a modified two-step anodization technique S.T. Nishanthi a , B. Sundarakannan a , E. Subramanian b , D. Pathinettam Padiyan a, * a Department of Physics, Manonmaniam Sundaranar University, Tirunelveli 627012, Tamil Nadu, India b Department of Chemistry, Manonmaniam Sundaranar University, Tirunelveli 627012, Tamil Nadu, India article info Article history: Received 11 July 2014 Accepted 15 December 2014 Available online Keywords: Band gap Oxygen vacancy Photoluminescence Photocurrent density Kinetic rate constant abstract Bamboo like TiO 2 nanotubes were successfully fabricated by a modified two-step anodization technique under different voltage and time. X-ray diffraction patterns and Raman spectra confirmed the anatase crystal phase of TiO 2 nanotubes. Optical studies reveal that the band gaps of TiO 2 nanotubes shrinked to 2.517(7) eV by varying the anodization parameters. The decrease in band gap was due to the improved crystallinity and the defect states formed in the forbidden gap. The photoluminescence spectra clearly revealed the formation of intermediate defect states, oxygen vacancies and color center defects. Bamboo like nanotubes exhibited higher photocurrent density than stack layered nanotubes due to the increase in crystallinity with higher surface area, enhanced light penetration depth and better scattering within the tubular structure. The photocatalytic activity of TiO 2 nanotubes for methyl orange degradation fol- lowed pseudo first order kinetics. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction TiO 2 is considered as a superior material for potential applica- tions such as water splitting, photocatalytic degradation, dye sensitized solar cells (DSSC), bio-sensors, etc. Among the various forms, the nanotube morphology of TiO 2 has advantages like larger surface area, efficient electrolyte diffusion and unidirectional charge carrier transport for high photoelectrochemical (PEC) per- formance in hydrogen production [1e5]. However, anatase TiO 2 absorbs only UV light corresponding to its band gap of 3.2 eV that limits its potential applications. In order to improve its PEC activity, different approaches are used for the fabrication of TiO 2 nanotubes (TNTs) with controlled morphologies. Application of low and high alternating voltages (AV) helps in fabricating bamboo like mor- phologies and also in shifting the band gap in the visible region. Double layered bamboo like and smooth walled TiO 2 nanotubes were fabricated by Li et al. [6] using two-step anodization with alternating voltages of 30 V and 60 V. Lin et al. [7] synthesized bamboo like nanotubes and the morphology was tuned by varying the anodization time, voltage and water content of glycerol elec- trolyte. Schmuki group [8,9] fabricated stratified bamboo type TNTs under an AV condition (120 V and 40 V) and reported a DSSC efficiency of 2.96% using HF electrolyte which is hazardous for the environment. Endut et al. [10] fabricated the bamboo like nano- tubes using glycerol mixture electrolyte at a constant voltage of 60 V. The length of the nanotube increased with increase in anodization time and its super capacitance was reported. Xie et al. [11] synthesized bamboo like nanotubes by alternating currents and voltages and reported a DSSC efficiency of 3.47%. Luan et al. [12] reported DSSC efficiency of 5.64% from bamboo like nanotubes fabricated by AV condition (60 and 10 V) which was higher than that of smooth walled nanotubes (3.90%) prepared by constant voltage (60 V). The high efficiency of bamboo like nanotubes was due to the larger surface area in which considerable amount of dye could be adsorbed. In the literature, most of the studies of bamboo like nanotubes of TiO 2 are on DSSC application. The present work reports a facile fabrication of stack layer and bamboo like TNTs using non-toxic electrolytes of ethylene glycol containing NH 4 F and H 2 O for enhanced photoelectrochemical and photocatalytic activities. In addition, the fabrication of TNTs is ach- ieved at relatively low voltage pairs (10 V/30 V) and shorter time. 2. Experimental details Titanium (Ti) foil of 99% purity and 0.25 mm thickness (Sigma Aldrich) was degreased with acetone, followed by sonication in double distilled (DD) water for 10 min. In the first step, anodization was carried out in an aqueous electrolyte bath consisting of 0.3 wt.% * Corresponding author. Tel.: þ91 9442063155; fax: þ91 462 2334363. E-mail address: dppadiyan@msuniv.ac.in (D. Pathinettam Padiyan). Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene http://dx.doi.org/10.1016/j.renene.2014.12.038 0960-1481/© 2014 Elsevier Ltd. All rights reserved. Renewable Energy 77 (2015) 300e307