Post-annealing treatment for Cu-TiO₂ nanotubes and their use in photocatalytic methyl orange degradation and Pb(II) heavy metal ions removal

¹ School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia Engineering Campus, 14300 Nibong Tebal, Seberang Perai Selatan, Pulau Pinang, Malaysia
² Nanotechnology & Catalysis Research Centre (NANOCAT), 3rd Floor, Block A, Institute of Postgraduate Studies (IPS), University of Malaya, 50603 Kuala Lumpur, Malaysia
³ Department of Civil and Environmental Engineering, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, Niigata 940-2118, Japan

^a e-mail: srimala@eng.usm.my

Received: 6 October 2013
Revised: 2 March 2014
Accepted: 4 March 2014
Published online: 11 July 2014

Abstract

TiO₂ nanotubes were synthesized via electrochemical anodization of Ti foil at 60 V for 1 h in a bath with electrolytes composed of ethylene glycol containing 5 wt.% of NH₄F and 1 vol.% of H₂O₂. The incorporation of optimum Cu²⁺ ions (1.30 at.%) into TiO₂ nanotubes were prepared by using wet impregnation method to improve their photocatalytic methyl orange degradation and Pb(II) heavy metal removal. The small Cu²⁺ ions were successfully diffused into lattice of TiO₂ nanotubes by conducting post-annealing treatment at 400 °C for 4 h in argon atmosphere after wet impregnation. In this manner, optimum Cu²⁺ ions played a crucial role in suppressing the recombination of charge carriers by forming inter-band states (mismatch of the band energies) within the lattice of Cu-TiO₂. The experimental results showed that a maximum of 80% methyl orange removal and 97.3% Pb(II) heavy metal removal at pH 11 under UV irradiation for 5 h. Besides, it was noticed that photocatalytic Pb(II) heavy metal removal was strong dependence on pH of the solution because of the amphoteric character of Cu-TiO₂ in an aqueous medium.