Long-Length Titania Nanotubes Obtained by High-Voltage Anodization and High-Intensity Ultrasonication for Superior Capacity Electrode Jose ́ R. Gonza ́ lez, † Ricardo Alca ́ ntara, †, * Francisco Nacimiento, † Gregorio F. Ortiz, † Jose ́ L. Tirado, † Ekaterina Zhecheva, ‡ and Radostina Stoyanova ‡ † Laboratorio de Química Inorga ́ nica, Campus Universitario de Rabanales, Edificio C3, Universidad de Có rdoba, 14071 Có rdoba, Spain ‡ Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev Str. Bldg. 11, 1113 Sofia, Bulgaria ABSTRACT: To modify the morphology and electrochemical properties of the resulting titanium oxide layer, we have applied high-intensity ultrasonication during the potentiostatic anodization of metallic titanium, and the applied voltage and anodizing time has been changed. The influence of the imposed voltage, anodizing time, and ultrasonication on the nanotubes growth has been studied. Additional dissolution process takes place under ultrasonication, as is observed in the anodizing curves (current density vs time) that show values on the order of ca. 200 A/m 2 . After only 30 min of ultrasound-assisted anodization at 42 V, the resulting nanotubes length is ca. 4 μm and, in contrast, in the case of non ultrasound-assisted anodization, the length is only ca. 1 μm. Further prolonged anodization under ultrasound induced the complete dissolution of the titanium. After anodization at 60 V during 20 h (no ultrasounds), the observed length of the nanotubes is as long as ca. 45 μm. The nanotube TiO 2 aspect ratio has been tailored between 40 and 320. The obtained nanotubes of TiO 2 exhibit high areal capacity (up to ca. 2 mAh/cm 2 and stabilized around 0.3 to 0.5 mAh/cm 2 ) and good cycling behavior in lithium batteries. A nonlinear relationship between the nanotubes length and the resulting capacity has been revealed. ■ INTRODUCTION The abundant reserves and low toxicity of titanium are some of the features that let us to look at titanium oxide as a very attractive electrode material for lithium ion batteries. In addition, the higher intercalation voltage of titanium oxides may render safer batteries in comparison with graphite electrode. Unfortunately, titanium oxide usually has low electrical conductivity and slow Li + insertion-extraction kinetics. In order to overcome this problem TiO 2 can be nanostructured to achieve shorter solid state path lengths for both Li-ion and electron transport, 1-7 however, poor electronic conduction network due to aggregation of nanopowders and loss of interparticle connection during electrochemical cycling can be suffered. In a try to improve the electrochemical behavior, titania nanotubes can be grown by anodization of Ti foil under the optima experimental conditions and the resulting areal capacity values are usually below 0.1 mAh/cm 2 . 3,6,7 Batteries based on 3D micro or nanostructures can offer advantages, such as small areal footprint, in comparison with 2D structures such as thin films. 8 To achieve a high-energy density and a small areal footprint requires the use of 3D batteries. 9 These batteries may improve the power-density of the usual batteries and reduce the recharging time as is claimed for further developing electric vehicles. Therefore, a 3D electrode with an anatase TiO 2 layer covering aluminum nanorods showed areal capacity values of ∼0.01 mAh/cm 2 , which is one order of magnitude higher than that for the equivalent 2D geometry. 9 Having all of these features in mind, we have considered that making the TiO 2 nanotubes longer may yield a higher capacity and power density while retaining the same areal footprint. To study this hypothesis, we first explored the preparation method by modification of several experimental parameters, characterized the physical-chemical properties of the resulting nanotubes, and finally studied the electrochemical behavior in lithium cells. Sonochemistry uses the application of high-intensity ultra- sound and the so-called acoustic cavitation effect in chemical reactions. Zhang et al. reported that the ultrasound-assisted anodization of aluminum enhances the pores growth and proposed that the ultrasonication can release the oxygen bubbles (2O = =O 2 + 4e - ) from the interface, which is helpful for the nanotubes growth. 10 The key processes responsible for anodic formation of nanoporous alumina and TiO 2 appear to be the same. 11-14 In this work, we have applied high-intensity ultrasonication to the anodization of titanium to grow titania nanotubes, and Received: May 23, 2012 Revised: September 4, 2012 Published: September 4, 2012 Article pubs.acs.org/JPCC © 2012 American Chemical Society 20182 dx.doi.org/10.1021/jp3050115 | J. Phys. Chem. C 2012, 116, 20182-20190