Nonstoichiometry-Induced Enhancement of Electrochemical Capacitance in Anodic TiO 2 Nanotubes with Controlled Pore Diameter V. C. Anitha, † Arghya Narayan Banerjee,* ,† G. R. Dillip, † Sang Woo Joo,* ,† and Bong Ki Min ‡ † School of Mechanical Engineering, Yeungnam University, Gyeongsan 712-749, South Korea ‡ Center for Research Facilities, Yeungnam University, Gyeongsan 712-749, South Korea * S Supporting Information ABSTRACT: We report the fabrication of self-organized titania (TiO 2 ) nanotubes (TNTs) with controlled pore diameters (140-20 nm) by anodization for the application of electrochemical capacitor electrodes. The areal capacitances obtained for 140 nm TNTs as 0.23/0.13 mF cm -2 at a scan rate of 1/5 mV s -1 and it is enhanced to 5.5/2.9 mF cm -2 (at the same scan rates) by controlling the pore diameter to 20 nm. In this study, role of pore diameter in the capacitance behavior of TNTs is explained on the basis of effective surface area and presence of oxygen vacancies/titanium interstitials. With a decrease in the pore diameter, the surface area-to-volume ratio (and hence, active surface sites) increases, which leads to greater dissociation of Ti 4+ into Ti 3+ under high temperature annealing and thus brings more nonstoichiometric defects like Ti 3+ interstitials and oxygen deficiency within the lower dimensional TNTs. This manifests higher charge conductivity and greater electrochemical performance of TNTs with lower diameters. The simplicity of anodization method and the excellent electrochemical properties make these vertical TNTs as an alternative candidate for use in energy storage applications. 1. INTRODUCTION Supercapacitors are being increasingly explored as a feasible charge storage technology in recent years. 1-6 On the basis of the mechanism of charge storage, supercapacitors can be categorized into three general groups: (i) nonfaradic super- capacitors (electric double layer capacitors-EDLCs) 7 that are based on electrostatic charge diffusion and accumulation at the electrode/electrolyte interface, (ii) faradic supercapacitors (pseudocapacitors) 7,8 that are dominated by faradaic reactions on electrode materials, 9,10 and (iii) hybrid supercapacitors. 11 Additionally, the speci fic capacitance for both storage mechanisms can be enhanced by using a material with a high specific surface area, such as nanostructured conducting polymer and metal oxides porous or carbonaceous materi- als. 12-14 Thus, understanding the surface and interface characteristics is critical for improving the performance of supercapacitors. Nanostructured materials have significant role in the field of electrochemical capacitors due to the combination of nanoscale features with a highly defined geometry and high surface area. Over the past decade, TiO 2 has been considered as a supercapacitor electrode material because of its semiconducting properties and chemical stability. 15-19 Self-organized titania (TiO 2 ) nanotubes (TNTs) have been explored for use as binder-free supercapacitor electrodes. 18,20 This nanostructured TNT surfaces have been fabricated using the optimized electrochemical anodization process using metallic titanium (Ti) foil as the substrate in fluoride containing electro- lytes. 19-21 Electrochemical anodization method offers suitably back-contacted nanotube layers on the substrate, which can be employed directly as an electrochemical device. 15,16 Ideally, Ti foil is used directly as a current collector, which provides direct and uninterrupted charge transport pathways, while TiO 2 nanotubes provide the active area for charge storage activity. 19 The TNT structures also form surface electrical fields and reduce recombination by confining the injected electrons to the central zone of the tubes which is observed in dye-sensitized solar cell (DSSCs) applications. 19,22 Vertically oriented TNTs have attracted much attention in charging storage systems because of their capacity to offer high surface area and greatly improved electron transfer pathways in comparison to nonoriented structures, which favor higher charge propagation in active materials. 19-21,23 Generally, titania capacitors resembles to conventional EDLCs, which act by a nonfaradic mechanism with a very low specific capacitance of 10-40 μF cm -2 in the process of Received: February 3, 2016 Revised: April 20, 2016 Published: April 25, 2016 Article pubs.acs.org/JPCC © 2016 American Chemical Society 9569 DOI: 10.1021/acs.jpcc.6b01171 J. Phys. Chem. C 2016, 120, 9569-9580