The Morphological Characterization of Anodic TiO₂ Nanotube Arrays

The distinct properties of TiO2 (semiconductor, photocatalytic, non-toxic, stable and easy to fabricate) make it highly attractable for a wide range of applications, such as sensors, H2 generation by water photoelectrolysis and dye-sensitized solar cells (DCSs) [1–3]. In the particular case of DCSs, the use of TiO2 photoanodes presents advantageous characteristics when compared with common photovoltaic devices [1]. Nanostructures such as TiO2 nanotubes (NTs) have been widely investigated, since they were shown to provide direct pathways for electrons transport up to the charge collector and reduce the electron recombination when compared with TiO2 nanoparticles or thin films [1,2]. TiO2 NTs can be produced by anodization techniques. Parameters like electrolyte composition, pH, applied potential, time and temperature significantly influence the growth and morphology of the TiO2 NTs and control the corresponding geometrical features [1,2,4]. In this work we performed the morphological characterization of the TiO2 NTs with different anodization conditions. We studied the effect of applied potential (V) and anodization time (t) on the geometrical parameters barrier layer thickness (δb), pore diameter (Dp), interpore distance (Dint), outer diameter (Douter), wall (W) and template thickness (L) of the TiO2 NTs. To this end, TiO2 NTs templates were obtained by electrochemical anodizations of Ti foils (99.99 % high-purity from AlfaAesar, 0.127 mm). Prior to anodization, the samples were ultrasonically cleaned with ethanol and deionized water. The electrolyte used was an ethylene glycol solution containing NH4F (0.3 wt%) and H2O (2 wt%), at room temperature [5]. Anodizations were conducted using a DC power supply Keithley Model 2004 Sourcemeter, monitored and controlled by a LabView application. Two sets of samples were prepared with (i) different applied potential (20 to 80 V; 3 hours); (ii) different anodization time (0.5 to 72 hours; at 60 V). The morphology of the resulting NTs was analyzed in detail by scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) techniques (FEI Quanta 400FEG Field Emission Scanning Electron Microscopy: FE-SEM). Optimization of the experimental conditions were performed after the STEM images presented NH4F contaminations within the NTs (Figure 1(a)). After anodization, the TiO2 NTs templates were sequentially cleaned with ethanol, deionized water and dried with a nitrogen stream, resulting high-quality contrast STEM images in order to observe the NTs morphology (Figure 1(b)). From the resulting STEM and SEM images (tops, bottoms and cross-section views), geometrical parameters δb, Dp, Dint, Douter, W and L were determined using the ImageJ open source software (Figure 2). These parameters revealed a different behavior regarding the conditions of V and t used. While they presented a linear relation with V, as usually reported in literature [2,4], a more irregular behavior was found with t, resulting from the microscopic mechanisms that are responsible for NTs formation, being strongly dependent on electrolyte pH variations through anodization. The use of microscopic techniques showed to be crucial to the NTs observation and characterization, since detailed information such as dimensions and morphological aspects can be taken from the well-defined structures provided by the SEM and STEM images [6]. ...