Morphology and Microstructure of As-Synthesized Anodic TiO2 Nanotube Arrays
© Cao et al. 2010
Received: 9 July 2010
Accepted: 16 September 2010
Published: 7 October 2010
The as-grown structure of electrochemically synthesized titania nanotube arrays is investigated by scanning electron microscope (SEM) in combination with transmission electron microscope (TEM) as well as X-ray diffraction (XRD). The analysis reveals a preferred growth direction of the nanotubes relative to the substrate surface and the well control on the nanotube arrays morphology. The crystal structure of the anatase phase is detected and exists in the tube walls without any thermal treatment, which makes it possible to realize the application of as-formed TiO2 nanotubes avoiding the degradation of the nanotube structures when sintering. In addition, a new growth, layered model of the anodic TiO2 nanotubes is presented to obtain further understanding of the growth mechanism.
Highly ordered and high surface area TiO2 nanotube arrays have attracted much attention for their potential use in several practical applications, such as photocatalytic applications [1, 2], sensing [3, 4], photoelectrolysis [5, 6], polymer-based bulk heterojunction photovoltaics [7, 8], dye-sensitized solar cells [9, 10], biofluids filtration, drug delivery and other biomedical applications [11, 12]. It is known that the as-fabricated nanotube arrays commonly have an amorphous crystallographic structure. After annealing at elevated temperatures in atmosphere, the nanotube walls transform into anatase phase, and a layer underneath the nanotubes converts into rutile [13–17].
TiO2 properties depend on the crystallinity and hence the utility of their applications also varies. For example, anatase phase is preferred in charge-separating devices such as dye-sensitized solar cells and in photocatalysis, while rutile is used predominantly as dielectric layers and in gas sensors. However, due to nucleation growth type of phase transformations, porosity and/or surface area reduction occur with the sintering . Yang and co-workers made the observations that the nanotube length decreased when calcination temperature is higher than 550°C and completely collapsed at 800°C . So, it is significant to fabricate TiO2 nanotube arrays with applicable crystallinity and isomorph type structures at low temperature. In this work, we report on the crystallinity in the nanotube layer after anodization without any thermal treatment. By analysis on current curve in conjunction with the SEM and TEM images, a layered growth model is put forth to obtain further understanding of anodic TiO2 nanotube arrays formation.
The Ti foils (0.1 mm, 99.6% purity) were degreased prior to anodization by sonicating in acetone, rinsed with deionized water (DI). The electrochemical set-up consisted of a high-voltage potentiostat Jaissle IMP 88 and a classical two-electrode cell, leaving the electrodes distance 1 cm and Ti surface 1 cm2 open to the electrolyte. The samples 1, 2 and 3 were anodized in solutions containing 0.175 M NH4F consisting of mixtures of DI water and glycerol (volume ratio 3:97%) at 30 V for 3, 6 and 12 h, respectively. The sample 4 was prepared in mixed electrolyte containing DI water and glycerol (volume ratio 50:50%) and 0.175 M NH4F at 20 V for 2 h. The sample 5 was anodized in glycerol: 0.175 M NH4F at 30 V for 3 h. A scanning electron microscope Hitachi FE-SEM S4800 and a transmission electron microscope (TEM) JEM-2100 were employed for the morphological characterization of the TiO2 nanotubular layers. The crystalline structures of the TNT arrays were checked by means of X-ray diffraction (XRD, MAC M18XHF).
Results and Discussion
The anodic conditions and morphology parameters of the nanotubes prepared in the electrolytes containing 0.175 M NH4F and different volume ratios of water and glycerol
Anodic time (h)
Tube length (nm)
Tube mouth diameter (nm)
Water/glycerol (Vol. 3:97%)
1,190 ± 10
1,240 ± 20
60 ± 5
1,720 ± 20
70 ± 5
Water/glycerol (Vol. 50:50%)
570 ± 10
80 ± 5
Water/glycerol (Vol. 0:100%)
830 ± 10
Figure 1f shows the SEM bottom and bottom side (inset) views of sample 1. The tube bottom exhibits the hexagon-like cell morphology. Que Anh S. Nguyen and co-workers reported that the separation of the nanotubes from the substrate most likely occurs along the barrier layer/nanotube interface rather than along the titanium substrate/nanotube interface . In this work, by mechanically bending the samples, the separated nanotube arrays were obtained, and the clear bottoms are shown in Figure 1f (inset). But from the bared Ti substrate, no oxide layer has been found.
Insights into Nanotube Growth Mechanism
The first oxide layer is formed in the initial stage of anodization, and the reaction is described by Eq. 1. This rough layer endures high potential and thus some randomly distributed breakdown take place. The fast chemical dissolution of TiO2 at these locations results in the pits formation, as described by Eq. 2.
With the increase in the pits size, the electrolyte has the chance to infiltrate into the interface of oxide layer/metal and the second oxide layer is formed and then broken down again by the electric field. Thereafter, the oxide is formed and broken down layer by layer and the current curve exhibits oscillations correspondingly. The pores are formed due to the longitudinal prolongation and the lateral expansion of the pits with the increase in oxide layer number.
With the pore size increasing, the internal surface area increases and the surface tension that causes the shrinking of the pore increases, too. If the adjacent pores become close adequately, the oxide among them would be pulled apart by the surface tension, eventually leading to the pore separation and the tube formation.
Because the pore size increasing in the latter layer lags that in the former layer, the pore separation in subsequent layer occurs later. Therefore, some oxide between adjacent layers would be remained and thus the ridges are formed. In Figure 5c, the schematic diagram of the tube structure illustrates the periodical unit between two ridge flats and the barrier layer formed by connected bottoms. In this case, the tube structure is the result of the barrier layer moving forward Ti substrate periodically.
Highly ordered TiO2 nanotube arrays were anodized in electrolytes, and the morphology can be well controlled by varying anodic conditions. By mechanically bending the samples, the nanotube arrays were separated from the Ti substrate, but no obvious oxide layer has been found on the bared substrate. The as-synthesized sample, prepared in more water (Vol. 50%) containing electrolyte, exhibits anatase phase crystalline structure and is further confirmed to exist in the tube wall by low glancing angle XRD trace. It can also be found slight anatase (101) peaks in XRD traces of the samples prepared in less water (Vol. 3%) containing electrolyte but with longer anodic time. The crystal growth maybe due to the faster ions mobility, which can offer adequate ions that favor crystal growth in more water containing electrolyte or the thinner tube walls that impose the less constraints for the wall reconstruction. By analysis on current curve in conjunction with the SEM and TEM images, a layered growth model is put forth to obtain further understanding of anodic TiO2 nanotube arrays formation.
Project supported by the National Natural Science Foundation of China (50872001), the Higher Educational Natural Science Foundation of Anhui Province, China (KJ2008B015, KJ2010A123), and the Open Foundation of Anhui Key Laboratory of Information Materials and Devices, China.
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