Fabrication of complete titania nanoporous structures via electrochemical anodization of Ti
© Ali et al; licensee Springer. 2011
Received: 24 February 2011
Accepted: 13 April 2011
Published: 13 April 2011
We present a novel method to fabricate complete and highly oriented anodic titanium oxide (ATO) nano-porous structures with uniform and parallel nanochannels. ATO nano-porous structures are fabricated by anodizing a Ti-foil in two different organic viscous electrolytes at room temperature using a two-step anodizing method. TiO2 nanotubes covered with a few nanometer thin nano-porous layer is produced when the first and the second anodization are carried out in the same electrolyte. However, a complete titania nano-porous (TNP) structures are obtained when the second anodization is conducted in a viscous electrolyte when compared to the first one. TNP structure was attributed to the suppression of F-rich layer dissolution between the cell boundaries in the viscous electrolyte. The structural morphologies were examined by field emission scanning electron microscope. The average pore diameter is approximately 70 nm, while the average inter-pore distance is approximately 130 nm. These TNP structures are useful to fabricate other nanostructure materials and nanodevices.
Macro-, nano-, and meso-porous structure gained a lot of attention of the scientific community in the last few decades due to their unique properties and potential application in various fields [1–4]. Particular attention was paid to the self-organized porous materials due to their self-ordered structure and ease of fabrication. One of the most extensively investigated porous materials is porous anodic alumina (PAA) . Highly ordered nano-porous structure can be fabricated on pure aluminum under optimized conditions via two-step electrochemical anodization . PAA are being used mostly as a membrane , as a biosensor , and as a template for fabrication of secondary nano-meter scale materials . Nano-porous structure formation on other value metals like Zr, Nb, Ta, W, Fe , and Al-Ti  alloy have been reported by Patrick and co-workers. The next porous material after aluminum which attracted the interest of researchers around the world in the last decade is titanium di-oxide due to the pioneer work of Fujishima and Honda  and Regan and Graztal .
Titanium di-oxide (TiO2, titania) is a semiconductor material and find their application in many areas like self-cleaning , solar cell [13, 14], photocatalysis , drug delivery , biomedical implant , and sensing . TiO2 nano-porous structure (TNP) was first reported by Zwelling et al.  via anodization of Ti and Ti alloy in chromic-HF electrolyte. Soon after, Grimes et al.  also reported TiO2 nanoporous structure in HF-containing aqueous electrolyte with limited thickness. Since then TiO2 nanostructure is the main focus of research. Among the various methods of TiO2 nanostructure fabrication, anodization is usually known a simple, versatile, and economical one. The nanotubes diameter, length, and smoothness can be easily controlled by varying the electrochemical parameters . TiO2 nanotubes have been fabricated in different electrolytes via anodization of pure Ti . A great breakthrough in the fabrication of TiO2 nanotubular structure was achieved by Macak et al. , and Grimes and co-workers , where they reported very smooth, regular, and very long nanotubes in organic viscous electrolytes. A lot of papers have been published so far on the morphologies and applications of TiO2 nanotubes. However, very little attention was paid to TiO2 nanoporous structure. TNP film was reported by Bu et al.  on glass substrate in polyethylene glycol (PEG) using sol-gel method; however, the pore diameter and pore density was not uniform. Beranek et al.  and Macak et al.  also fabricated TNP in H2SO4-HF and Na2SO4-NF electrolytes, respectively, through anodization of Ti. However, from their SEM results, the morphologies of TiO2 nanostructure are similar to tubular structure instead of porous structure. Choi et al.  also reported TNP structure at the top surface of Ti via nano-imprint and successive anodization of Ti. TiO2 nanoporous structures on Si substrates have also been reported by Yu et al.  via anodization, but they did not obtain well-defined and ordered pore morphologies. Zhang et al.  applied multi-step (3-step) anodization approach to Ti and obtained highly ordered TNP structure only at the top surface after third anodization. According to their report, ordered nano-porous titania showed much higher photocurrent when compared to titania nanotubes due to efficient separation of photo-generated electron-hole pair by nano-porous titania. Very recently, Patrik and co-workers  obtained TNP structure under optimized conditions. Although they successfully obtained TNP structure not only on the top surface, but also cross-sectional wise; however, the degree of ordering and uniformity of channels was not achieved. Hence an ideal nanoporous structures like PAA is scarcely obtained.
Here, in this study, we obtained highly orientated TNP structures with uniform and parallel nano-channels using a two-step anodizing method. By changing the nature of electrolyte during second-step anodization, we obtained different morphologies of TiO2 nanostructures. Furthermore, we also studied the effect of various electrolytes and prolonged anodizing time on the pore morphology during second-step anodization.
Titanium foil (Ti, Goodfellow, 0.1 mm thickness, 99.6% purity), ammonium fluoride (NH4F, Sigma-Aldrich, Germany, 98+%), hydro-fluoric acid (HF, Sigma-Aldrich, Germany , 98+%), ethylene glycol (Extra pure, Junsei Chemical Co. Ltd. Japan), and glycerol (Extra pure, Junsei Chemical Co. Ltd. Japan) are used in their as-received form without further treatment.
Highly ordered and smooth TiO2 nanotubes were fabricated by anodization of Ti foils in ethylene glycol (EG) electrolyte containing 0.5 wt% NH4F and 0.2 wt% H2O. Briefly before anodization, the Ti foils were degreased by sonicating in acetone, isopropyl alcohol, and methanol each for 10 min. Subsequently, the Ti foils were rinsed many times with deionized (DI) water and dried in gas stream. Two electrodes system with Ti-foil as a working electrode and a platinum gauze (15 × 25 × 0.2 mm3) as a counter electrode was used for anodization. The first-step anodization was carried out at 50 V in the above-mentioned electrolyte for 7 h using DC power supply system, producing highly ordered and smooth TiO2 nanotubes. It is worth mentioning that in this study the first nanotubes layer was separated from the underlying Ti substrates with the help of N2-blowing technique instead of using an ultrasonic treatment . This method not only provides a very clean, smooth, uniform, and oriented honeycomb-like a patterned substrates for further anodization but also helps to avoid possible mechanical damage to the substrates. Thus, as a result a high-quality TiO2 nanotubes arrays have been achieved. In order to study the effect of electrolytes on pore morphology, a set of experiments were performed in different electrolytes during the second-step anodization. The second-step anodization was conducted in the same EG-based and 0.5 wt% HF aqueous electrolytes under identical parameters for 20 h, producing TiO2 nanotubes covered with a thin nanoporous layer on the top surface. The second-step anodization conducted in an electrolyte consisting of glycerol with 0.5 wt% NH4F and 0.2 wt% H2O under identical parameters for 20 h led to a highly oriented TNP structure.
In addition, we also investigated the effect of anodizing time on the surface topologies of TiO2 nanotubes (TNT) and TNP structures. On the basis of our field emission scanning electron microscope (FESEM, Hitachi S-4800, Tokyo, Japan) results, EG and glycerol-based electrolytes were employed for further experiments. Two samples were anodized in the same EG-based electrolyte for different times (11 and 72 h) under identical parameters with the first-step anodization. In another set of experiment, one sample was first anodized in the same EG-based electrolyte and then re-anodized in the same glycerol-based electrolyte for 72 h via the second-step anodization. The structural morphology of the samples was characterized with the help of FESEM attached with energy dispersive X-ray spectroscopy (EDX). The cross-sectional studies were carried out on mechanically cracked samples.
Results and discussion
The first-step anodization in EG-based electrolyte
The second-step anodization in EG-based electrolyte
The second-step anodization in HF-based aqueous electrolyte
The second-step anodization in glycerol-based electrolyte and formation of TiO2 nano-porous structures
Energy dispersive X-ray spectroscopy (EDX) analysis of top and bottom surface of TNP
Effect of anodizing time on the morphologies of TNT and TNP
In summary, we have fabricated a complete titania nanoporous structure with uniform and parallel nanochannels using a two-step anodization process. The average pore diameter was approximately 70 nm and inter-pore distance was approximately 130 nm. Self-organized, highly ordered, and very smooth TNTs were fabricated in EG-based electrolyte by the first-step anodization. The top surfaces of TNTs were covered with an oxide layer irrespective of the anodizing time. Clean and homogeneous honeycomb-like patterned Ti substrates were left off after the detachment of TNTs from the underlying Ti-foil. The second-step anodization on the patterned Ti-substrate produced a uniform and closed packed TNTs with open end morphology. The second-step anodization in EG and aqueous HF-based electrolytes produced TNTs covered with a thin nanoporous layer on the top. Very rough and disordered morphology of TNTs were obtained in HF-based electrolyte unlike EG-based electrolyte via the second-step anodization. A highly oriented and complete TNP structure was obtained when the second-step anodization was conducted in glycerol-based electrolyte. TNP structure were attributed to the suppression of F-rich layer dissolution between the cell boundaries in the viscous electrolyte. In addition, we found that TNP structure retained in shape even in spite of a long anodizing time (72 h) after the second-step anodization and that its ordering was improved to a great extent. This study provides a simple route to fabricate highly oriented TNPs with parallel and uniform nanochannels, which may be useful for high performance applications such as sensors, filters, dye sensitized solar cells, and photocatalysis.
anodic titanium oxide
energy dispersive X-ray spectroscopy
field emission scanning electron microscope
porous anodic alumina
This study was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MEST) (No. 2010-0026150). The authors are greatly acknowledged the help of Emad-u-din.
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