Influence of Anodic Conditions on Self-ordered Growth of Highly Aligned Titanium Oxide Nanopores
© to the authors 2007
Received: 12 May 2007
Accepted: 15 June 2007
Published: 4 July 2007
Self-aligned nanoporous TiO2templates synthesized via dc current electrochemical anodization have been carefully analyzed. The influence of environmental temperature during the anodization, ranging from 2 °C to ambient, on the structure and morphology of the nanoporous oxide formation has been investigated, as well as that of the HF electrolyte chemical composition, its concentration and their mixtures with other acids employed for the anodization. Arrays of self-assembled titania nanopores with inner pores diameter ranging between 50 and 100 nm, wall thickness around 20–60 nm and 300 nm in length, are grown in amorphous phase, vertical to the Ti substrate, parallel aligned to each other and uniformly disordering distributed over all the sample surface. Additional remarks about the photoluminiscence properties of the titania nanoporous templates and the magnetic behavior of the Ni filled nanoporous semiconductor Ti oxide template are also included.
Nanodimensional structures including nanotubes, nanowires and nanoporous architectured materials based on semiconducting metal oxides have become of fundamental interest to the development of functional nanomaterials, nanodevices and nano-systems . Recently, the synthesis of nanostructured functional oxides based on transition metals, with controlled structure and morphology, has attracted a huge interest due to their potential applications in a broad research fields such as Nanoelectronic, Spintronic, Fuel Cells, Nano-biotechnological or Magneto-optoelectronic devices. These new materials have shown a broad range of novel and enhanced mechanical, optical, magnetic and electronic properties respect to those showed by their bulk analogues [2–5]. Actually, great efforts are made in order to obtain self-assembled nanostructures based on TiO2 nanoporous membranes prepared by sol–gel coating , nano-imprint , or electrochemical processes . The search is focused to low cost and efficient fabrication techniques of nanostructured transition metal oxides with high quality nanoporous structures over large surface areas and an accurate pore size control together with long range ordering to enhance the efficiencies of devices based on nanoporous titania (TiO2) templates . The principal advantages for using pure titanium and its alloys are, among others, their high corrosion and good oxidation resistances, low density, high yield strength in a wide temperature range and excellent biocompatibility, which becomes this metal in an outstanding candidate for its application in a wide scientific and technological areas, as e.g. in biocompatible biomaterials, semiconductor memory alloy devices, diluted magnetic semiconductors and materials for micro-optoelectronic applications, transparent oxides semiconductors and gas/humidity or conductivity sensors [10–15]. Otherwise, some of these properties adequately combined with the large band gap semiconductor properties, a high photo-catalytic activity and an excellent biocompatibility exhibited by the TiO2 converts it in a very promising material for applications in many scientific and technological areas, e.g., biocompatible biomaterials for bone implants  or transcutaneous hydrogen sensors , semiconductor memory alloy devices , materials for optoelectronic applications , gas/humidity or conductivity sensors , among others.
The low cost-effective obtention of nanoarchitectured semiconducting metal oxides with high quality nanoporous structures over large surface areas and with precise control of pore size and periodic ordered degree distribution, still remains as an open task. It is undoubted that the control of all these requirements must be fulfilled at the same time in order to optimize the efficiency of the devices based on the titania (TiO2) nanopore arrays [10, 12]. The existence of two unique structural features in these nanostructured semiconducting oxide such as, mixed cation valences and an adjustable oxygen deficiency put the basis for creating and tuning many novel material properties, as well which, allow to use them in the design of sensors and functional devices with superior performance [11–19].
In this work we report about the temperature parameter and acidic electrolyte media influence on the self-aligned and randomly disordered growth of titania nanopore arrays, synthesized by using a very simple and recently reported electrochemical anodization technique . We have focused our attention on the pore size distribution of titania nanopore arrays and the formation of stable and larger wall thicknesses on the wide nanoporous surface obtained, which greatly depend on the experimental anodic parameters. We have extensively studied the titania nanopore arrays growth with varying the anodization temperatures, under different ambient conditions and also, varying the chemical concentrations of the acid electrolytic media. Recently, we have also reported about the magnetic behavior of ferromagnetic nanowires grown inside semiconducting titania nanopores array by electrodeposition technique, using them as templates . Finally, some remarks about the main applications based on these nanoporous semiconductor oxide nanostructures are briefly discussed.
Small pieces of high purity Titanium foils (99.95+%, Goodfellow) with 2.25 mm2 surface area and 25 μm of thickness were employed as starting metal to obtain self-aligned TiO2 nanopores through an unique anodization process . Prior to the anodization, the Titanium sheets were smooth out mechanically up to mirror polishing, and afterwards the polished titanium foils were ultrasonically degreased and cleaned in isopropanol, acetone and ethanol, and finally they were rinsed out with abundant deionised water (18.2 MΩ).
Titania nanoporous array microstructures were studied by using an X’pert-APD Philips X-ray diffractometer. The morphology and atomic chemical microanalysis by energy dispersive X-ray spectroscopy (EDX), of the so-obtained anodized nanoporous titanium dioxide layers, were characterized with HITACHI S-800 and JEOL, JSM-6100, Scanning Electron Microscopes (SEM). Atomic Force Microscopy (AFM) in dynamic mode was also used for surface characterization.
The depth profiling analysis for the titania nanopores arrays were carried out by means of a radiofrequency Glow Discharge Optical Emission Spectroscopy (rf-GD-OES) technique operated with Ar by using a commercial JY 5000 RF instrument working at 13.56 MHz . The samples taken as cathodes were placed in a holder and submitted to a previous vacuum during 5 min. The arrays of the randomly disordered and self-aligned titania nanopores were sputtered in Argon atmosphere at 600 Pa and applied power of 40 W. Data were collected from a nanoporous array circular area of 4 mm in diameter. The wavelengths of spectral emission lines employed for detecting sputtered specimens of Ti, C, O, S, and Ar were 365, 156, 130, 181, and 404 nm, respectively. The sputtering rate and penetration depths were obtained from calibration by direct profilometer measurements, (Perth-o-meter S5P, Mahr Perthen) .
Afterwards, the nanoporous anodic TiO2 templates were also filled with Ni by means of an electrodeposition technique, by using an alternating pulse method reported elsewhere . The metallic Ni nanowire arrays were electrochemical grown into the titania nanopores employing the Watts-bath electrolyte , where the temperature and pH values where kept between 35–40 °C and 4.5, respectively and the electrodeposition time was ranged from 20 min to 1 h.
Magnetic characterization of the Ni electrodeposited samples were carried out through the hysteresis loops measurements in parallel and perpendicular configurations to the Ni nanowires axis at room temperature by using a VSM magnetometer in the range of the applied magnetic field up to 104 Oe (1 Tesla). The sample was mounted in the slot of a plastic holder which, likewise, was placed into the magnetic field of the magnetometer. Turning the sample holder allowed hysteresis loop measurements to be performed for the applied magnetic field aligned both along, and transverse to the long axis of the Ni nanowires.
Structure and Morphology of the Anodic Titanium Oxide Nanopores Array
Due to the solubility of the titanium oxide in HF-containing electrolytes the current starts to increase in the next stage leading to the random growth of titanium oxide nanopores. The presence of fluoride ions in the electrolyte has been previously demonstrated to be necessary for the occurrence of the titania porous structure growth [8, 11, 26, 27]. Along the last stage, a more stable and regular self-aligned titania nanopores growth is established and therefore the current starts slowly to decrease again. Periodical fluctuations exhibited by the current remaining after the reached quasi-steady-state can be also appreciated during the current transients. Such strong current oscillations occurred during titanium anodization have been ascribed to a passivation and depassivation reactions related to the TiO2 porous layer formation and oxide dissolution which are competing during the self-aligned titania nanopores growth .
The Fig. 2b shows the current transients (Idc-t) recorded during Ti anodizations at constant voltage of 20 Vdc in different mixtures of aqueous acidic solutions of HF and H2SO4 electrolytes, temperatures and times of the anodic process. It can be seen that for the different curves reported, similar processes take place to before described in Fig. 2a, briefly they are: during the first few seconds of the anodization a drastic current exponential drop due to the formation of compact oxide film which increases the resistance reducing the current densities, followed by a next stage where the current intensity starts to increase due to the solubility of the Ti oxide in acidic HF-containing solutions and nanopores begin to grow randomly. This is followed by a more stable and regular competition between the self-aligned titania nanopores growth and therefore, the current starts slowly to decrease again. The inset of Fig. 2b shows the typical current intensity fluctuations which are closely related to the growth and dissolutions of the oxide films, ascribed to a passivation and depassivation reactions related to the nanoporous TiO2 formation and oxide dissolution. In fluoride-containing electrolytes, the anodization of Ti is accompanied with the chemical dissolution of Ti oxide due to the formation of TiF6 2− . This dynamic oxide formation/dissolution equilibrium established during large periods of the anodization process, controls the time scale of the porosity developed, self-ordering effects and the thickness of the oxide layer. This mechanism of oxide growth is typical for the self-inhibited semiconducting oxides formation, where a potential drop is located in the space charge layer near the oxide surface during the oxide film thickness growth.
Self-assembled TiO2Nanopores Arrays as Templates for Applications
Other kind of interesting applications for the nanoporous titania templates are based on doping semiconductor titanium oxide with magnetic materials for diluted magnetic semiconductors and/or spintronics [13, 34].
It has been previously established that the pure TiO2 do not shows any ferromagnetic behavior [10, 21], therefore it appears evident that the peculiar ferromagnetic long-range order exhibited is due to the Ni electrodeposition filled titania nanopores. A paramagnetic contribution from the Ti substrate to the magnetic signal of the nanowires can also be clearly detected . Since the Ni nanowires were synthesized employing commercial electrolytes, the co-deposition of impurities from the electrolytes could cause the magnetization of the Ni nanowires array to be smaller than that of the pure metals.
The magnetic behavior exhibited by the peculiar shape of the hysteresis loops of the arrays of Ni nanowires embedded into the self-aligned and randomly disordered amorphous TiO2 nanopores, is very similar to the case of Ni nanowire arrays with large diameter nanowires and quite high aspect ratio (length/diameter) about 2.3–2.4, as reported from others . The coercivity and remanence given values would correspond to a vortex-state nanowire, where the magnetization adopts a helical structure beginning to tilt in the circumferential direction, starting at the ends of the wire, giving rise to a circumferential magnetization at the wire’s perimeter while the magnetization at the center of the wire remains oriented along its axis. The vortex-type remanent magnetization distribution in these flat nanowires can be created due to the strong magnetic interactions between the larger-diameter particles, helped by the minor role that plays the weak magnetocrystalline anisotropy of the policrystalline Ni at room temperature . In fact, reducing the wires height to diameter ratio favors the M perpendicular magnetized state. Besides that, when the inter-wires spacing makes small as compared to the nanowires diameter, the magnetostatic interactions between the coupled nanowires can be so strong as to make the M perpendicular direction as the easy magnetization axis. Additionally, the M perpendicular coercivity of these Ni nanowires is low due to the vortex magnetic configuration can cause partial flux closure which reduces it, since it results relatively easy to move a vortex or domain wall through the particles . Magnetic nanowires exhibiting vortex domain state have typical switching field values around hundreds of Oe, as a result of differences in the shapes, heights, microstructures, roughness or inhomogeneities of the particles, that can act as pinning centers of the magnetization as it rotates in plane. Moreover, vortex states in the nanowires formed by short, but high enough aspect ratio, and closely spaced interacting wires can give rise to a larger saturation field in the M parallel direction and low remanence, which is consistent with strong interactions. These novelty nanocomposites, based on ferromagnetic nanowires embedded in anodic nanoporous semiconducting TiO2 templates, can become in a promising candidates for many applications in a broad range of scientific and technological areas, such as functionalized arrays for magnetic sensing, ultrahigh density magnetic storage media or spin-based electronic devices.
Financial support under Spanish MEC, CICyT and FICyT research projects numbers FC-04-EQP-28, PCTI_PC-06-048, MAT2006-03356 and MAT2004-00150 are grateful acknowledged. The authors whish to thank to Drs. A. Menendez, N. Bordel and R. Pereiro from “Grupo de Espectrometria Analitica” of Universidad de Oviedo for their help with the rf-GDOES measurements. Dr. J. Riba is gratefully acknowledged for the SEM images in the SCT´s equipment of Universidad de Oviedo. The authors also acknowledge to Dr. O. M. Sacristan for the photoluminescence and Raman spectra measurements.
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