AMERICAN JOURNAL OF SCIENTIFIC AND INDUSTRIAL RESEARCH © 2011, Science Huβ, http://www.scihub.org/AJSIR ISSN: 2153-649X doi:10.5251/ajsir.2011.2.6.852.859

Electrochemical formation of Titania Nanotubes in non-aqueous

electrolyte Abdulkarim M.A.Alsamuraee1, Qassim Mohammed Doss Al-Ittabi2, and Yousif

A.Mohammed2

1College of Science ,Baghdad University ,Chemistry Department Email:samuraee2000@hotmail.com,mobil:009647901350456

2College of Engineering, Baghdad University, Mechanical Eng. Department

ABSTRACT

Self organized titania (TiO2) nanotubes were grown by simple but optimized anodization process in nonaqueus eletrolyte composed of 95%glycerol and 0.5% ammonium fluoride .Different constant cell voltages 2 to 25v (dc) used to obtain nanotubes diameter in the range 8 to 42 nm and lenght of 50 to 820 nm ,all experiments conducted at room temperature ~ 25 Degree centigrade for a period of 1 hour. Scanning Electron Microscopy (SEM) was used to obtain information on the surface morphology and the SEM images were statistically analyzed by Java image processing program, where Atomic Force Microscopy (AFM) conducted to obtain quantitative information on the grain sizes of the films as well as the surface roughness, and X-Ray Diffraction (XRD) to obtain information on the crystalline structure of the surfaces.

Keywards: titania nanotubes, anodization, non-aqueous electrolyte

INTRODUCTION

Nano-sized objects show a great variety of interesting properties and this is the main driving force of the research in the field of nanotechnology today. The anodization of some metals, (ex;Ti,Al),under suitable conditions, leads to the formation of ordered arrays of pores in an oxide matrix. This technique has implemented and are now able to tune pore diameter and period. In the case of TiO2, comparative studies have shown that highly ordered, vertically oriented nanotube arrays outperform their colloidal counterpart for applications including sensors[1],water photoelectrolysis[2], dye-sensitized and solid-state heterojunction solar cells[3].In addition to biomedical applications including biosensors, molecular filtration, drug delivery and tissue engineering[4]. Furthermore, initial investigations indicate that they also may be useful for energy storage devices such as Li-ion batteries, and super-capacitors[5]. The widespread technological use of titania is impaired by its wide band gap(3-3.2eV) which can be narrowed by different methodologies.The anodization method has attracted the most intrest among the other fabrication methods due to its ability to produce integrative ,vertically oriented highly ordered

nanotube arrays with controllable dimentions[6].This study aims at investigation of fabricating titania nanotubes via anodizing electrochemical process using nonaques electrolyte, and the effects of cell voltages on the morphology of the fabricated titania nanotube[7].

Experimental: Titanium (Ti) metal foils (99.7% purity) ,0.2-1 mm thick, 1.6 cm diameter specimens, were punched out of a sheet. A titanium disc is loaded into a Teflon specimen holder which exposes 1 cm2 of the sample (1 side), copper conductor provides electrical contact over the sample, and minimizes contamination of the electrolytes from metal fixtures. Before electrochemical anodization, the Ti foils were degreased by ultrasonic treatments in acetone, ethanol, and deionized (DI) water for 10 min. The Ti foils were then anodized in 0.5 wt % NH4F (98% + ACS reagent,Sigma-Aldrich) in glycerol (Anhydrous, 99.8%, Sigma Aldrich). Nanotubular TiO2 arrays were formed by anodization using a two-electrode configuration with Ti foil as anode and a platinum electrode (thickness; 0.1 mm, served as a cathode. the separation between the two electrodes was kept at2.0 cm in all the experiments. Anodization was carried out for different voltages at room temperature (~25 ºC) with anodizing duration of 1Hr. In each case the voltage was held constant using a DC voltage supply while anodization the current (mv

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throw 1Ω resistance) & voltage were recorded for time monitoring using two channels millivoltmeter X-Y recorder (siemense8800) as shown in schematic diagram in fig.1, The anodized samples were cleaned with distilled water to remove the occluded ions from the anodized solutions, and dried in a nitrogen gas. Scanning Electron Microscopy (Hitachi 4500) was used to obtain information on the morphology of titania nanotube, SEM images were analyzed with Java Image processing program ,where Atomic Force Microscopy (AFM-AA3000 of Angstrom Advanced

Inc.,USA) conducted to obtain quantitative information on the grain sizes of the titania films as well as the surface roughness, and X-Ray Diffraction (Hitachi 600-XRD) to obtain information on the crystalline structure of the surfaces.

Fig.1 Schematic diagram of Ti anodizing apparatus.

RESULTS & DISCUSSION TiO2 nanotubes were grown with diameters varying from 8 nm to 42 nm. No nanotubes detected with anodizing voltage of 2.5v, above this magnitude of potential the radii of the tubes were found to be

proportional to the applied voltage. Furthermore, the length of the tubes was significantly small or absent when low voltage was used and long tubes achieved with increasing cell voltages to 25vdc, as shown in table.

1 Table 1.Titania nanotubes diameter and length at different anodizing potentials in Glycerol /0.5%NH4F electrolyte at 25ºC for 1Hr.

Potential V

Mean pore Diameternm

Standard deviationnm

Tube lengthnm

2.5 - - - 5.0 8 ±1.02 50

10.0 12 ±1.03 68 15.0 18 ±0.9 200 25.0 25 ±1.0 820

The above table was established using SEM images, by MBF_Image J. program. It is a public domain Java image processing program inspired by NIH Image for the Mac-intosh. It runs, either as an online applet or as a downloadable application, on any computer with a Java 1.1 or later virtual machine. Maryland, USA.

This program has the ability to find and measure the dimensions in either nano or micro scale and make a comparison with a known scale from a specific image, such as SEM images or optical micrometer images .

Potential V

Mean pore Diameter

nm

Standard deviation

nm

Tube length nm

2.5 - - - 5.0 8 ±1.02 50 10.0 12 ±1.03 68 15.0 18 ±0.9 200 25.0 25 ±1.0 820

1Ω

A VTeflon body

cell

Copper conductor

Ti disk

DC power supplyMechanical stirrer

X‐Y Mill voltmeter recorder

Cathode

Anode

Pt foil

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Fig.2 Menu bar of ImageJ program.

Figures (3 to 4) shows the Top-view SEM images for the self-organized nanoporous titanium oxide templates obtained by anodization processes.

Fig.3, SEM micrographs of TiO2 nanotube grown at 5V for 1 hr at 25ºC ; (a)20.0kV5.7mm X90k SE(V),(b) 20.0kV5.7mm X501k SE(V).

Fig.4, SEM micrographs of TiO2 nanotube grown at 25V for 1 hr at 25ºC ; (a)10.0kV7.1mm X100kSE(V),(b) 10.0kV7.1mm X300kSE(V).

The SEM images revealed that Nanotubes which near or contact to the boundary of edge or any stressed sites they were grew larger in size than the

other, figure 5, this may be reseaned to that tubes in the edge are free and affection to Anodizing conditions (electrolyte and current) more than other .

a

a

b

b

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Fig.5,SEM images showing TiO2 nanotubes grown near edges and stressed sites When the voltage is high (25v), some separated nanotubes became obvious and lied horizontally on the surface of

Ti, figure 6.

Fig.6,SEM images shows TiO 2 nanotubes fabricated at high voltage (25v) (a) lied horizontally on Ti surface,(b) separated.

a b

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More accurate statistical results obtained by AFM analysis ,figure 7 ,which showed a significant match with the

SEM images.

7(a) 7(b)

7(c)

Fig.7,AFM results; (a)2D image,(b)3D image ,and (c) normal granularity distribution report of TiO2 fabricated in(glycerol/0.5%NH4F ) at 25v,1hr

,and 25ºC.

Also AFM scans revealed information on the surface roughness of the fabricated titania films ,as shown in figure 8;(a) for TiO2 nanotubes grew at lowest

voltage(5v),and(b) at highest voltage(25v),again they show matching with SEM analysis which detect some

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separated nanotubes led to increasing the roughness of the film surfaces.

Fig.8,AFM section line roughness analysis reports;(a)titania grew at 5v,1Hr,25ºC

, and(b)at 25v,1Hr,25ºC showing . Different colors have been grown on the surface of samples depending on the

voltage of the anodization process of titanium. (2.5, 5, 10, 15, and 25) five different colors were appeared as shown in fig (8).

Fig.9, Photograph show the colors of the anodized Ti foils at different voltages.

Ra(Roughness Average) 4.37 [nm] Rq(Root Mean Square) 5.45 [nm] Ry(Peak-Peak) 23.6 [nm] Rz(Ten Point Height) 22.3 [nm] Sm(Mean Peak Spacing) 927 [nm] Rk(Core Roughness Depth) 12 [nm] Rvk(Reduced Valley Depth) 4.71 [nm]

b

Ra(Roughness Average) 3.53 [nm] Rq(Root Mean Square) 4.25 [nm] Ry(Peak-Peak) 18.7 [nm] Rz(Ten Point Height) 18.2 [nm] Sm(Mean Peak Spacing) 605 [nm] Rk(Core Roughness Depth) 9.43 [nm] Rvk(Reduced Valley Depth) 2.81 [nm]

a

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The XRD patterns of Titania films were investigated as received without any treatment, it show a sharp peaks at 2θ ;35,38,40,70, 93 of the Ti (hexagonal)–

base metal and some broad and weak peaks at 2θ ; 25,37,55,86, belongs to the amorphous and anantase (tetragonal) TiO2 nanotubes[8].

Fig.10,XRD paterns of titania nanotubes as received after fabrication

via anodizing in glycerol/0.5%NH4F at 25’C. The mechanism of the formation of TiO2 nanotubes is proposed by many authors [9, 10],they assumed that nanotubes formed as a result of competition between field-assisted anodic oxidation, defined as the formation of the anodic layer under an applied electric field: Ti + 2H2O TiO2 + 2H2 and chemical/field assisted dissolution of the forming oxide: TiO2 + 6F- + 4H+ TiF6

2- + 2H2O

which can be regarded as dissolution promoted by the presence of fluoride ions (chemical dissolution) and by the electric field weakening the bond between Ti and O (field assisted dissolution). The presence of fluoride ions in the electrolyte leads to the formation of fluoro complexes. The most stable is TiF6

2-, which has a large negative Gibb’s free energy of formation ( ∆G0 =-2118.4kJ / mol ). For comparison, the Gibb’s free energy of 298 0 formation of TiO2 is ∆G298 =-821.3kJ / mol.The formation of fluoro-complexes is also aided by the applied voltage, which drives fluoride ions (F-) toward the metal along with Ti4+ toward the electrolyte: Ti+4 + 6F- TiF6

2- The barrier layer at the bottom of the nanotubes (at the metal/oxide interface) acts as a resistance to the flow of ions such as Ti4+ and O2-, which need to move through the anodic film to maintain active the

oxidation. Greater resistances (i.e. thicker barrier layers) can be overcome by increasing the applied anodization voltage. The presence of fluoride ions in the electrolyte allows creation of channels in which current can flow and keeps the oxidation process active. The thickness of the barrier layer underneath the pores/tubes is constantly reduced by dissolution and in turn regenerates by oxidation. The nanotubes length increases until the oxidation rate at the metal/oxide interface equals the chemical dissolution rate of the top surface of the tubes. After this point, the nanotube length is independent of time.

CONCLUSIONS The results of this study demonstrated the possibility of fabricate aligned titania nanotubes in non-aqueous glycerol electrolyte containing 0.5% NH4Ft. The diameter and length of the nanotubes vary with anodization voltages for fixed other process conditions; temperature, time, Smaller and shorter nanotubes were obtained at the low voltages, where the nanotubes became wider and longer on increasing anodization potentials. TiO2 nanotubes with diameter of the range 8 to 42 nm and length of the range 50 to 870 nm were fabricated using potential of the range 5 to 25 volts dc ,while no nanotubes detected below this range of potentials. Grate matching of statistically analysis of the SEM and AFM scans upon the pore diameters and roughness’s of the fabricated TiO2 nanotubes.

Ti‐hexagonal TiO2‐amorphus‐weak‐anatase

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XRD of the as-anodized titania revealed amorphous with very weak anatase titania structures.

ACKNOWLEDGMENT

The authors acknowledge Dr. Peng Zhang (Dalhousie University, Halifax, NS, Canada) and his research group to do some of the work at their labs. and many thank to Mrs. Patricia Scallion (SEM Technician, Dalhousie University) for the SEM test.

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