Effect of annealing temperature on structural and morphological stability of Ti02 nanotube arrays Rajini P Antony, Tom Mathews, Sitaram Dash, A K Tyagi Thin Film and Coating Section Material Science Group Indira Gandhi Center for atomic Research Kalpakkam, Tamil Nadu, India
[email protected]
was chosen as the electrolyte and platinum sheet as the cathode. The electrodes were kept 1.5 cm apart. Potentiostatic anodization was carried out at 20, 30, 40, 50 and 60V. From the above anodization experiments it was found that smooth walled nanotube arrays of high aspect ratio were obtained at 40 V. Hence the nanotube arrays synthesized at 40V were chosen for all studies. The time dependent anodization current was recorded with a computer controlled multimeter (HP 34401A). The pH of the electrolytic solution was measured by using a pH meter. After each anodization the samples were sonicated in ethanol and then dried in nitrogen stream. The dried samples synthesized at 40V were annealed at 400, 500, 600 and 700°C for 2 hours in air, followed by natural cooling. The heating rate was 3° per minute.
Abstract- Self aligned Ti02 nanotube arrays were synthesized on
Ti
foils by
electrochemical
anodization
technique.
The
qualitative mechanistic aspect of nanotube formation is discussed based on the results of potentiostatic transient measurements. The structural and morphological stability of the
nanotube
arrays synthesized were investigated by annealing the anodized foils at various temperatures and then carrying out the XRD and FESEM analysis. The phase transformation and crystallization of nanotubes are discussed briefly.
Keywordsfilms
Ti02
nanotubes, electrochemical anodization, thin
I.
INTRODUCTION
Nano structured Ti02 have found interesting applications in the field of photo catalysis and gas sensors because of their large surface area [[1], [2]]. Very large surface area and better electron transport properties, because of their directional nature, have made self aligned Ti02 nanotube array films one of the most sought after material for gas sensing, photocatalytic and dye sensitized solar cell applications. Large surface area self aligned Ti02 nanotube arrays are generally synthesized by electrochemical anodization technique [[3]]. It is crucial to analyze the crystallinity and structural stability of nanotube arrays with temperature as different phases are preferred for specific applications. For example anatase phase of Ti02 is preferred for photocatalytic applications [4] whereas rutile phase is desired for the gas sensing applications [5]. From the application point of view analysis of the morphological stability of the nanotubes (which provides the high surface area) with temperature is important. In this study Ti02 nanotubes were synthesized by electrochemical anodization technique, subjected to various high temperature annealing programmes and characterized to investigate the morphological and structural changes with temperature II. A.
B.
t = K A / B Cos 9. Where K is a dimensionless constant having value 0.9, A=1.5406 A° and 9 is the diffraction angle in radians. Broadening B was calculated by taking the full width half maximum of the diffraction peak corresponding to each plane.
EXPERIMENTAL
Anodization
The anodization was perfonned in a two electrode configuration. Titanium sheet sonicated in ethanol, acetone and then thoroughly washed with distilled water and finally dried in nitrogen stream was used as the anode. Ethylene glycol containing 0.5 wt% ammonium fluoride and 2.5 wt% of water
978-1-4673-0074-21111$26.00 @2011 IEEE
Characterization
The surface morphology of the as prepared and annealed samples was analyzed by using scanning electron microscope (FEI Serion, Philips, Netherlands). From the partial lift off of the nanotubular layers, the cross sectional images were taken and the thickness variations were calculated. The composition of the nanotubular layers were characterized by an energy dispersive X-ray Analyzer fitted to the SEM chamber. X- ray diffraction studies were carried out to identify the crystal structure of the as prepared and annealed samples using a glancing angle X-ray diffractometer (STOE, Germany). Diffraction experiments were done using Cu Ka (A=1.54060AO) radiation at 40 kV and 30 rnA with a scan step of 1° over 20 to 70 degrees. The XRD pattern of the samples were compared with the JCPDS file 21-1272 and 21-1276 corresponding to anatase and rutile phases respectively. The crystallite sizes of the samples were calculated using Scherer formula [6].
326
III. A.
formation of hemispherical closed bottom end is because of the combined effect of formation of oxide and dissolution of Ti metal and oxide, by fluoride ions present in the non aqueous medium, during anodization. During the initial stage of anodization an insulating oxide layer is formed. As a result further oxide growth as well as dissolution of Ti metal and oxide occurs by field assisted anodization. As illustrated by various researchers [7]-[9] the nanotube formation is the result of steady state equilibrium between field assisted chemical dissolution and field assisted chemical oxidation reactions.
RESULTS AND DISCUSSION
Chemistry ofnanotube Formation Abbreviations and Acronyms
Fig.l (a) and (b) shows the SEM image of the Ti foil before and after anodization respectively. From the figure it is clear that anodization process has resulted in nanotube formation. Fig.l (c and d) represent the SEM image of the backside (bottom) of the nanotube arrays and the cross section respectively. The backside view of the nanotube arrays reveals that the nanotubes are having closed ends.
Fig. 3 depicts the typical variation of current density with time, during anodization. The transient can be explained as the non ideal case of potentiostatic transient where surface reactions makes the deviation from ideal behavior. This deviation explains the formation of nanotube. The main electrochemical reactions taking place during anodization are formation of barrier oxide layer on Ti substrate due to reaction with oxygen generated by electrolysis of water present in the electrolyte, field assisted chemical etching of Ti as well as Ti02 by fluoride ions to form pits on the substrate, attack of fluoride ions as well as H30+ ions on this pits where the field is very high, and evolution of H2 at the platinum electrode. 0 .012 ,..----,
0.010
Figure I. SEM image of Ti foil before anodization. (b) SEM image of Ti foil after anodization (reveals the formation of nanotube). (c) and (d) Micrographs showing the closed ends (bottom end) and cross sectional view of nanotubes respectively
Fig. 2 shows the morphology of Ti substrate after removing the nanotube layers by ultrasonication. The periodically arranged dimple morphology indicates the potential of the substrates obtained after removing the nanotube layers, as templates for growing one dimensional structures of various materials by different techniques.
Figure 2.
0.002
1000
2000
3000 Time (5)
4000
5000
Figure 3. Potentiostatic transient obtained during the anodization process, inset shows the variation in the current density at the initial stages of nanotube formation
Since anodization was carried out in organic medium (ethylene glycol containing 0.5 wt% ammonium fluoride and 2.5 wt% of water) the pH was �7±0.1. It is reported in the literature that in acidic electrolytes like HF or HCI the rate of chemical dissolution is high and the nanotube length restricts to a few hundreds of nanometer [2]. The rate of dissolution is determined by the localized concentrations of H+ and F- ions nearer to the bottom end of the nanotube, where electrochemical reaction proceeds leading to the continuous growth of the nanotube. Correlating with the current density versus time graph the mechanism of the nanotube formation can be explained as follows. Initially, when Ti metal is subjected to anodization, a passive oxide layer is formed, according to the reaction:
Template pattern of Ti Foil after anodization
Ti + 2H20 � Ti02 + 4H+ + 4e-
The closed ends of the nanotubes and the pattern observed on the titanium sheets after removing the nanotube layers revealed that the bottom end of the nanotube is always hemispherical in nature. It is to be pointed out that the
The initial sharp decrease in current density is due to the formation of this barrier oxide layer. By this reaction, H+ ions
327
are generated at the electrolyte / metal oxide interface. At the same time, in the presence of the H+ ions, the fluoride ions nearer to the substrate will react with Ti02 as well as Ti metal to form [TiF6]2- complex according to the reactions:
Ti
Ti- Titanium Ti
Ti02 + 6NH4F + 4H+ .... [TiF6 f + 2H20 + 6NH + and .... 20
Both H+ and F- ions enhance the rate of chemical dissolution at the interface. The simultaneous chemical oxidation and chemical dissolution, assisted by the field (E= V/d, where V is the applied voltage and d the thickness of the oxide layer), occurring at the active area, will move the metal oxide/metal interface as well as the electrolyte/metal oxide more inside the metal substrate. The inset of Fig.3 shows variation in current density with time. The decrease in current density in the initial stage is due to the barrier oxide layer formation. This is followed by an increase in current density because of chemical dissolution of Ti02 and Ti according to the above reactions. After some time the current density approaches a constant value signifying steady state / equilibrium between the competitive oxidation and dissolution reactions.
Figure 4.
60
50
20(degree)
XRD pattern of the as prepared Ti02 nanotubes (shows that the samples are amorphous)
from the pattern that the nanotube arrays are amorphous in the as prepared condition, which is consistent with the results obtained by Oomman et al [10]. The peaks seen correspond to Ti substrate. In order to study the crystallinity of the nanotube arrays, XRD pattern of the samples annealed in air at 400°C, 500°C, 600°C and 700°C for 2 hours were recorded. Fig. 5 shows the XRD pattern of the annealed samples. At 400°C, there were peaks corresponding to tetragonal anatase phase of Ti02 in addition to the substrate peak. When the samples were annealed at 500°C the anatase peaks got sharpened and intensified indicating increase in anatase phase crystallinity and crystallite size. The observed low intensity peak corresponding to rutile phase indicates the beginning of phase transformation. The XRD pattern of the sample annealed at 600° C revealed the presence of rutile and anatase phase mixture. At 700°C, anatase and rutile phases were still present but with higher concentration of rutile phase. The phase transformation behavior observed here was slightly different from those reported by Oomman et al [10], where phase transformations took place at higher temperatures. These changes can be attributed to the difference in the heating rate.
Since the anodization is carried out at constant potential, the field generated should be uniform throughout, i.e.; the active area should have equifield surface. The shape acquired by the active area should be hemispherical in order to satisfy the equifield concept. If the active area would have square or any other geometry the field will not be uniform and will be maximum at the corners thereby increasing the chemical dissolution rate at the corners. This will prevent the continuous growth of nanotube. The irregularity in the current density observed at the initial stage indicates that the active area is rough and therefore the field is not equivalent everywhere. At steady state the rough surface transforms into an equifield surface (hemispherical shape) because of attainment of equilibriwn between the oxidation and dissolution reactions. B.
40
30
A
A- Anatase R-Rutile Ti-Tltanlum A A
Effect a/Temperature on crystallinity and morphology Units
The crystallinity of the synthesized nanotubes has to be thoroughly studied as different phases of the material are preferred for specific applications. Crystallinity, crystallite size, surface area and composition plays major role in the material properties. In order to verify the crystallinity and crystallite size, XRD studies were carried out for the as prepared and annealed samples. Figure 4 depicts the XRD pattern of the as prepared Ti02 nanotube arrays. It is clear
20
40
30
29 Figure 5.
328
50
(degree)
60
70
XRD pattern of Ti02 nanotubes annealed at different temperature
The occurrence of anatase to rutile phase transformation at 500 to 6000 C inferred from XRD results can be correlated with the scanning electron micrographs. Phase transformation from anatase to rutile requires crystal reorientation and growth [11] and is achieved with increase in temperature. It has been reported that phase stability, crystal growth and nucleation is dependent on crystallite size [12]. Anatase phase of crystallite size less than 14 run is stable and will not undergo phase transformation [12]. In our study the crystallite size of the nanotube arrays annealed at the lowest temperature (400°C) is �48 run, and therefore is prone to phase transformation. Shannon in 1964 investigated the defect mechanism of phase transformation in vacuum and hydrogen reduced Ti02 samples [13]. The anatase to rutile transformation involves the break up of anatase structure with a volume change of about 8%. This collapse occurs by the deformation of the oxygen frame work and shifting of the Ti4+ ions by rupturing two of the six Ti-O bonds to form new bonds [13]. Hebrard and Nortier investigated the effect of hydroxyl species in the sintering and crystallite growth of Ti02 powders and found that surface diffusion of hydroxyl species is the most likely rate limiting step of the particle growth of anatase Ti02 powder [14] and hence the phase transformation is affected by the presence of hydroxyl ions chemisorbed on the Ti02 surface. Fig. 8 illustrates the variation of anatase crystallite size with annealing temperature. The 101 diffraction peak of the anatase phase is considered here for the crystallite size calculation. From 6000C to 7000C there is a drastic change in the crystallite size variation which evidently depicts the fast crystal growth in this temperature range. This can be attributed to the increase in the oxygen diffusion rate due to the presence of hydroxyl species present on the Ti02 surface. The presence surface OH groups were clearly identified by X-ray photoelectron analysis [15]. An increase in the crystallite size of anatase phase Ti02 will enhance the rutile phase transformation and occurs by consuming the anatase crystallites in the nanotube walls. As the process continues, complete transformation occurs leading to the destruction of the tubular morphology there by generating thick rutile films [10].
In order to verify the effect of temperature on tube morphology, FESEM analysis of the nanotube arrays were carried out. Fig. 6(a-d) and 7(a-c) shows the surface morphology and cross sectional views respectively of the nanotube arrays annealed at different temperatures, clearly depicting the effect of calcination temperature on tube morphology. From the surface topography of the nanotube arrays, it is clear that the samples annealed at 400 and 500°C retains the tubular structure. Whereas, the samples annealed at 600 and 700°C have tubes with closed mouth, due to wall thickening. However, the cross sectional view clearly indicate retention of tubular morphology even up to 700°C.
Figure 6. SEM images showing the surface morphology of annealed Ti0 2 nanotube arrays (a) at 400°C (b) at 500°C (c) at 600°C and (d) at 700°C
62 ,------, 60
E 58 r::::
-
CLI
N 'iii
� =;:
'Iii �
56 54
52 50
o 48 400
450
500
550
600
Temperature Figure 7. SEM images showing the cross sectional views of annealed Ti02 anotube
650
100
(DC)
Figure 8. Variation of anatase crystallite size with temperature
arrays (a) at 400°C (b) at 600°C and (c) at 700°C
329
CONCLUSIONS
Self aligned Ti02 nanotube arrays were synthesized on Ti substrates by electrochemical anodization technique using fluoride ion containing non aqueous electrolyte. Highly ordered nanotubes of about 1 micron length were obtained in neutral pH within 1 hour of anodization. The potentiostatic transients qualitatively revealed the mechanism of nanotube formation. From the potentiostatic transients, correlating with reported results, it can be said that field assisted chemical oxidation of Ti and field assisted chemical dissolution of Ti02 and Ti by fluoride ions played the key role in nanotube formation.
[4]
N. G. Park, J. V. Lagemaat, and A. J. Frank, "Comparison of Dye Sensitized Rutile- and Anatase-Based Ti02 Solar Cells", J. Phys. Chern. B, vol. 104 pp. 8989-8994, September 2000.
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Q. A. S. Nguyen, Y. V Bhargava and T. M. Devine, "Initiation of Organized NanoporeiNanotube Arrays in Anodized Titanium Oxide", Journal of the Electrochemical Society, vol. 156, pp. E55-E61,
[9]
K. Yasuda, 1. M. Macak, S. Berger, A. Ghicov and P. Schmuki, "Mechanistic Aspects of the Self-Organization Process for Oxide Nanotube Formation on Valve Metals", Journal of the Electrochemical Society, vol. 154, pp. C472-C478, June 2007.
September 2000
The as prepared nanotubes were amorphous. XRD analysis revealed amorphous to anatase phase transformation at 4000 C. As the annealing temperature is increased the anatase phase started converting to rutile phase. The crystallite size as well as the presence of hydroxyl group favored the anatase to rutile transformation. At 7000C the XRD peaks of nanotube arrays showed a mixture of phases with high rutile content. Although the surface morphology was found to be destroyed at 6000 C and above, the cross sectional SEM micrographs revealed retention of nanotubular morphology up to 7000C.
December 2008.
[10] O. K. Varghese, D. Gong, M. Paulose, C. A. Grimes and E. C. Dickey "Crystallization and high-temperature structural stability of titanium oxide nanotube arrays", 1. Mat. Res, vol. 18, pp. 156-165, January 2003 [II] R. D. Shannon, "Phase transformations studies in Ti02 supporting different defect mechanisms in Vaccum reduced and hydrogen reduced rutile", 1. Appl. Phys,vol. 35, pp.3414-3416, November 1965.
ACKNOWLEDGMENT
[12] H. Zhang and J. F. Banfield, "Phase transformation of nanocrystalline anatase-to-rutile via combined interface and surface nucleation", 1. Mater. Res, vol. 15, pp. 437-448, February 2000.
Authors acknowledge SAIF, lIT Madras for FESEM analysis. We also acknowledge Mrs S. Kalavati for carrying out the XRD investigations.
[13]
P. 1. Gouma and M. J. Mills, "Anatase-to-Rutile Transformation in Titania Powders", 1. Am Cer. Soc, vol. 84, pp 619,
622, March 2001
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