Preparation and Characterization of TiO2 Nanotube Arrays via Anodization of Titanium Films Deposited on FTO Conducting Glass at Room Temperature

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1 ACTA PHYSICO-CHIMICA SINICA Volume 24, Issue 12, December 2008 Online English edition of the Chinese language journal Cite this article as: Acta Phys. -Chim. Sin., 2008, 24(12): ARTICLE Preparation and Characterization of TiO2 Nanotube Arrays via Anodization of Titanium Films Deposited on FTO Conducting Glass at Room Temperature Yuxin Tang, Jie Tao*, Yanyan Zhang, Tao Wu, Haijun Tao, Zuguo Bao College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing , P. R. China Abstract: Self-organized TiO 2 nanotube arrays with micro-scale length were prepared on fluorine-doped tin oxide (FTO) conducting glass in NH 4F/glycerol electrolyte by electrochemical anodization of pure titanium films deposited by radio frequency magnetron sputtering (RFMS) at room temperature. The samples were characterized by means of field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and photoelectrochemistry methods. The results showed that Ti films prepared at the condition of Ar pressure 0.5 Pa, power 150 W, and 0.5 h at room temperature possessed the zone T model structure with good homogeneity and high denseness. When the anodization time was prolonged from 1 to 3 h at the voltage of 30 V, the pore diameter of TiO 2 nanotubes increased from 50 to 75 nm, and the length increased from 750 to 1100 nm and then gradually decreased to 800 nm, while their wall morphology changed from smooth to rough. Also with increasing the anodization voltage, the pore diameter became larger, and the remaining oxide layer reduced, which could be easily removed by ultrasonic- chemical cleaning in 0.05% (w, mass fraction) diluted HF solution. Moreover, the photocurrent response curves and electrochemical impedance spectroscopy (EIS) results indicated that UV-illumination clearly enhanced the effective separation of the electron-hole pairs and the crystallized electrodes from the annealing treatment of as-anodized electrodes at 450 C exhibited a better photoelectrochemical performance. Key Words: NH 4F/glycerol Radio frequency magnetron sputtering; Room temperature; FTO conducting glass; TiO 2 nanotube arrays; The growth of metal film in vacuum is a deposition process of massive atoms or atomic groups arriving on the substrate, which is affected by the deposition parameters, such as substrate temperature, sputtering pressure, and power. The microstructure of the films is dominated by the relative substrate temperature and the energy of deposition atoms [1 4]. Mainly, there are evaporation method and sputtering method to prepare metal films by vacuum deposition. The former easily conducted in simple equipment can manufacture the films with poor compactness and several bubbles. The latter can produce firm and dense films with good homogeneity and reproducibility in a large area owing to high-energy bombarding particles. Therefore sputtering method has become an important way that has been used very widely. Recently, titanium film has drawn extraordinary attention for its applications in microelectronics, machinery, aerospace, and medical industry owing to its remarkable photoelectric performance and corrosion resistance. Mor group [5] has successfully developed the technology to grow TiO 2 nanotube arrays from titanium thin films deposited on glass substrates at 500 C by anodization. The well-orderly and perpendicularlyoriented TiO 2 nanotubes possessed not only large specific surface area and strong absorption capacity, but also superior electron lifetimes owing to providing excellent pathways for electron percolation. Thus they exhibited high photoelectric conversion efficiency [6 8] and good photocatalytic properties [9,10]. Some other groups [11 16] also used the same way to deposit Ti films at high temperature on a variety of substrates (e.g. silicon). However, the length of nanotube arrays prepared in the electrolytes was less than 700 nm since the increasing of Received: July 14, 2008; Revised: September 17, *Corresponding author. taojie@nuaa.edu.cn; Tel: The project was supported by the Natural Science Foundation of Jiangsu Province (BK ) and the Aeronautical Science Foundation of China (04H52059). Copyright 2008, Chinese Chemical Society and College of Chemistry and Molecular Engineering, Peking University. Published by Elsevier BV. All rights reserved. Chinese edition available online at

2 the thickness of Ti-sputtered films resulted in the degradation of the films quality [5,6]. Obviously, it is not appropriate to deposit Ti film at high temperature on flexible and temperaturenonresistant substrates, e.g. polyethylene terephthalate (PET) or polyethylene naphthalate polymer (PEN). Moreover, to prepare TiO 2 nanotubes on transparent substrates (e.g. conducting glass, PET substrate) can enhance the light-converting efficiency of optical-electric devices based on the TiO 2 nanotubes and make these devices microminiaturized and flexible. Although Eu [17] and Neale [18] et al. have prepared the porous titania films on the transparent substrate, the distribution of the pores is not ordered and highly oriented. According to our previous work [19], the quality (dense, uniform) of the titanium film and the anodization parameters are critical to the formation process of titania nanotubes. Therefore, on the basis of the fabrication of high-qualify Ti films, it is necessary to select an electrolyte with low chemical dissolution rate to increase the thickness-conversion ratio of Ti film. In this study, we attempt to deposit compact and uniform Ti films on conducting glass at room temperature, and then fabricate large aspect ratio TiO 2 nanotube layer (1100 nm) in NH 4 F/glycerol electrolyte. Additionally, the growth process and photoelectrochemical characteristics of the TiO 2 nanotubes are investigated, and a method for eliminating the cover layer on the nanotube surface is also discussed. 1 Experimental 1.1 Deposition of the titanium film at room temperature on conducting glass Ti films (ca 1.47 µm) were deposited on fluorine-doped tin oxide (FTO) conducting glass at room temperature by RFMS (radio frequency magnetron sputtering) (JPG500, China). A Ti disk (99.9% purity, TianYuan Co., Ltd., Shenzhen, China) with 60 mm diameter and 5 mm thickness was used as the target. The base chamber pressure was below Pa and the distance between the target and sample was 60 mm. To remove pinhole defects in the titanium film, the specimen was cleaned by ultrasonic in acetone, ethanol, and deionized water for 15 min, respectively. Before deposition, the target was prior sputtered for 10 min to remove the oxide layer or other impurities on the titanium target surface. During the deposition process, the argon gas (99.999% purity) pressure (p Ar ) was 0.5 Pa with gas flowing at 10 ml min 1 and the sputtering power (P s ) was 150 W at room temperature. After deposition for 0.5 h, the samples were kept in vacuum for 6 h. 1.2 Preparation of TiO2 nanotubes on FTO glass using an anodization method Anodization was carried out using a two-electrode configuration. Titanium film/fto with an exposed surface area of 1.0 cm 2 was used as the working electrode. A platinum sheet (2 cm 1.5 cm) with a copper wire was used as the counter-electrode. The distance between the working and counterelectrodes was kept at 3 cm. A DC power source (WYK-1502, Eksi Electronic Co., Ltd., Jiangsu, China) supplied the required anodization potential (10 40 V) in a single step (without ramping). Anodization was conducted in 0.5% (w) NH 4 F/glycerol at room temperature using a digital multimeter interfaced with a computer. After anodization, the samples were immediately washed with distilled water and subsequently dried in air. Some samples were heat treated for 3 h at different temperatures ranging from 300 to 550 C in air at a heating rate of 5 C min 1, and then the samples were cooled in the furnace. 2 Results and discussion 2.1 Characterization of titanium films deposited at room temperature The atom deposition process can be divided into three steps, namely the gas phase atom s deposition or adsorption, the surface diffusion, and the bulk self-diffusion. The morphological features of metal films are given on the basis of the relative substrate temperature T s /T m (T s is substrate temperature, T m is melting point of the deposited material) and the energy of deposition atoms. The morphological features were named as zone 1, zone T, zone 2, and zone 3 for high melting point metal film by taking into account the T s /T m and the pressure [1 4]. Based on the titanium film (T m(ti) =1660 C) deposited at room temperature (T s(fto) <150 C) in this experiment and hence T s(fto) /T m(ti) <0.3, the Ti film structure depended on the sputtering pressure and power will be formed between zone 1 and zone T. Compared with zone 1, the critical nucleus size of Ti grains is still small and the structure is also consisted of an array of fibrous grains in the zone T, however, the surface diffusion of the atom is significant, causing the relatively compact structure at grain boundaries without holes and tapered crystals. Apparently, it is necessary to choose the appropriate sputtering parameters to promote the appearance of zone T structure. At suitable low sputtering pressure and high sputtering power, the Ti atom will obtain higher energy. Therefore, the surface diffusion is improved and the substrate temperature increases, yielding a movement of the structure to zone T region. FESEM images, XRD pattern, and EDX spectrum of Ti film deposited under the conditions of p Ar =0.5 Pa, P s =150 W, and t=0.5 h at room temperature are shown in Fig.1. In Fig.1(a c), it is found that a close-packed hexagonal structure of α-ti film with (002) preferred orientation appears when 150 W power is applied. The microstructure of the film is uniform with the grain size of 100 nm, and a banding distribution of columnar structure belonged to the typical zone T structure with high density is seen from the cross-section of the film. The Ti atom has high energy to migrate and occupy the equilibrium sites of titanium crystal lattice when the Ti

3 Fig.1 (a) Top and (b) cross-sectional FESEM images of Ti film and the corresponding (c) XRD pattern and (d) EDX spectrum film is deposited at appropriate high sputtering power and low sputtering pressure, which results in the growth of the columnar structure (zone T). Moreover, the result of the chemical analysis by energy dispersive X-ray (EDX) spectroscopy (Fig.1(d)) indicates that an almost pure Ti film is formed with a low oxygen contamination level (below 1%). Fig.2 depicts FESEM images of TiO 2 nanotube arrays via anodization of titanium films deposited at different conditions in 0.5% (w) HF solution at 10 V. When Ti film is deposited at 500 C (T s(fto) /T m(ti) > 0.3) [14], the structure of the film belongs to the zone 2 form. In this condition, the surface and bulk diffusion of the film are significant and the deposited atoms migrate adequately, so that the shadowing effect is weakened, leading to a columnar structure of the film with high denseness. Therefore, the substrate is frequently heated at high temperature to create a dense, uniform, and crystal titanium film suitable for anodization [5 7,11 16]. In this experiment, the morphological feature of uniform and organized titania nanotubes (Fig.2(a)) anodized from Ti films deposited at room temperature is similar to Fig.2(b), which indicates that the dense and uniform Ti film obtained directly at room temperature is favorable for the formation of ordered nanotube arrays. 2.2 Fabrication and characterization of TiO2 nanotube arrays on FTO glass substrates Fig.3 depicts the current time behaviors recorded at 10 V for 0.5% (w) HF/1 mol L 1 H 3 PO 4 and 0.5% (w) NH 4 F/glycerol electrolytes. It can be seen that the behaviors are not different from that of the Ti films anodized in HF solution [19]. However, this process is relatively slow for NH 4 F/glycerol electrolyte owing to its high dielectric constant and coefficient of viscosity. Also, a low current density is found in the viscous electrolyte, which is only one-tenth of HF/H 3 PO 4 electrolyte, indicating that the anodization process is controlled by diffusion. Hence a dependence of the diffusion constant on the viscosity is in a Stokes-Einstein manner: D=k B T/6πηr, where D is the diffusion constant, k B is Boltzmann s constant, T is the absolute temperature, η is the dynamic viscosity, and r is the radius of a spherical body. According to this formula, D is inversely proportional to η. Therefore, a lower growth and chemical dissolution rate of the nanotubes is obtained in the viscous electrolytes. Also, it takes a long time to reach the stable-state. As the anodization proceeds, the steep rise of current in HF/H 3 PO 4 electrolyte occurs earlier than in glycerol electrolyte after going through the stable stage. The current increase is because the electrolyte interacts with the FTO surface as the last of the Ti film is consumed. Meanwhile, the sample becomes translucent and should be quickly removed from the electrolyte; otherwise it would be consumed by the HF. Moreover, compared with smooth current curve in glycerol electrolytes, occasional fluctuations of the current curve are found in HF/H 3 PO 4 electrolyte, which is caused by oxygen bubbles (2H 2 O 4e 4H + +O 2 ) produced on the pinholes in Ti film or any exposed FTO substrate at the condition of high potentials and low viscosity of the aqueous HF/H 3 PO 4 electrolyte. However, this phenomenon does not appear in the Fig.2 FESEM images of TiO 2 nanotube arrays via anodization of titanium films deposited at different conditions (a) p Ar=0.5 Pa, P s=150 W, T substrate=room temperature, t=0.5 h; (b) p Ar=0.5 Pa, P s=150 W, T substrate=500 C, t=1 h Fig.3 Current density anodizing time curves of titanium films at 10 V in different electrolytes (a) 0.5% (w) NH 4F/glycerol, (b) HF/H 3PO 4

4 Yuxin Tang et al. / Acta Physico-Chimica Sinica, 2008, 24(12): glycerol electrolyte. The effect of anodization time and voltage on the formation of TiO2 nanotubes in 0.5% (w) NH4F/glycerol electrolyte is studied. Fig.4(a g) shows FESEM images of the samples anodized for different times at the voltage of 30 V. At the initial stage of anodization (Fig.4(a, b)), the surface of nanotubes is almost covered with a porous oxide layer. After anodization for 1 h, the pore diameter of TiO2 nanotubes is about 50 nm, with the length of 750 nm, which is longer than that of aqueous HF/H3PO4 electrolyte for the same time. In this case, the wall morphology of TiO2 nanotubes is smooth. When the anodization time is 2 h (Fig.4(d, e)), the morphology of the nanotube arrays is getting clear owing to the obvious thinning of the porous structure. The pore diameter and the length of the nanotubes with roughness and ripples at the bottom of tube walls increases to 75 nm and 1100 nm, respectively, while the thickness of Ti film decreases to 200 nm. During the anodization process, the glycerol electrolyte keeps absorbing moisture in the air, leading to the drop of the resistance of the electrolyte. As a result, the rise of effective potential on the Ti/FTO electrode results in the diameter of TiO2 nanotubes to increase with the increasing amount of water in the electrolyte. Simul- Fig.4 taneously, the variation of the amount of water in glycerol electrolyte causes the difference in the dissolution and the oxide formation rate of TiO2 nanotubes, bringing about the change of the wall topography. These phenomena are the same for the anodization of Ti foil[20,21]. When the anodization time is prolonged to 3 h, the pore diameter of nanotubes is no longer changed. And the porous structure is consumed completely because of the chemical dissolution during the anodization (Fig.4(f, g)). The length of the nanotubes is shortened to 800 nm with some collapsed nanotubes on the surface owing to the serious corrosion of the electrolyte. The whole formed tubes have rough walls with ripples. Fig.4(h, i, f) shows FESEM images of samples anodized for 3 h at the voltages of 10, 20, and 30 V, respectively. Obviously, the potential has a large effect on the tube diameters, ranging from about 35 nm at 10 V to 75 nm at 30 V. However, the porous layer existed after 3 h anodization reduces gradually with the increase of anodizing potential. This indicates that the higher potential not only increases the electric field intensity of the oxide layer, but also speeds up the diffusion of ions, leading to the higher growth rate of nanotube, and accelerating the fieldenhanced dissolution rate of the upper oxide layer. Therefore, Top and cross-sectional FESEM images of TiO2 nanotubes anodized in 0.5% (w) NH4F/glycerol electrolyte at different conditions (a, b) 0.5 h, (c) 1 h, (d, e) 2 h, (f, g) 3 h at 30 V, and top view at 10 V (h), 20 V (i) for 3 h

5 Fig.5 Top view of the surface of TiO 2 nanotubes in Fig.4(d) after ultrasonic cleaning by immersing in 0.05% (w) HF solution for different times (a) 15 s, (b) 30 s, (c) 60 s the increase of the potential can promote dissolution of the cover oxide layer. In order to remove the porous oxide layer on the surface, a series of as-prepared samples are cleaned in 0.05% (w) HF solution by chemical etching with ultrasonic. Fig.5 exhibits the variation on the surface topography of the Fig.4(d) samples (1100 nm) cleaned at different times. Clearly, the asprepared sample without cleaning is almost covered by a thin porous oxide layer which is dissolved gradually as time passes. After 15 s treatment (Fig.5(a)), a small part of the uniform nanotubes are exposed due to the porous oxide film dissolving under the combined action of the hydrofluoric acid chemical dissolution and ultrasonic oscillation, and then clear nanotubes (Fig.5(b)) appear when the time reaches to 30 s. Furthermore, the porous layer is removed completely after 60 s treatment (Fig.5(c)), while the topography and the length of nanotubes are not influenced. Therefore, the ultrasonic-chemical cleaning is an efficient method for the removal of the covered oxide layer. The XRD patterns of the nanotubular TiO 2 /FTO electrodes before and after heat treatment at different temperatures are shown in Fig.6. Most of Ti film has been transformed to TiO 2 nanotube layer (1100 nm) after anodization. The characteristic peaks of the as-anodized sample (Fig.6(a)) do not arise from the TiO 2 but from the conducting film (SnO 2 and SnO) of FTO and Ti, contributing to the form of the amorphous sample. After heat treatment at 300 C for 3 h (Fig.6(b)), the amorphous sample transforms to anatase structures characterized with (101) preferred orientation. At higher temperature (450 C, 550 C), the peaks of anatase in Fig.6(c, d) become stronger and the (110) characteristic diffraction peak of rutile phase appears, which indicates that the crystal type of TiO 2 nanotubes is a mixed structure of anatase and rutile. Also, the rutile phase grows at the interface between the barrier layer of the nanotubes and titanium film where the thermal oxidation occurs ( 430 C) [5,14]. 2.3 Photoelectrochemical characterization of nanotubular TiO2/FTO electrodes Figs.7 and 8 show the photocurrent response curves and electrochemical impedance spectra (EIS) of nanotubular TiO 2 /FTO electrodes annealed at different temperatures under UV illumination, respectively. All electrochemical measurements are carried out in electrolyte solution of 0.1 mol L 1 Na 2 SO 4 under ultraviolet lamp (UV, 254 nm, 8 W) illumination using three-electrode system at room temperature with the TiO 2 /FTO electrode, a platinum plate, and a saturated calomel electrode (SCE) served as the working, counter, and reference electrodes, respectively. The distance between the working electrode and the UV lamp is 3 cm. The EIS measurement is Fig.6 XRD patterns of nanotublar TiO 2/FTO electrodes annealed at different temperatures (a) as-anodized, (b) 300 C, (c) 450 C, (d) 550 C Fig.7 Photocurrent response curves of nanotubular TiO 2/FTO electrodes annealed at different temperatures in 0.1 mol L 1 Na 2SO 4 solution under UV pulsed-illumination

6 response test. 3 Conclusions Fig.8 EIS of nanotubular TiO 2/FTO electrodes annealed at different temperatures in 0.1 mol L 1 Na 2SO 4 solution under UV illumination carried out by applying 100 khz to 0.01 Hz frequency range with oscillation amplitude of 5 mv on a CHI660 electrochemical workstation. As seen in Fig.7, the photoelectric current transiently increases and then tends to be stable when UV light is turned on, which indicates that the nanotube electrode has a good photoelectric current stability. However, once UV light is turned off, the photocurrent intensity quickly decreases to initial value as dark current. This phenomenon illustrates that the composite electrodes have the n-type semiconductor characteristic. It is clear that the photocurrent increases along with the increasing of heat treatment temperature and the responsive currents are constant at 0.03, 0.38, 1.55, and 4.88 µa under dark condition and then drastically increase to 1.74, 6.48, 9.78, and 7.95 µa under UV illumination, respectively. The increase of photocurrent for TiO 2 nanotubes under the UV illumination is owing to the efficient photo-generated electrons conducting through interfacial region of the nanotubes under an electric field, and the photocurrent level is affected by the crystallinity and isomorph type of the titania. When the heat treatment temperature increases, the crystallinity of TiO 2 nanotubes is enhanced obviously, leading to the improvement of photoelectrochemical performance. It is found that the sample shows better performance at 450 C. However, when the temperature is higher than 550 C, the performance of the electrode degrades due to the decreasing of the specific surface area of TiO 2 nanotubes which results from the structural damage and collapse of the nanotubes (not shown here). This is in line with the heat treatment of TiO 2 nanotubes on Ti foil [22 24]. Fig.8 presents that larger radius of the semicircle is found in the amorphous electrode corresponding to higher TiO 2 resistance value. At the same frequency, the larger the radius of semicircle, the smaller will be the capacitance constant, which results in higher value of the impedance of Faraday current. Therefore, it is more difficult for the chemical reaction to take place on the electrode owing to the higher energy barrier. However, it is found that the radius of the semicircle decreases after heat treatment. Moreover, the smallest radius corresponding to the optimal performance appears at 450 C. This result is consistent with that of the photocurrent The Ti films were deposited on FTO by RFMS under 0.5 Pa Ar pressure and 150 W sputtering power at room temperature for 0.5 h, which possessed a uniform and dense zone T structure. Then TiO 2 nanotubes of 1100-nm length and 75-nm pore diameter were successfully prepared by potentiostatic anodization of the Ti films in 0.5% (w) NH 4 F/glycerol electrolyte at the voltage of 30 V for 2 h. With the increasing of the anodization voltage (from 10 to 30 V for 3 h), the pore diameter became larger (from 35 to 75 nm), and the remaining oxide layer reduced, which could be easily removed by ultrasonicchemical cleaning in 0.05% (w) diluted HF solution. Furthermore, the photoelectrochemistry measurements indicated that the crystallized electrodes from the annealing treatment of as-anodized electrodes at 450 C exhibited better photoelectrochemical performance. 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