Facile fabrication, characterization, and enhanced photoelectrocatalytic degradation performance of highly oriented TiO 2 nanotube arrays

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1 J Nanopart Res (2009) 11: DOI /s z RESEARCH PAPER Facile fabrication, characterization, and enhanced photoelectrocatalytic degradation performance of highly oriented io 2 Qidong Zhao Æ Xinyong Li Æ Ning Wang Æ Yang Hou Æ Xie Quan Æ Guohua Chen Received: 20 November 2008 / Accepted: 23 June 2009 / Published online: 10 July 2009 Ó Springer Science+Business Media B.V Abstract Highly ordered io 2 were successfully fabricated using ethanol and water mixture electrolytes (40 vol% ethanol and 0.2 wt% hydrofluoric acid) by a facile electrochemical anodization method. he as-prepared were grown perpendicular to the titanium substrate with about 90 nm in diameter, 20 nm in wall thickness, and around 500 nm in length. he formation mechanism of the samples is briefly discussed. A blue shift in the spectrum of UV Vis absorption was observed with respect to a piece of the sol gel derived io 2 film. Moreover, photocurrent response and photoelectrocatalytic degradation of methyl orange under ultraviolet light irradiation were adopted to evaluate the photoelectrocatalytic properties of the io 2. We demonstrate that the highly ordered io 2 possess much better photoelectrocatalytic activity than the sol gel derived io 2 film and good stability. Q. Zhao X. Li (&) N. Wang Y. Hou X. Quan Key Laboratory of Industrial Ecology and Environmental Engineering (MOE) and State Key Laboratory of Fine Chemical, School of Environmental and Biological Science and echnology, Dalian University of echnology, Dalian , Liaoning, China xinyongli@hotmail.com X. Li G. Chen Department of Chemical and Biological Engineering, he Hong Kong University of Science and echnology, Clear Water Bay, Kowloon, Hong Kong, China Keywords itania Nanotube Photocurrent Photoelectrocatalytic degradation Nanomanufacturing Introduction During the past two decades, titania nanomaterials have received intensive attention due to their useful optical and electric properties, stability, biological benignity, and most of all, their excellent photocatalytic performance for environmental and energy applications (Chen and Mao 2007; Fujishima et al. 2008; Ghicov and Schmuki 2009; Shankar et al. 2009). Especially, one-dimensional io 2 has been paid a great deal of attention due to its technological potentials, such as sensing devices (Seo et al. 2009; Varghese et al. 2003), biomaterials and therapy (Macak et al. 2005a, b, c; Shrestha et al. 2009), photocatalysts (Ghicov and Schmuki 2009; Lin et al. 2008; Liu et al. 2008; Mor et al. 2005; Quan et al. 2005; Raja et al. 2006; Shankar et al. 2009; Wang et al. 2009; Zhang et al. 2008), solar cells (Bwana 2008; Chen and Xu 2009; Kim et al. 2008; Kuang et al. 2008; Mor et al. 2006; Yu et al. 2008; Zhu et al. 2007), photoelectrochemical splitting of water (Mohapatra et al. 2007), energy storage (Wang et al. 2008), etc., which is mainly attributed to their high specific surface area, oriented charge transfer channel, shrinked transport path, and possible quantum size effect. Plenty of methods have been developed to

2 2154 J Nanopart Res (2009) 11: fabricate the typical one-dimensional nanostructure, io 2 nanotubes, including sol gel (Wijnhoven and Vos 1998), hydrothermal synthesis (Kasuga et al. 1998), anodic oxidation (Bhargava et al. 2009; Choi et al. 2004; Diamanti and Pedeferri 2007; Ghicov and Schmuki 2009; Lin et al. 2008; Mura et al. 2009; Prakasam et al. 2007; Shankar et al. 2009; Su and Zhou 2009; Yoriya et al. 2008; Zwilling et al. 1999), templated-deposition (Na et al. 2008), sonoelectrochemical method (Mohapatra et al. 2007), and microwave irradiation (Wu et al. 2005). he derived nanotubes with flexible parameters of chemical composition and spatial textile structures are still inspiring further investigations due to their promising and attractive applications. Among the reported techniques, electrochemical anodization of titanium is a relatively convenient route to fabricate highly oriented io 2. ypically, electrolyte composition and applied potential are the most important anodization variables for the formation of io 2 besides temperature. Electrolyte composition determines the rate of titania formation as well as the rate of titania dissolved, and influences the resultant morphologies and properties of the products. Many researchers have been devoting great efforts to a wide range of inorganic electrolytes, such as HF (Quan et al. 2005), HF/H 2 SO 4, HF/chromic acid (Zwilling et al. 1999), KF/NaF (Macak et al. 2006), H 3 PO 4 /HF, (NH 4 )H 2 PO 4 /NH 4 F (Ghicov et al. 2005), etc. Besides, electrolytes containing organic species have been proved to facilitate the fabrication of io 2 better than the inorganic ones. Ruan and coworkers (2005) prepared highly ordered io 2 in fluorinated dimethyl sulfoxide (DMSO) and ethanol mixture electrolytes by electrochemical etching of titanium under potentiostatic conditions. However, long anodization time (70 h) and high concentration of hydrofluoric acid were required to produce the desirable io 2 nanotubes. Raja and coworkers (2007) reported ordered arrays of io 2 nanotubes anodized in fluoride-containing polyhydric alcohols. Satoshi and coworkers (Kaneco et al. 2007) obtained io 2 nanotubes in methanol and water mixture (containing hydrofluoric acid) organic solvents. hey emphasized that as the ratio of methanol to water increased, the io 2 nanotube number density decreased, and resulted in larger spaces between the nanotubes. More recently, ethylene glycol-based electrolyte has become an attractive choice for fabrication of well-aligned and smooth io 2 nanotubes (Shrestha et al. 2009; Su and Zhou 2009; Wang et al. 2009), because ethylene glycol effectively reduces hydroxyl ion injection from the electrolyte to the oxide layer in addition to its lower conductivity, which favors the growth rate of nanotubes (Paulose et al. 2006; Prakasam et al. 2007). Albu and coworkers reported on the formation of double-walled io 2 nanotubes in ethylene glycol-based electrolyte and demonstrated that tube layers could be converted to an ordered io 2 nanoporous membrane by a simple and rapid thermal annealing process (Albu et al. 2008a). hey also found that some novel morphology, such as bamboo-type reinforced nanotubes and two-dimensional (2D) nanolace sheets, could be obtained if the anodization process was carried out under specific alternating-voltage conditions (Albu et al. 2008b). Lin and coworkers obtained the open-ended high aspect-ratio anodic io 2 nanotube films by the electrochemical anodization of i foil at 80 V in an electrolyte composed of 0.5 wt% NH 4 F and concentrated H 2 O 2 solution in ethylene glycol for 24 h with post-peeling (Lin et al. 2008), which is claimed as a green method. In addition, other organics, such as formamide-based electrolytes, have been found useful in achieving longer nanotubes (Liu et al. 2008; Shankar et al. 2007). he glycerol-based electrolytes have also been shown to be efficient in obtaining smooth io 2 with high aspect-ratio and length of several micrometers (Macak et al. 2005a, b, c; Wang et al. 2008; Wang and Lin 2009; Yang et al. 2008; Yu et al. 2008; Zhuang et al. 2007). hese findings have provided a wider perspective for synthesizing highly oriented io 2. Meanwhile, it is equally important to explore the properties and potential applications of the io 2 derived from the novel mixed media, such as the degradation of organic contaminants of dyes. In this report, we successfully fabricated the highly oriented io 2 by the facile potentiostatically anodization with ethanol and 0.2 wt% hydrofluoric acid mixture electrolytes. he structural and optical properties of the highly oriented io 2 were characterized in detail. hen, we evaluated their photoelectrocatalytic activity with respect to the photocurrent and the degradation of

3 J Nanopart Res (2009) 11: contaminant methyl orange dye, which was compared with the sol gel derived io 2 film. Significantly enhanced photoelectrocatalytic performance of the highly oriented io 2 is demonstrated. Experimental Materials and regents itanium sheet (99.6% purity, in thickness 1.0 mm) was purchased from Aldrich Chemical Company. Methyl orange (purchased from the National Fine Chemical Laboratory of DLU) was used as target compound for degradation, and its molecular structure is shown in Fig. 1. Other chemicals and solvents were of analytical grade from Aldrich Chemical Company. All the compounds were used as received without further purification. Pretreatment of the titanium sheet A piece of titanium sheet was first mechanically polished with different emery papers, rinsed in an ultrasonic bath of cold distilled water for 10 min, and then chemically etched for 30 s in a mixture of HF and HNO 3 acids to form a fresh surface. he acid mixture ratio of HF:HNO 3 :H 2 O is 1:4:5 in volume. he last step of pre-treatment was rinsed with acetone and distilled water, and then dried in air at room temperature. Preparation of the highly oriented io 2 nanotube arrays Highly oriented io 2 were fabricated by the electrochemical anodization method. he anodization was performed in an ethanol and water mixture electrolyte (40 vol% ethanol, 0.2 wt% HF) using a voltage potentiostat (3646 A DC power supply 0 72 V/0 1.5 A) and a conventional twoelectrode configuration with platinum foil as a counter electrode. he electrochemical treatment Fig. 1 Molecular structure of methyl orange dye started with a potential ramp from an open-circuit potential to 20 V by a ramp rate of 50 mv s -1, followed by holding the applied potential at 20 V for tens of minutes. Afterward, the samples were rinsed with DI water and dried in a nitrogen stream. he obtained highly ordered titania were crystallized by annealing in an oxygen atmosphere for 1 h at 500 C with heating and cooling rates of 2 C min -1. A piece of titania film of the same area as the io 2 was also fabricated by a sol gel method on a likewise titanium sheet for comparison (Watanabe et al. 2000). In detail, 20 ml i(oc 4 H 9 ) 4 was added drop by drop into the vigorously stirred 75 ml of dehydrated ethanol. And, then, 0.5 ml glacial acetic acid was slowly added into the continuously stirred solution. After 5 min, 4.5 ml HCl (1.0 mol L -1 ) was added into the solution. Light yellowish and transparent io 2 sol was obtained after stewing the solution for 0.5 h. he io 2 film was prepared on one pre-treated i sheet by a dippingcoated method. he cleaned i sheet was first kept immerged in the obtained sol for 40 min, and then lifted out of the sol with the speed of 6 cm min -1. he wet film was dried for 5 min at 100 C then cooled down. Five times of the above dipping-coated procedure were carried out to reach the desired thickness of film. Finally, the dried i sheet with precursor was heated to 500 C with 10 C min -1 in an oven, kept for 1 h, and then cooled down to room temperature naturally. he obtained white film was thick enough (thicker than 2 lm) to absorb the entire incident UV light in photocatalysis test. Characterization of the highly oriented io 2 he morphology of the io 2 was characterized using an environmental scanning electron microscopy (ESEM, FEI QUANA 200 FEG, Holand FEI Company) operating an accelerating voltage of 30 kv. X-ray diffraction (XRD, RIGAKU, Dmax22000) measurements were carried out on a diffractometer fitted with Cu Ka radiation over the 2h range of he accelerating voltage and the applied current were 40 kv and 30 ma, respectively. he nitrogen adsorption desorption isotherms were measured at 77 K on a SA3100 surface area and pore size analyzer. Samples were degassed in a vacuum at

4 2156 J Nanopart Res (2009) 11: C for 3 h prior to each measurement. he Brunauer Emmett eller (BE) method was utilized to calculate the specific surface areas S BE. he ultraviolet visible (UV Vis) diffuse reflection spectra (DRS) of the samples were taken on a UV Vis spectrometer (Jasco V-550) with the wavelength range of nm. Photoelectrocatalytic experiment he photoelectrocatalytic activities of the as-prepared samples were evaluated by the photocurrent and degradation of methyl orange in a single photoelectrochemical compartment shown in Fig. 2. he photoelectrocatalytic reactor consisted of a quartz cell with the size of 50 mm in diameter, 70 mm in height, and an effective volume of 100 ml, in which io 2 nanotube arrays electrode was configured as the working electrode, the platinum foil as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. he experiment was performed under the conditions of UV-light irradiation (a 200 W highpressure mercury lamp with a principal wavelength of 365 nm, light intensity = 2.0 mw cm -2 ), 0.6 V (vs. SCE) of bias potential, vigorous stirring, and 0.1 M Na 2 SO 4 as supporting electrolyte. he initial concentration of methyl orange solution was 20 mg L -1 during the experiment. he concentration of the methyl orange was monitored at different time intervals by measuring the absorbance of samples at 465 nm using a UV Vis spectrometer (UV1100). Results and discussion he morphology of the highly oriented io 2 Scanning electron microscopy (SEM) images of a typical io 2 nanotube array with different magnifications are shown in Fig. 3. hese io 2 nanotubes are well-aligned and organized into a highly oriented array. From the inset of the enlarged image, the average diameters of these tubes are about 90 nm and the wall thickness is about 20 nm. he highly oriented fabricated in this study are directly related to the ethanol contained in the electrolyte. he effect of ethanol is similar to the results reported by Kaneco and coworkers (2007). It could reduce the size of hydrogen bubbles, which were produced on the cathode surface, and then the current density could be finely controlled. he BE specific surface area of the io 2 nanotubes was about 285 m 2 g -1. his value is much higher in comparison to the io 2 particulate film of 50 m 2 g -1 approximately. he formation process of the highly oriented io 2 he growth of io 2 were monitored by taking SEM images at various intervals as shown in Fig. 4. After chemical etching, a smooth surface can be seen on i substrate in Fig. 4a. An oxide layer forms on the surface of titanium as a result of the pretreatment. After 5 min of anodization (Fig. 4b), due to Fig. 2 he photoelectrocatalytic reactor with three electrodes: (1) high-pressure mercury lamp; (2) working electrode (i/ io 2 ); (3) stirring bar; (4) counter electrode (Pt foil); (5) salt bridge; (6) reference electrode (SCE); (7) potentiostat Fig. 3 SEM images of the highly oriented io 2 nanotube arrays with different magnifications

5 J Nanopart Res (2009) 11: the presence of HF in solution, the oxide layer dissolves locally (Zhao et al. 2005), and small pits start to form in the oxide layer. After anodization for 10 min, as shown in Fig. 4c, the chemical dissolution reduces the thickness of the oxide layer at the bottom of the pits and allows pits formation to continue. Until the dissolution rate of titanium oxide matches the formation rate of titanium oxide (Fig. 4d), the nanotubes can form in a constant thickness. And, no further change in nanotubular morphology was observed, when the anodization was carried out for up to 40 min, as shown in Fig. 4e. Figure 4f is a tilted plot of highly oriented io 2. It can be seen that the length of io 2 nanotubes is around 500 nm. More comprehensive investigations on the mechanisms of the nanotube array growth can be found in some recent reports, which focused on the initiation of organize arrays (Bhargava et al. 2009), galvanostatic pre-treatment (Mura et al. 2009), cation effect (Shankar et al. 2007), chloride and bromide containing electrolytes (Nguyen et al. 2008), electrolyte temperature (Wang et al. 2009), the effect of the fluoride on inter-tube separation (Yoriya et al. 2008), pulsed anodization (Chanmanee et al. 2008), the relations between porosity and the anodization conditions (Su and Zhou 2009), crystallization and phase transition (Yang et al. 2008), etc. hese exploitations have provided in-depth understanding Fig. 4 SEM images of the io 2 formation process during anodization at 20 V in a ethanol and water mixture (40 vol% ethanol, 0.2 mass% HF) electrolyte with different durations: (a) 0 min; (b) 5 min; (c) 10 min; (d) 25 min; (e) 40 min; (f) tilted plot view

6 2158 J Nanopart Res (2009) 11: of the underlying mechanisms and will undoubtedly boost the development of more well-controlled nanostructures. he crystalline phase of the highly oriented io 2 X-ray diffraction patterns of io 2 are displayed in Fig. 5. Before calcination, there are only peaks of titanium substrate in the XRD patterns, as shown by curve a. Obviously, an amorphous phase is dominant. After calcination, the highly oriented io 2 were annealed at 500 C for 1 h, the peak with 2h values of 25.4 emerged, corresponding to the crystal plane of (101) of anatase phase. herefore, the high-temperature thermal treatment is a necessary step to achieve desired crystallization from amorphous to anatase phase for the io 2. We also examined the SEM images of as-prepared io 2 before and after calcinations as shown in Fig. 6. Remarkable was the fact that, even after annealing the io 2 nanotube arrays retained their ordered structure. UV Vis DRS analysis of the highly oriented io 2 Fig. 6 SEM images of the highly oriented io 2 nanotube arrays. a Nanotubes before calcinations. b Nanotubes after calcinations at 500 C for 1 h Relative intensity (a.u.) (b) A (a) θ (degree) A-anatase -itanium Fig. 5 XRD patterns of the titania nanotubes formed by anodic oxidation. (a) Nanotubes before calcinations. (b) Nanotubes after calcinations at 500 C for 1 h Figure 7 shows the UV Vis diffuse reflection spectra of the io 2 compared with the io 2 film. It can be observed from Fig. 7 that their bandgap absorption edges of the samples are around 378 and 393 nm, respectively. On the basis of Kubelka Munk function (Sene et al. 2003), band-gap energy of the highly oriented io 2 and io 2 film are about 3.28 and 3.16 ev, respectively. For the, the band-gap absorption edge is slightly above that of the bulk anatase io 2 (3.2 ev). In other words, higher band-gap energy was obtained with the highly oriented io 2 due to the quantum size effect of the small grains composing the nanotubes, which implies that the photo-induced charges in the highly oriented titania have stronger redox capacity. herefore, higher photoelectrocatalytic performance under ultraviolet light irradiation could be expected for the nanotube arrays. For the visible absorption for both the io 2

7 J Nanopart Res (2009) 11: Absorbance (a.u.) a-io 2 film b-io 2 nanotube Wavelength (nm) Fig. 7 UV Vis DRS of the highly oriented io 2 nanotube arrays b a Photocurrent density (ma/cm 2 ) 0.8 (a) (b) (c) Potential (V vs SCE) Fig. 8 Photocurrent density curves of methyl orange on different electrodes. a io 2 electrode; b io 2 film electrode; c dark current film and nanotubes, that is partially attributed to some color centers or defect structures in the nanoparticles (Kuznetsov and Serpone 2006; Zhuang et al. 2007), which leads to sub-bandgap transition of electrons either from the deep trapping level to the bottom of the conduction band or from the top of valence band to the shallow trapping level. Carbon contaminants from the organic electrolyte can also lead to nonnegligible visible absorption (Mohapatra et al. 2007; Richter et al. 2007). he photocurrent of the highly oriented io 2 Degradation ratio (%) (d) (c) (b) (a) Irradiation time (min) he photocurrent measurements for the highly oriented io 2 and io 2 film were performed in the presence of 20 mg L -1 methyl orange, 0.1 M Na 2 SO 4 solution (Fig. 8). In the dark, the anodic current was low, but in the presence of UV irradiation, the photocurrent strengthened with increasing bias applied to the anode. here was higher photocurrent on the io 2 than that on the io 2 film under the same bias potential below 1.0 V. his feature should be attributed to the fact that the io 2 possess more regular transport channel with high integrity and less barriers for the photogenerated charges, which is deficient for the io 2 film. he unique structure of the highly oriented will benefit the photoelectrochemical process through the electrode, as confirmed below. Fig. 9 Degradation of methyl orange by the io 2 nanotube arrays and io 2 film by different processes. a io 2 nanotubes with photoelectrocatalytic process; b io 2 film with photoelectrocatalytic process; c io 2 nanotubes with photocatalytic process; d without io 2 he photoelectrocatalytic properties of the highly oriented io 2 he photoelectrocatalytic degradation of methyl orange solution with either the highly oriented io 2 or the io 2 film was carried out. he results are shown in Fig. 9. Obviously, they exhibited discrepant photoelectrocatalytic activities. he concentration of target contaminant was monitored at the wavelength of 465 nm. In 5 h, 90.6% of the methyl orange dye was degraded by the io 2 nanotube array electrode, while 83.6% of the methyl orange dye was

8 2160 J Nanopart Res (2009) 11: degraded by the io 2 film electrode. his result could be attributed to the difference of specific surface area as well as the band-gap energy between the oriented titania and the titania film. he io 2 possess larger specific surface area than the io 2 particulate film, which could increase their adsorption capacities and promote the rate of charge transfers. hese features favor the photocatalytic reactions occurring in the system. In addition, the degradation of methyl orange using photocatalytic and photoelectrocatalytic processes were comparatively carried out on the highly oriented io 2. Almost no methyl orange removal was observed without titania nanotube arrays, as shown by curve d of Fig. 9. Under photocatalytic process, about 71.7% of the methyl orange solution was degraded in 5 h of irradiation (Fig. 9, curve c). For the photoelectrocatalytic degradation process, it was revealed that the degradation efficiency reached 90.6% by applying a bias potential of 0.6 V (vs. SCE), which was faster than the degradation efficiency of photocatalytic process. he UV Vis spectral change of the 20 mg L -1 methyl orange as a function of irradiation time during the course of photoelectrodegradation was shown in Fig. 10. With the increase in irradiation time, the characteristic absorption band at 465 nm shrunk gradually and disappeared in about 7 h. his variation indicated that methyl orange was completely removed from the solution. C t /C ime (min) Fig. 11 Methyl orange photoelectrocatalytic degradation efficiencies of five repeated experiments with the highly oriented io 2 (experiment condition: 20 mg L -1 initial concentration of methyl orange, 0.1 M Na 2 SO 4, 5 h irradiation time, bias potential = 0.6 V, light intensity = 0.6 mw cm -2 ) he mechanism for improved photoelectrocatalytic degradation of methyl orange with respect to photocatalytic way can be explained as follows. he electric bias supplied to the io 2 contributes to the separation of the photo-induced electron-hole pairs. hen, the electrons and the holes have more opportunities to participate in degradation reaction, leading to increased photocurrent, and larger quantity of active species. Consequently, the degradation efficiency increases. he stability of the highly oriented io 2 nanotube arrays as photoelectrocatalytic electrodes Absorbance (a.u.) h 7 h In order to examine the stability of highly oriented io 2 used in photoelectrocatalytic degradation, the sample was investigated by repeating the experiment of methyl orange degradation for five times. he io 2 were washed in deionized water with ultrasonication after each batch. As shown in Fig. 11, the results of the five repeated batches for photoelectrocatalytic degradation of methyl orange in 5 h indicated that the degradation efficiency was very stable, despite of a little decrease of about 8.5% Wavelength (nm) Fig. 10 UV Vis absorption spectra of photoelectrocatalytic degradation methyl orange recorded at different time intervals by the io 2 Conclusions he present study has demonstrated that the electrochemical anodization method using water ethanol

9 J Nanopart Res (2009) 11: mixture as the solvent can be used to fabricate highly oriented io 2. he pore diameter and the nanotube length can be facilely tuned by changing the anodization time. Compared with the sol gel derived titania film, the titania nanotube arrays showed an increased photocurrent. hey behaved with higher efficiency in degrading methyl orange through photoelectrocatalytic process, which is mainly attributed to the larger specific surface area and higher band-gap energy of the titania nanotubes as well as the well-oriented channel for charge transfer. Furthermore, the application of a bias potential was demonstrated to promote the degradation efficiency of methyl orange in the process of photocatalysis by the. he films also showed fine stability in repeated degradation. herefore, these highly oriented titania derived from the ethanol-containing electrolyte will play an important role in various applications, such as environmental photoelectrocatalytic devices. Acknowledgements he generous financial support by the National Nature Science Foundation of China (No , ), National High echnology Research and Development Program of China (863 Program; No. 2007AA ), Major State Basic Research Development Program of China (973 Program; No. 2007CB613302), and Ph.D. Program Foundation of Ministry of Education of China (No ) is gratefully acknowledged. References Albu SP, Ghicov A, Aldabergenova S, Drechsel P, LeClere D, hompson GE, Macak JM, Schmuki P (2008a) Formation of double-walled io 2 nanotubes and robust anatase membranes. Adv Mater 20:1 5 Albu SP, Kim D, Schmuki P (2008b) Growth of aligned io 2 bamboo-type nanotubes and highly ordered nanolace. Angew Chem Int Ed 47: Bhargava YV, Nguyen QAS, Devine M (2009) Initiation of organized nanopore/ in titanium oxide. J Electrochem Soc 156:E62 E68 Bwana NN (2008) Comparison of the performances of dyesensitized solar cells based on different io 2 electrode nanostructures. J Nanopart Res. 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