Synthesis and Characterization of Self-Organized Oxide Nanotube

Nov 24, 2008 - Shikai Liu, Wuyou Fu, Haibin Yang*, Minghui Li, Peng Sun, Baomin Luo, Qingjiang Yu, Ronghui Wei, Mingxia Yuan, Yanyan Zhang, Dong Ma, ...
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19852

J. Phys. Chem. C 2008, 112, 19852–19859

Synthesis and Characterization of Self-Organized Oxide Nanotube Arrays via a Facile Electrochemical Anodization Shikai Liu,† Wuyou Fu,† Haibin Yang,*,† Minghui Li,† Peng Sun,† Baomin Luo,‡ Qingjiang Yu,† Ronghui Wei,† Mingxia Yuan,† Yanyan Zhang,† Dong Ma,† Yixing Li,† and Guangtian Zou† State Key Laboratory of Superhard Materials, Jilin UniVersity, Changchun 130012, P.R. China, and School of Materials Science & Engineering, Henan Polytechnic UniVersity, Jiaozuo 454003, P.R. China ReceiVed: April 21, 2008; ReVised Manuscript ReceiVed: October 9, 2008

Self-organized oxide nanotube arrays have been prepared by a facile two-electrode electrochemical anodization on Ti-2Al-1.5Mn alloy in a 0.5 wt % NH4F aqueous electrolyte. The surface morphology, structure, elemental analysis, and optical and photoelectrochemical behaviors of the nanotubular films are considered. The morphology greatly depends on the applied voltage and anodization time. The as-formed nanotubes under the optimized condition, at 20 V for 3 h, are highly ordered with ∼500 nm in length and the average tube diameter is about 90 nm. The possible “oxide growth and dissolution” mechanism is also discussed. By annealing the initially amorphous films at different temperatures, the importance of the crystalline nature is confirmed. A continuously remarkable red-shift of the absorption edge has been observed with increasing annealing temperature, which is related to the increasing crystallization and the possible new energy bands formed in the TiO2 band gap. The photoelectrochemical properties are investigated and the highest photocurrent of 3.11 mA/cm2 is obtained under AM1.5 100 mW/cm2 illumination at 0.65 V (vs Ag/AgCl). Significantly, a considerable and sustained water splitting behavior has also been observed, and the present convenient synthesis technique can also be extended to other binary or ternary oxide compositions for various applications. 1. Introduction In the past several decades, TiO2 and TiO2-based semiconductors have been extensively studied due to their excellent chemical stability, nontoxicity, and high photocatalytic activities. They are considered as particularly versatile materials with technological application as efficient photocatalysts, photovoltaic materials, separations, gas sensors, optical emissions, electrical circuit varistors, structural ceramics, smart surface coatings, ultraviolet blockers, and functional filling materials in textiles, paints, cosmetics, biocompatible materials, and more. Among these semiconductors, TiO2 nanostructure has drawn much more attention, and numerous efforts have been devoted to the synthesis of nanosized TiO2. Recently, increasing studies demonstrate that tailoring the structure and morphology of semiconductor materials on a nanometer size scale is of fundamental and practical importance. TiO2 self-organized nanotube arrays via electrochemical anodization of titanium (Ti) in fluorine-containing electrolyte, which are highly ordered, high-aspect ratio structures with nanocrystalline walls oriented perpendicular to the substrate, have attracted tremendous efforts because of their outstanding charge transport properties that enable a variety of applications.1-8 The nanotube arrays can create a better opportunity to harvest sunlight more efficiently7 and improve the photoexcited charge carrier lifetime by more than an order of magnitude.8-10 Up to now, three generations of TiO2 nanotube arrays have been developed. Gong and co-workers11 pioneered the synthesis of the first generation by using dilute hydrofluoride acid (HF) * Corresponding author. Tel.: +86 431 85168763. Fax: +86 431 85168763. E-mail: [email protected]. † Jilin University. ‡ Henan Polytechnic University.

aqueous solution as electrolyte with the length of nanotubes limited to a few hundred nanometers.12,13 The second generation of TiO2 nanotube is fabricated in fluorine-containing buffer solution with tube length up to several micrometers.14-16 And the third generation refers to nanotubes with an ultrahigh aspect ratio of the length/diameter by using a nonaqueous electrolyte with further reduced water content.3,5,17 In parallel to activities on pure Ti anodization, this surprisingly simple nanostructuring approach was implemented also for many Ti alloys such as Ti-Al,18 Ti-8Mn,19 Ti-Nb,20 Ti-Zr,21 Ti-45Nb,22 Ti-28Zr-8Nb,23 Ti-29Nb-13Ta-4.6Zr,24 Ti-6Al-4V, and Ti-6Al-7Nb.25 Compared with the single phase of pure Ti, many Ti alloys show a dual phase R + β microstructure. The R-phase, which has a hexagonal closed-packed (hcp) structure, is enriched with hcp stabilizing elements such as Al; whereas β-phase, with a body-centered Cubic (bcc) structure is enriched with bcc stabilizing elements such as V, Mn, Nb, Mo, and Ta. Because of the difference in chemistries of these phases, the selective dissolution and the different reaction rates are two intractable problems. It is still a challenge to form highly ordered nanotubular oxide layers on Ti alloys. Despite all this, the growth of nanotubes on various alloys is particularly interesting for the incorporation of doping species in the oxide structure, which can increase drastically the potential functionality of the tubes. Except for being applied as biomedical materials or surface coatings, the nanotubular oxide layers should also be modulated for a wide variety of other applications, for example, high efficient photocatalysts, solar cell, and photoelectrochemical hydrogen generation. However, little information was available about these aspects. Until very recently, Mor and co-workers reported on the fabrication and visible spectrum photoelectrochemical properties of Ti-Fe-O nanotube arrays by anodization of Ti-Fe metal films.26 Very importantly, they paved the

10.1021/jp803516d CCC: $40.75  2008 American Chemical Society Published on Web 11/24/2008

Self-Organized Oxide Nanotube Arrays via Anodization possible way for shifting the band gap of TiO2 while maintaining its excellent charge-transfer properties and photocorrosion stability, which had primarily focused on metal doping.27-29 It is well-known that metal ion doping introduces midgap energy levels in a minimal concentration, or else the metal ions will serve as recombination centers for the photogenerated electron-hole pairs and thus diminish the overall activities. Hence, the concentration of the metal ion doping is one of the critical factors that must be recognized. As one of the important engineering Ti alloys, Ti-2Al-1.5Mn is commercially available and inexpensive with low contents of Mn (1.5 wt %) and Al (2.0 wt %). Furthermore, it has been known that the energy band gap of manganese oxide with semiconductor characteristic is from 0.26 to 0.7 eV,30 which is small enough to stimulate electrons to a conduction band. Because both Ti-Al18 and Ti8Mn19 can be anodized in fluoride ion containing electrolytes, the possibility exists for obtaining highly ordered oxide nanotube arrays by the electrochemical anodization of Ti-2Al-1.5Mn alloy, which are desired for good photoelectrochemical properties. In the present work, we investigate the feasibility and fabrication of self-organized nanotubular oxide layers formed on Ti-2Al-1.5Mn alloy by a simple two-electrode anodization technique. It is well-known that the diameter and length of the nanotube made by anodization method are proportional to the anodization voltage and anodization time.15,31 Accordingly, they are regarded as the two main factors herein. Surface morphology, structural characterization, elemental analysis, X-ray photoelectron spectroscopy determining chemical states of Ti and Mn, optical measurements, and photoelectrochemical properties under simulated solar light of these oxide nanotubular films have been studied and discussed. 2. Experimental Section All chemicals were the highest purity available purchased from Beijing Chemicals Co. Ltd. and used as received without further purification. Deionized water (18 MΩ cm) was used in all cases. Ti-2Al-1.5Mn alloy foil (0.35 mm thick, Jinsheng Weiye Rare Metal Co., Ltd., China) was used as the substrate for growth of mixed oxide nanotube arrays. The sheets (4 cm × 3 cm) were cut and ground with silicon carbide abrasive paper of successively finer roughness (800, 1000, 1500, and 2000 grit). They were then cleaned in an ultrasonic bath with acetone, isopropanol, and ethanol successively, followed by rinsing with deionized water and drying in air. Electrochemical anodization was performed in a simple twoelectrode cell at room temperature by using a direct current (dc) stabilized voltage and current power supply (WYJ60V3A, Pingguo instrumentation Co. Ltd., China), in which Ti-2Al1.5Mn alloy sheet served as anodic electrode and a graphite plate as the cathode. The backside of Ti-2Al-1.5Mn sheet was protected by coating a layer of waterproof rubberized fabric. The electrolyte was 0.5 wt % NH4F aqueous solution, anodization voltages ranged from 10 to 40V, and anodization times were varied from 10 min to 6 h. After anodization, the sample surface was washed by immersion in deionized water and ethanol followed by drying in air. The amorphous as-anodized samples were subsequently crystallized by annealing at different temperatures, vary from 300 to 600 °C, for 3 h in air with a heating rate of 5 °C /min. The morphologies and elemental analysis of the samples were studied by using a field emission scanning electron microscope (FESEM; JEOL JEM-6700F) fitted with an energy dispersive X-ray spectrometer (EDX). The cross-sectional thickness measurements were carried out on mechanically bent cracked

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Figure 1. Voltage dependent current-time behavior during anodization of Ti-2Al-1.5Mn alloys in an electrolyte containing 0.5 wt % NH4F at 10 V (a), 15 V (b), 20 V (c), 25 V (d), and 30 V (e).

samples. The crystal structure of the samples was identified by X-ray diffractometer (XRD, Rigaku, D/max-rA) using Cu KR radiation (λ ) 1.5418 Å). X-ray photoelectron spectroscopic (XPS, Surface Science Instruments, Al KR: 1486.6 eV) studies were performed on the selected as-synthesized films. Optical characterization of the films was performed using a UV-3150 double-beam spectrophotometer. The photoelectrochemical properties were investigated using a conventional three-electrode system made of quartz cell linked with the electrochemical workstation (CH Instruments, model CHI 601C). As-prepared oxide nanotube arrays photoanode, platinum (Pt) mesh, and saturated Ag/AgCl electrode were used as working electrode, counter electrode, and reference electrodes, respectively. The electrolyte was 1.0 M NaOH solution. The CHI electrochemical workstation was used to measure dark and illuminated current at a scan rate of 10 mV/s. Sunlight was simulated with a 500 W xenon lamp (Spectra Physics). The light intensity was calibrated by using a laser power meter (BG26M92C, Midwest Group), equivalent to AM 1.5 light at 100 mW/cm2. Photoelectrochemical water splitting experiments were conducted at a potential of -0.4 V (vs Ag/AgCl) in a 3 M KOH electrolyte. The illuminated area of the working electrode is about 1.5 cm2, and the measured light irradiance is 100 mW/cm2. The gas collection was based on the principle of simple and well-known draining means. 3. Results and Discussion Figure 1 shows the current-time behavior during the first 30 min of the constant voltage anodization of Ti-2Al-1.5Mn samples in 0.5 wt % NH4F aqueous electrolyte at different applied voltages. Before recording the current transient, the voltage was slowly increased to the target voltage. A systematic variation in anodization behavior is seen with increasing applied voltage. As the applied voltage changes, the measured current density is oscillated with the similar trace. At higher applied voltage (Figure 1c-e), the strong current decay in the first 50 s is due to the passivation of the active electrode by formation of an initial electrically insulating oxide layer, followed by an increase in the current due to oxide pitting by the fluoride ions. In Figure 1c,d, the current then gradually changes to plateau at a steady-state value indicating the balance between oxide formation and oxidative dissolution. However, the steady-state of current cannot be obtained when the voltage is higher than

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Figure 2. FESEM images of the obtained oxide films (top-view and cross-section) prepared by anodizing Ti-2Al-1.5Mn alloy in 0.5 wt % NH4F solution at 10 V (a), 15 V (b), 20 V (c), 25 V (d), 30 V (e), and 40 V (f) for 3 h.

30 V (Figure 1e). While at lower voltage (Figure 1a,b), the current drop is not so sharp, and then it gradually declines to a steady value. The significantly higher current densities could be ascribed to additional chemical oxide dissolution due to the preferential corrosion attack of the β-phrase in the aqueous fluoride solution.19 In line with the numerous previous reported results, the optimizing applied voltage should be 20 V, which will be confirmed by the further investigation. Figure 2 shows the top-view FESEM images of the anodic oxide layer grown on Ti-2Al-1.5Mn by a simple anodization in an aqueous electrolyte containing 0.5 wt % NH4F. The insets show the corresponding cross sections of the obtained oxide films. Anodization is carried out under applied voltages ranging from 10 to 40 V for 3 h. It is obvious that the applied voltage influences the nanotube formation process and morphology of the as-anodized samples greatly. As shown in Figure 2a, anodization at a lower voltage results a compact oxide layer, on which no exact tubular structure is formed and the nanopores are in high disorder with average pore diameter smaller than 15 nm. It can be seen from Figure 2b-e that, at a higher voltage, nanotubular structures can be identified, the resulting average

tube diameters are, respectively, 55, 90, 100, and 120 nm for 15, 20, 25, and 30 V voltages. The small disordered small particles on top of the nanotubes can be attributed to the insufficient growth of the oxide film (Figure 2b), and the partial irregular nanotubes should be related to general chemical etching of the tubular walls (Figure 2d,e). When the voltage is too high, the nanotubular structure will be almost destroyed, as shown in Figure 2f. It is also observed that the layer thickness of the resulting films changes with applied voltage, which is depicted in the insets of Figure 2. The layer thickness increases from 120 to 1000 nm when the anodization voltage changes from 10 to 30 V (cross-sectional images of Figure 2a-e insets) in 3 h of anodization. It is found that most of the nanotubes are straight and highly compact (Figure 2b--e insets); however, some of the tubes are observed to be slightly bent (Figure 2e inset) and are formed due to the strained environment. As to the condition of 40 V, although serious etching has taken place, the hollow and individual characters of the nanotubular structure can still be identified (Figure 2f inset). The most interesting observation is in the inset of Figure 2c, where individual straight nanotubes,

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Figure 3. Top view FESEM images of as-prepared samples by constant-voltage anodization of Ti-2Al-1.5Mn alloys at 20V in 0.5 wt % NH4F aqueous solution for 5 min (a), 10 min (b), 30 min (c), 1 h (d), 3 h (e), and 6 h (f).

the closed bottoms (black arrow), and exposed alloy substrate part (white arrow) are clearly presented, which match well with the previous studies on self-organized nanotube arrays by anodization of pure Ti. By combining the findings of the experiments of Figures 1 and 2, it can be deduced that the optimized applied voltage for self-organized nanotube arrays formation is established for 20 V in the 0.5 wt % NH4F aqueous electrolyte. Therefore, all the following further detailed investigations were performed at 20 V. At this applied voltage, the influence of different anodization times on the anodized surfaces was investigated in a range from 5 min to 6 h, as will be shown in Figure 3. As apparent in Figure 3a after 5 min anodization, an initial oxide top layer has formed with small particles that underneath just show an irregular porous structure. After this initial stage, the transition to the ordered nanotubes has been clearly observed when the time increases to 10 min (Figure 3b), and the average diameter of the inadequate growth nanotubes is about 70 nm. With further increasing anodization time, the nanotube growth becomes increasingly ordered, and a self-organized nanotube structure is observed as shown in Figure 3c,d. After 3 h

anodization (Figure 3e), highly ordered layer is apparent with a less rough surface. However, when the time reaches 6 h, deformation of the resulting film has been observed and the nanotubes are not well self-organized mainly due to the immoderately chemical dissolution (Figure 3f). According to the experimental results and discussion above, the main growth mechanism governing the formation of nanotubes on Ti-2Al-1.5Mn alloy in NH4F electrolyte is suggested to be consistent with those of tubes synthesized on pure Ti in F- containing solutions on the whole. Nanotube formation in fluoride ion bearing electrolytes occurs as a result of the interplay between three simultaneously occurring processes, namely, the field assisted oxidation of alloy to form oxides, the field assisted dissolution of metal ions in the electrolyte, and the chemical dissolution of metal and oxides due to etching by fluoride ions, which is substantially enhanced by the presence of H+ ions. The initial H+ ions in the electrolyte are mainly from the NH4+ hydrolysis. And the formation of self-organized nanotube arrays are ultimately determined by the dynamic equilibrium between the growth and the dissolution processes.17 As the alloy contained two different phases, it is

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Figure 4. (A) XRD patterns of as-formed and annealed (range from 300 to 600 °C, in air, 3 h) nanotube samples prepared at 20 V for 3 h in 0.5 wt % NH4F aqueous electrolyte. (B) EDX spectra of sample in part A annealed at 600 °C.

important to restrain the selective dissolution of the phases, which can result in a nonuniform surface layer. It is profitable to select subacid electrolyte and to optimize anodization voltage and time. Herein, the increased anodic current densities indicate the increased oxidative corrosion rate. Although the both contents of Al and Mn are quite small in the Ti-2Al-1.5Mn alloy, especially for Mn, it is believed that the related side anodic reactions during anodization could affect the resulting nanotubular oxide formation, which will be presented in another work. It may also be expected that both the oxide of Al and the oxide of Mn have to grow based on the TiO2 for their very small contents during the anodization. Al and Mn elements may distribute over the TiO2 surface or growth interface as very thin films in the as-anodized film. The as-formed nanotubular films are amorphous, but they can be crystallized upon thermal annealing. The reason for annealing is to increase the conductivity of the layers by minimizing the structural defects (by converting amorphous nature) that act as electron traps.32 Figure 4A shows XRD patterns of as-formed and annealed samples prepared by anodization at 20 V for 3 h in an aqueous electrolyte containing 0.5 wt % NH4F (Figure 2c). The annealing experiments were carried out in air ranging from 300 to 600 °C for 3 h with a ramp up rate of 5 °C min-1. After annealing at 400 °C, clearly a predominantly anatase TiO2 structure can be identified. When the annealing temperature is higher than 550 °C, the sample will be found with a small amount of rutile. With further increasing temperature, the rutile TiO2 phase gradually acquires a little predominance at 600 °C. It is quite clear that higher annealing temperature will be of

Liu et al. great benefit to the crystallization process. The crystalline nature of the nanotube walls is critical to applications involving light absorption, electrical carrier generation, and carrier transport.26 In all the patterns, no Al or Mn and their oxides can be identified, which may mainly be because that the Al or Mn contents in the anodized samples were too low to be detectable. The fact is no Al nor Mn can be identified even in the XRD of Ti-2Al-1.5Mn alloy itself (see Figure S1 in the Supporting Information). And no oxide or alloy can be identified from XRD except TiO2, which is also consistent with Raman spectroscopy analysis of the films annealing at different temperatures (see Figure S2 in the Supporting Information). Energy dispersive X-ray analyzer (EDX) fitted to the FESEM chamber was used for determining the composition. Figure 4B shows example of an EDX spectrum acquired on the nanotube layer annealed at 600 °C. Clearly, Ti, O, Mn, and Al peaks are detected,and their valence states will be studied by XPS survey. XPS survey spectra reveal the annealed nanotubular oxide films at 600 °C to contain Ti, O, Mn, and Al. Figure 5a is the Ti 2p region spectra showing the presence of the main doublet composed of two symmetrical peaks at Eb(Ti 2p3/2) ) 458.25 eV and Eb(Ti 2p1/2) ) 464.07 eV, assigned to Ti4+ in the spectrum of the annealed oxide film indicating presence of a TiO2 stoichiometry. Figure 5b shows the O 1s spectrum. The peak at 529.47 eV could be mainly attributed to the Ti-O structures for which the generally reported peak binding energy is about 529.8 eV. The spectrum of the Mn 2p region is shown in Figure 5c. The peak at Eb (Mn 2p3/2) ) 642.21 eV indicates the existence of Mn4+. The XPS investigation also confirmed that aluminum exists as Al3+ state. No component related to zerovalent Ti, Mn, and Al can be extracted. After more specific XPS tests, Mn4+ can be also found at lower temperatures than 600 °C. And no Mn3+ can be identified when the annealing temperature is higher than 500 °C, which is different from the results reported by Mohapatra and co-workers.19 Figure 6 shows the UV-vis spectra of the as-formed and annealed samples; as expected, the absorption edge shows a significant red-shift with the annealing temperature increased. The absorption edges are 361, 379.01, 433.04, and 472.67 nm, respectively, corresponding to the annealing temperatures of 300, 400, 500, and 600 °C as shown in Figure 6b-e. This may relate to the increasing crystallization with annealing temperature as shown in Figure 4a. However, the band gap of TiO2, ∼3.2 eV for anatase and 3.0 eV for rutile, fixes on its absorption band edge of not more than 413 nm, which is much smaller than 433 nm (Figure 6d) and much less than 472.67 nm (Figure 6e). According to the band gap structure of TiO2, the π bonding orbit and π/ antibonding orbit formed by the t2g orbit in Ti4+ ions and the pπ orbit in the O2- of TiO2 crystal lattice are, respectively, the valence band and conduction band.26 Since the ionic radius (0.054 nm) of Mn4+ is smaller than that of Ti4+ (0.0605 nm), the Mn4+ cation could occupy the Ti4+ sites without creating additional charge defects, which may introduce structural disorder, especially in the position of oxygen.19 The overlap of the conduction band due to Ti(d) of Ti oxide and the Mn d orbit may be essential to decreasing the energy gap between Ti(d) and O(p) orbitals of Ti oxide to enable the visible light to be adsorbed. It is known that Al3+ cannot enter the crystal lattice of TiO2 to form an alloy state when the annealing temperature is lower than 800 °C. So the aluminum oxide is sure to be amorphous and can do nothing good to the photoabsorption. These results shown in Figure 6 clearly demonstrate that such annealed nanotubular oxide films may work effectively not only under UV light irradiation but also under visible light

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Figure 5. XPS results of the annealed oxide nanotubular array on Ti-2Al-1.5Mn at 600 °C: (a) Mn 2p photo emission spectrum; (b) O 1s spectrum; (c) Ti 2p spectrum; and (d) Al 2p spectrum.

Figure 6. UV-vis absorbance spectra of as-formed (a) and annealed nanotube films at different temperatures: (b) 300 °C; (c) 400 °C; (d) 500 °C; and (e) 600 °C.

Figure 7. Photocurrent density vs potential in 1 M NaOH solution for as-formed and annealed nanotube array samples under AM 1.5 100 mW/cm2 illumination. Dark currents are also shown for each sample.

irradiation. And the interesting photoelectrochemical properties can also be expected. The direct growth of oxide nanotube arrays on Ti-2Al-1.5Mn substrates allows us to further study their photoelectrochemical activity. Figure 7 shows the photocurrent density vs applied potential curves for the as-formed and annealed oxide nanotubular film electrodes under dark and simulated sunlight irradiation (AM1.5 100 mW/cm2). The electrolyte used in the photoelectrochemical studies was 1.0 M NaOH. The dark current in each case is negligible up to 0.65 V (vs Ag/AgCl) beyond which the dark currents for water oxidation dominate; therefore, no photocurrent saturation is observed. The photocurrent increases with the increase of the applied electrode potential as the applied positive electrode potential reduces the recombination of the photogenerated electrons and holes. This is consistent

with the previous literature results.33 It is confirmed that the crystalline nature of the nanotubular film is critical to the photocurrent. The amorphous as-anodized nanotube film only shows a maximum photocurrent of 0.11 mA/cm2 at 0.65 V (vs Ag/AgCl) which is pretty much the same thing with those dark currents. After annealing at 300 °C, the maximum photocurrent increased remarkably to 1.25 mA/cm2. With further annealing at 400, 500, and 600 °C, respectively, the corresponding maximum photocurrents of 2.88, 3.0, and 3.11 mA/cm2, under 100 mW/cm2 at 0.65 V (vs Ag/AgCl), which are higher than that recently reported for iron oxide related materials by Grimes26 of 2.0 mA/cm2 and by Gratzel34 of 2.2 mA/cm2, although there are some small differences on the irradiation lights. It is believed that the presence of amorphous aluminum

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oxide will dramatically degrade photoelectrochemical properties. In the current work, we believe that the possible very thin Al2O3 films on the TiO2 surface or growth interface could act as a buffer layer to reduce back recombination.35 Two electrode measurements were performed to determine the photoconversion and light energy to chemical energy conversion efficiency. Photoconversion efficiency η is calculated as

η(%) ) jp[(1.23-V)/I0] × 100 2

(1)

where jp is the photocurrent density (mA/cm ), I0 is the intensity of incident light (mW/cm2), and V is the potential applied between the anode (oxide nanotubular film sample) and cathode (platinum mesh). The efficiency of the 600 °C annealed sample was determined to be 1.61%. The experiments of hydrogen generation were also done at a potential of -0.4 V vs Ag/AgCl in 3 M KOH solution. Under 100 mW/cm2 illumination, the sample of as-prepared oxide nanotubular arrays annealed at 600 °C demonstrated a considerable and sustained water splitting behavior. No obvious degradation in sample performance was observed under illumination over a course of several days. By collecting the gases at the photoanode consisting of a 600 °C annealed oxide nanotube array sample and Pt mesh counter electrode during the photoreaction, we observed a ∼2:1 volume ratio of hydrogen and oxygen, which further confirmed water splitting. It is a pity that we cannot give the exact hydrogen evolution rate currently. Overall, it was observed that a simple anodization of Ti-2Al1.5Mn alloy in an aqueous electrolyte containing 0.5 wt % NH4F resulted in the formation of a well ordered nanotubular oxide film. During the anodization process, much cheaper and familiar graphite was used as cathode instead of platinum (Pt), safe and facile 0.5 wt % NH4F aqueous electrolyte was adopted in the two electrode system, and larger effective area (∼10 cm2) of the alloy was exposed to the electrolyte. All of these factors make the present method of preparation a simple, facile, available, and reliable route. The growth process can be explained by the “oxide growth and dissolution”, mechanism. The higher anodization current density indicated increased oxidative corrosion rate, which could be mainly attributed to the preferential corrosion of the β-phase (with higher Mn content). Despite that the different two phases in the alloy almost did not affect the formation or ordering of the oxide nanotubes, compared with pure Ti, the presence of Al and Mn should cause additional reactions which will be investigated in another work. It may be noteworthy that in general the mixed oxide layers on Ti-2Al-1.5Mn have a higher mechanical stability than comparable pure TiO2 nanotubes. Although the crystalline nature of the nanotubes is critical for applications involving light absorption, electrical carrier generation, and carrier transport, confirmed by the XRD, UV-vis and photoelectrochemical studies, the highest annealing temperature was 600 °C to avoid the spoiling of the nanotubular structure, resulting in as-prepared films of high series resistance demonstrating essentially nil photoelectrochemical properties.26 The real situation of higher annealing temperatures is still unknown for the as-formed nanotube array films. The films prepared by anodization may also achieve the nanotubular structures in other kinds of electrolytes, such as the ethylene glycol solution containing water and F-. These possible nanotubular structures are expected to show new characteristics in photoabsorption and photoelectrochemical properties. The present convenient synthesis technique can also be extended to other binary or ternary oxide compositions for various applications. Our future efforts will mainly focus on these aspects.

4. Conclusions In summary, we demonstrated the synthesis of self-organized oxide nanotube films on Ti-2Al-1.5Mn alloy by a facile and effective two-electrode anodization process in an aqueous electrolyte containing 0.5 wt % NH4F. The structure and morphology of the resulting films can be influenced over wide ranges by the applied voltage and the anodization time. Under the optimized condition, at 20V for 3 h, the as-formed nanotubes are highly ordered with ∼500 nm in length, and the average tube diameter is about 90 nm. The growth process can be explained by the “oxide growth and dissolution” mechanism. Importantly, the different two phases in the alloy almost did not affect the formation of the oxide nanotubes. It may be noteworthy that in general the nanotubular oxide layers on Ti2Al-1.5Mn have a higher mechanical stability than comparable pure TiO2 nanotubes. It is proved that the crystalline nature of the nanotubes is critical to their properties including light absorption, electrical carrier generation, carrier transport, and so on. Significantly, with increasing annealing temperature, the absorption edge shows a continuously remarkable red-shift, which may relate to the increasing crystallization and the new energy bands formed. More importantly, we obtain the highest photocurrent of 3.11 mA/cm2 under AM1.5 100 mW/cm2 at 0.65 V (vs Ag/AgCl). Furthermore, a considerable and sustained water splitting behavior has been identified, and the present convenient and effective synthesis technique can also be extended to other binary or ternary oxide compositions for various applications. Acknowledgment. We wish to thank Z. X. Guo for FESEM investigation of our samples, G. Peng for XRD characterization, L. N. Zhang, T. T. Wang, and J. Cao for UV-vis tests, and Hari-Bala for helpful discussions. The authors also thank and appreciate the reviewers for their helpful comments and suggestions. Supporting Information Available: XRD pattern of Ti2Al-1.5Mn alloy and Raman spectra of the as-anodized samples at different temperatures. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Varghese, O. K.; Gong, D. W.; Paulose, M.; Ong, K. G.; Dickey, E. C.; Grimes, C. A. AdV. Mater. 2003, 15, 624–627. (2) Ruan, C.; Paulose, M.; Varghese, O. K.; Mor, G. K.; Grimes, C. A. J. Phys. Chem. B 2005, 109, 15754–15759. (3) Shankar, K.; Mor, G. K.; Prakasam, H. E.; Yoriya, S.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nanotechnology 2007, 18, 065707. (4) Albu, S. P.; Ghicov, A.; Macak, J. M.; Schmuki, P. Nano Lett. 2007, 7, 1286–1289. (5) Paulose, M.; Shankar, K.; Yoriya, S.; Prakasam, H. E.; Varghese, O. K.; Mor, G. K.; Latempa, T. A.; Fitzgerald, A.; Grimes, C. A. J. Phys. Chem. B 2006, 110, 16179–16184. (6) Mor, G. K.; Varghese, O. K.; Paulose, M.; Shankar, K.; Grimes, C. A. Sol. Energy Mater. Sol. Cells 2006, 90, 2011–2075. (7) Mohapatra, S. K.; Misra, M.; Mahajan, V. K.; Raja, K. S. J. Phys. Chem. C 2007, 111, 8677–8685. (8) Zhu, K.; Neale, N. R.; Miedaner, A.; Frank, A. J. Nano Lett. 2007, 7, 69–74. (9) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2006, 6, 215–218. (10) Grimes, C. A. J. Mater. Chem. 2007, 17, 1451–1457. (11) Gong, D.; Grimes, C. A.; Varghese, O. K.; Hu, W.; Singh, R. S.; Chen, Z.; Dickey, E. C. J. Mater. Res. 2001, 16, 3331–3334. (12) Mor, G. K.; Varghese, O. K.; Paulose, M.; Mukherjee, N.; Grimes, C. A. J. Mater. Res. 2003, 18, 2588–2593. (13) Varghese, O. K.; Gong, D. W.; Paulose, M.; Grimes, C. A.; Dickey, E. C. J. Mater. Res. 2003, 18, 156–165. (14) Cai, Q. Y.; Paulose, M.; Varghese, O. K.; Grimes, C. A. J. Mater. Res. 2005, 20, 230–236.

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