Double-Wall Anodic Titania Nanotube Arrays for Water Photooxidation

May 19, 2009 - Chemical and Materials Engineering/MS 388, University of Nevada, Reno, Nevada ... ration of H2 from water using solar light is one of t...
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Double-Wall Anodic Titania Nanotube Arrays for Water Photooxidation Shiny E. John, Susanta K. Mohapatra, and Mano Misra* Chemical and Materials Engineering/MS 388, University of Nevada, Reno, Nevada 89557 Received February 3, 2009. Revised Manuscript Received April 29, 2009 Vertically oriented double-wall titania nanotube (external diameters of 82 and 206 nm) arrays are synthesized by a sonoelectrochemical anodization technique in combination with a unique room-temperature ionic liquid and organic electrolyte. Compared to similar single-wall nanotubes (0.638 mA/cm2) and commercial nanoparticles (0.365 mA/cm2), these double-wall nanotube arrays show 2-4 times more photoactivity to split water under solar light illumination to generate hydrogen and oxygen. Partial doping of B and C into the TiO2 matrix gives rise to these double-wall nanotubes which absorb visible solar light more efficiently than the intrinsic TiO2. The structural properties of these novel structures have been studied extensively using various spectroscopic, analytical, and electrochemical techniques.

*To whom correspondence should be addressed. E-mail: [email protected].

of applications such as photocatalysis,9 Li ion battery,10 membrane11 solar cells,12 and H2 generation.13 Over the past few years, TiO2 nanotubes (NTs) with various morphologies and physical properties have been developed for the applications described above.14,15 The morphology and orientation of these self-standing NTs play a great role in their applications. In this paper, we report the synthesis of double-wall TiO2 NT arrays via the sonoelectrochemical anodization process using room-temperature organic ionic liquid [1-butyl-3-methylimidazolium tetraflouroborate (BMIM+-BF4-)]. This provides a simple way to grow vertically oriented double-wall TiO2 NTs on titanium (Ti) foil. We have investigated the structural properties of these new structures, as well as their use for photooxidation of water to generate hydrogen (H2) under solar light illumination. Generation of H2 from water using solar light is one of the great challenges in solving the current energy crisis.16 We found that the double-wall TiO2 NTs split water quite efficiently compared to the conventional single-wall NTs and commercial TiO2 nanoparticles (NPs). This process not only produces new metal oxide architecture and double-wall NTs but also helps the material to absorb visible light more efficiently compared to the intrinsic TiO2. The electrochemical anodization process is a facile route to growing a variety of metal oxide NT arrays on a metal surface using a fluoride-containing acid electrolyte. This technique is developed from the pitting corrosion of metals (e.g., Ti) in the presence of halides such as fluoride. This process generally

(1) (a) Iijima, S. Nature 1991, 354, 56. (b) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (c) Ajayan, P. M. Chem. Rev. 1999, 99, 1787. (d) Sharma, S.; Sunkara, M. K. J. Am. Chem. Soc. 2002, 124, 12288. (e) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. Adv. Mater. 2003, 15, 353. (2) (a) Cui, Y.; Lieber, C. M. Science 2001, 291, 851. (b) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Webber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (c) Martinson, A. B. F.; Elam, J. W.; Hupp, J. T.; Pellin, M. J. Nano Lett. 2007, 7, 2183. (3) Kim, S. W.; Kim, M.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642. (4) Tsai, C.-C.; Teng, H. Chem. Mater. 2004, 16, 4352. (5) (a) Sander, M. S.; Cote, M. J.; Gu, W.; Kile, B. M.; Tripp, C. P. Adv. Mater. 2004, 16, 2052. (b) Cho, Y. H.; Cho, G.; Lee, J. S. Adv. Mater. 2004, 16, 1814. (6) Li, D.; Xia, Y. N. Nano Lett. 2004, 4, 933. (7) Xiong, Y.; Wiley, B.; Chen, J.; Li, Z.-Y.; Yin, Y.; Xia, Y. Angew. Chem. 2005, 117, 8127. (8) Zwilling, V.; Aucouturier, M.; Darque-Ceretti, E. Electrochim. Acta 1999, 44, 921. (9) (a) Mohapatra, S. K.; Kondamudi, N.; Banerjee, S.; Misra, M. Langmuir 2008, 24, 11276. (b) Liu, Z.; Zhang, X.; Nishimoto, S.; Jin, M.; Tryk, D. A.; Murakami, T.; Fujishima, A. J. Phys. Chem. C 2008, 112, 253. (10) Lee, W.-J.; Alhoshan, M.; Smyrl, W. H. J. Electrochem. Soc. 2006, 153, B499.

(11) Wang, J.; Lin, Z. Chem. Mater. 2008, 20, 1257. (12) (a) Shankar, K.; Bandara, J.; Paulose, M.; Wietasch, H.; Varghese, O. K.; Mor, G. K.; LaTempa, T. J.; Thelakkat, M.; Grimes, C. A. Nano Lett. 2008, 8, 1654. (b) Kuang, D.; Brillet, J.; Chen, P.; Takata, M.; Uchida, S.; Miura, H.; Sumioka, K.; Zakeeruddin, S.; Gra¨tzel, M. ACS Nano 2008, 2, 1113. (c) Jennings, J. R.; Ghicov, A.; Peter, L. M.; Schmuki, P.; Walker, A. B. J. Am. Chem. Soc. 2008, 130, 13364. (d) Park, J. H.; Lee, T.-W.; Kang, M. G. Chem. Commun. 2008, 2867. (13) (a) Mohapatra, S. K.; Misra, M.; Mahajan, V. K.; Raja, K. S. J. Catal. 2007, 246, 362. (b) Chanmee, W.; Watcharenwong, A.; Chenthamarakshan, C.; Kajitvichyanukul, P.; de Tacconi, N. R.; Rajeswar, K. J. Am. Chem. Soc. 2008, 130, 965. (c) Mor, G. K.; Varghese, O. K.; Wilke, R. H. T.; Sharma, S.; Shankar, K.; Latempa, T. J.; Choi, K.-S.; Grimes, C. A. Nano Lett. 2008, 8, 1906. (14) Macak, J. M.; Tsuchiya, H.; Taveira, L.; Aldabergerova, S.; Schmuki, P. Angew. Chem., Int. Ed. 2005, 44, 7463. (15) Mohapatra, S. K.; Misra, M.; Mahajan, V. K.; Raja, K. S. J. Phys. Chem. C 2007, 111, 8677. (16) (a) Turner, J. A. Science 1999, 285, 687. (b) Nowotny, J.; Sorrell, C. C.; Sheppard, L. R.; Bak, T. Int. J. Hydrogen Energy 2005, 30, 521. (c) Kamat, P. V. J. Phys. Chem. C 2007, 111, 2834. (d) Blanchette, S.Jr. Energy Policy 2008, 36, 522. (e) Osterloh, F. E. Chem. Mater. 2008, 20, 35. (f ) Sahayam, U.; Norton, M. G. J. Mater. Sci. 2008, 43, 5395.

1. Introduction Over the past decade, a considerable number of studies have focused on the synthesis of one-dimensional (1D) porous materials. The quantum confinement and low dimensionality of these materials give them unique optical and electronic properties compared to bulk materials.1 These types of materials have great prospects as the building blocks of next-generation photoelectrochemical and photovoltaic devices.2 Porous structures also have great potential in catalysis.3 Porous structures have been synthesized by various methods such as hydrothermal treatment,4 template-assisted deposition,5 and electrospinning.6 The corrosion-based process is one of the simple and advanced processes for synthesizing ordered porous materials from metals. Xia and coworkers developed a galvanic replacement process to make silver (Ag) and palladium (Pd) nanoporous materials.7 These nanoporous metal and alloy structures possess catalytic activities higher than those of nonporous analogues. Zwilling and co-workers8 have developed an electrochemical anodization process to make metal oxide arrays on a metal surface. This process has attracted many researchers because of its simplicity and possible application of these materials in various research fields. This process for synthesizing 1D metal oxide-based porous structures, in particular titania (TiO2), is very attractive for a number

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produces highly disordered porous NTs (NTs form by the combination of rings). This technique is further developed to obtain highly ordered and smooth NTs with a high aspect ratio (second-generation NTs).14,15,17 Here we report third-generation NTs with a unique double-wall structure. A combination of organic electrolyte (ethylene glycol) and organic roomtemperature ionic liquid forms these types of novel nanotubular structures on Ti foil (Figure 1). Synthesis, characterization, and application of the novel NTs for a solar light-driven water splitting reaction are reported here. Because of their particular structure and morphology, these double-wall TiO2 NTs may have unique applications in the pharmaceutical industry, catalysis, drug delivery, and dye-sensitized solar cells.12,18

2. Materials and Methods Preparation of TiO2 Photoanodes. All chemicals were used in the as-received condition without any further purification. Double-wall TiO2 NT arrays were prepared via anodic oxidation of Ti foil (ESPI, 0.2 mm thick, 99.9% pure) using an aqueous (10 vol % distilled water) solution of ethylene glycol (Acros, 99%) and 0.6 vol % BMIM-BF4 (Fluka, 97%). The pH of the solution was around 6.4. The anodization process was similar to the conventional two-electrode [Ti as an anode and platinum (Pt) as a cathode] system and was conducted within the potential difference range of 20-100 VDC using a rectifier (Agilent, E3640A). Anodization of a flag-shaped Ti foil of 1 cm2 (1 cm  1 cm) was conducted using a 300 mL electrolytic solution in an ultrasonic bath (100 W, 42 kHz, Branson 2510R-MT) for a maximum of 1 h. The size of the Pt foil used was 3.75 cm2. The distance between the two electrodes was kept at 4.5 cm in all experiments. The anodization current was monitored continuously using a digital multimeter (METEX, MXD 4660A). The anodized samples were rinsed with distilled water to remove the occluded ions and then air-dried. The samples were annealed under a N2 atmosphere at 500 °C for 6 h in the CVD furnace (FirstNano) at a heating and cooling rate of 1 °C/min. Ultra-high-purity gases (UHP) with a flow rate of 200 sccm were introduced into the furnace to achieve the desired atmosphere. The furnace was purged with UHP argon (10000 sccm) for 30 s before the heat treatment with reducing gases (hydrogen and nitrogen) was begun. This resulted in the conversion of the amorphous TiO2 NTs to the crystalline phase. The annealed samples were then subjected to characterization and photoelectrochemical measurements. For the sake of comparison, NTs were also prepared using an ammonium fluoride (NH4F, Fischer, 100%) solution under similar experimental conditions. These results were also compared with those of commercial P25 TiO2 nanoparticles (Degussa). A dip coating method was adopted to coat the P25 NPs on Ti foil. For this purpose, the P25 NPs (10 mg) were dispersed in a solution containing ethylene glycol (10 mL), ethanol (5 mL, Aldrich), and polyvinylpyrrolidone (5 mg, Aldrich). The Ti foil (after P25 coating) was dried in an air oven (120 °C) overnight, followed by annealing in an oxygen atmosphere for 3 h at 500 °C. This process removed the organics from the thin layer of P25 and made a uniform film on the Ti foil. The TiO2 NTs prepared under fluoride ions and BF4- ions were noted as F-TiO2 and BF4-TiO2 NTs, respectively. Characterization. All the nanomaterials were characterized by various analytical and spectroscopic techniques as discussed below. A scanning electron microscope (SEM; Hitachi, S-4700) was used to analyze the morphology of the NTs. The images were taken with an operating voltage of 20 kV. Energy-dispersive X-ray (EDX) analysis was conducted using (17) Prakasam, H. E.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. J. Phys. Chem. C 2007, 111, 7235. (18) (a) Gubbala, S.; Chakrapani, V.; Kumar, V.; Sunkara, M. K. Adv. Funct. Mater. 2008, 18, 2411. (b) Watanabe, H.; Kunitake, T. Chem. Mater. 2008, 20, 4998.

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Figure 1. Schematic showing the growth of double-wall TiO2 NTs on Ti foil via the sonoelectrochemical anodization process.

an Oxford detector. Diffuse reflectance ultraviolet and visible (DRUV-vis) spectra of the samples were measured (to understand the solar light harvesting properties of the materials) from the optical absorption spectra using an UV-vis spectrophotometer (UV-2401 PC, Shimadzu). Fine barium sulfate (BaSO4) powder was used as a standard for the baseline measurements, and spectra were recorded in the range of 200-800 nm. A scanning transmission electron microscope (STEM; JEOL 2100F) equipped with ESVision software was used for mapping and crystal distribution of the samples. Transmission electron microscope (TEM) measurements were carried out by scratching a portion of the TiO2 NTs from the Ti disk in ethanol, followed by ultrasonication for a few minutes for proper distribution. A drop of ethanol (containing sample) was placed on a carbon-coated copper (Cu) grid and subjected to high-resolution TEM (HRTEM), scanning TEM (STEM), and fast Fourier transformation (FFT) measurements. Glancing angle X-ray diffraction (GAXRD) was conducted using a Philips-12045 B/3 diffractometer. The target used in the diffractometer was Cu (λ = 1.54 A˚), and the scan rate was 1.2o/min. Raman spectra were obtained using a Bruker Senterra confocal microscope. An integration time of 300 s was used to run the samples. The system was checked with a reference silicon sample with the peak position at 520 cm-1. Photoluminescence (PL) studies were conducted by irradiating the samples with a light having a wavelength of 325 nm. The surface composition of the TiO2 NTs was analyzed with an X-ray photoelectron spectroscope (XPS; Thermo-VG Scientific ESCALab 250) with a monochromatic Al Ka X-ray source (1486.6 eV). The Mott-Schottky analysis was carried out using a standard electrochemical impedance spectroscope at 3000 Hz in a 1 M potassium hydroxide (KOH, Sigma Aldrich, 85%) solution by scanning the potential from the positive to the negative direction in steps of 50 mV/s under both dark (without light illumination) and illuminated conditions (by the simulated solar light using an AM 1.5 filter). The flat band potential (UB) and charge carrier density (ND) of the TiO2 nanotubular anode were calculated from the Mott-Schottky plots with and without the addition of organic additives. Photoelectrochemical Activity. Experiments that aim to evaluate the H2 generation via photoelectrolysis of water were conducted in a glass cell with photoanode (TiO2 sample) and cathode (Pt foil) compartments, which were connected using a fine porous glass frit. Silver/silver chloride (Ag/AgCl) microelectrodes were used as the reference electrode. The cell was provided with a 60 mm diameter quartz window for light incidence. The electrolyte used was 1 M KOH. A computer-controlled potential was used to control the potential and to record the generated photocurrent. A 300 W solar simulator in combination with global AM 1.5 was used to irradiate the samples. The samples were anodically polarized at a scan rate of 5 mV/s under illumination, and the photocurrent was recorded. A long pass filter (Edmund optics) was used to investigate the visible photoactivity of the TiO2 samples by filtering the UV light. The intensity of the light was measured by a thermopile sensor (model 70268, Newport) using a radiant power and energy meter (model 70260, Newport Corp., Stratford, CT). DOI: 10.1021/la900426j

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3. Results and Discussion Structure Analysis. In the sonoelectrochemical anodization process, the applied potential and electrolytic solution control the morphology (diameter, wall thickness, and length) of the NTs. Here a complex fluoride source (BF4-) was used which gave a smooth NT structure which was evidenced from the current transient (Figure 2). Because of the low corrosion potential of Ti in BF4-, no NTs were formed below 60 V. The growth kinetics of this process were observed to be slower (steady state current density of 0.15 mA/cm2) compared to those of the F-TiO2 NTs (steady state current density of 10.4 mA/cm2). The higher anodization current density observed during anodization in fluoride solution was due to the fact that Ti and TiO2 are more prone to react with fluoride than BF4-. The formation of NTs was monitored by SEM at different time intervals of the anodization process. Initially, nanorod types of structure formed, and slowly, the 1D structure split into double-wall NT arrays (Figure S1 of the Supporting Information). In this process, NT arrays with double walls were formed after anodization for approximately 20 min (Figure 3). Each NT consisted of concentric NTs with average outer diameters of 82 ( 2 and 206 ( 4 nm. This was the largest ever diameter observed for NTs prepared by the anodization process. These NTs were quite thick with average wall thicknesses of 27.5 and 49 ( 1 nm compared to the F-TiO2 NTs (∼20 nm). A cross-sectional SEM image showed that both the NTs had a single root (tube end). Unlike the fluoride media, these NTs did not grow more than 350 nm under these experimental conditions; rather, they formed a layer-by-layer structure on the Ti foil. Increasing the applied potential from 60 to 80 V also did not affect the length of the NTs; rather, it formed multiporous NTs (Figure S2 of the Supporting Information). A single NT was found to be made of a cluster of NTs due to multiple pitting on the nanowalls and in the barrier layer at higher applied potentials. This type of structure might be a potential candidate for drug delivery and catalysis.19 The open-pore structure can enhance the diffusion of reactants and effusion of the products compared to a closed-pore structure. At a potential difference of >80 V, the chemical dissolution dominated the deposition process, which did not help in the formation of any nanostructure on the metallic foil. Pure ionic liquid as the solvent produced a feature similar to the conventional single-wall NTs with ripples.20 This indicated that the electrolyte composition took a lead role in the formation of the double-wall NT arrays. To understand the impact of the electrolyte on the anodization behavior of Ti metal, potentiodynamic polarization of Ti was conducted in both BMIM-BF4 and NH4F electrolytes (Figure 4). The higher current density of the Ti in NH4F indicated that the corrosion rate of the Ti in NH4F was faster than that in the BMIM-BF4 medium, which could be correlated to the slower layer-by-layer formation and dissolution of BF4-TiO2 NT arrays. The formation of double-wall TiO2 NTs by the anodization process indicated that there were two types of pitting taking place on the Ti surface. It is believed that BF4- ions dissociated to release free fluoride ions in the presence of metallic ions such as Ti4+.21 The inner tube may have been formed by the BF4- ions, and the walls were simultaneously anodized again to produce (19) (a) Selvam, P.; Mohapatra, S. K. Microporous Mesoporous Mater. 2004, 73, 137. (b) Liong, M.; Lu, J.; Kovochich, M.; Xia, T.; Ruehm, S. G.; Nel, A. E.; Tamanoi, F.; Zink, J. I. ACS Nano 2008, 2, 889. (20) Paramasivam, I.; Macak, J. M.; Selvam, T.; Schmuki, P. Electrochim. Acta 2008, 54, 643. (21) (a) Jordan, R. F.; Dasher, W. E.; Echols, S. F. J. Am. Chem. Soc. 1986, 108, 1718. (b) Jordan, R. F.; LaPointe, R. E.; Bajgur, C. S.; Echols, S. F.; Willett, R. J. Am. Chem. Soc. 1987, 109, 4111. (c) Wark, T. A.; Stephan, D. W. Can. J. Chem. 1990, 68, 565.

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Figure 2. Current transient obtained during anodization of Ti at 60 V in an aqueous solution of ethylene glycol containing BMIM-BF4. The inset shows the current transient of anodization of Ti under similar conditions using NH4F.

Figure 3. (A-C) Double-wall TiO2 NTs prepared at 60 V for 20 min using ethylene glycol and a BMIM-BF4 solution and (D) single-wall TiO2 NT arrays prepared at 60 V for 20 min using ethylene glycol and NH4F. The inset of panel A shows the crosssectional image of the double-wall NT arrays. (B) Layer-by-layer double-wall TiO2 NTs. (C) STEM surface view of the double-wall NT arrays.

Figure 4. Potentiodynamic polarization behavior of Ti metal in NH4F and BMIM-BF4 solutions. This indicates that Ti metal is more prone to corrode in F- solution than BF4-.

a unique double-wall nanostructure. To understand this growth process, we took SEM images of the sample in various intervals during the anodization process (Figure S1 of the Supporting Information). The initial single-wall TiO2 nanostructure split into a double wall after anodization for 5 min. A complete growth of double-wall NT took place in 20 min. Further increasing the anodization time led to the removal of TiO2 film, and a new layer of TiO2 film started forming. Further investigation is necessary to Langmuir 2009, 25(14), 8240–8247

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understand the growth mechanism. To confirm the results obtained as mentioned above, it was necessary to check the effect of the cations of the electrolyte on NT formation. Two different complex salts were used in this study, namely, sodium tetraflouroborate (NaBF4) and nickel tetraflouroborate [Ni(BF4)2]. SEM images of the samples (not shown here) anodized in these electrolytes showed the double-walled NTs. This clearly indicates that the peculiar morphology of the TiO2 NTs formed was influenced by the anion part of the electrolyte BF4- and not by the cation part (Na, Ni, and BMIM). To understand the crystal structure of the NTs, we conducted GAXRD, TEM, and fast FFT measurements. The GAXRD pattern of as-anodized double-wall TiO2 NT arrays was found to be amorphous (figure not included here); however, the crystalline pattern was observed after annealing under a N2 atmosphere at 500 °C for 6 h. A representative XRD pattern of double-wall TiO2 NTs is shown in Figure 5A, which shows predominantly an anatase phase (I41/amd). The formation of the anatase phase during annealing under a N2 atmosphere was further supported by HRTEM and FFT patterns (Figure 5B). A lattice spacing of 0.355 nm was obtained corresponding to the (101) plane of the anatase phase. The FFT pattern also showed the planes corresponding to an anatase phase.22 EDX analysis confirmed the formation of TiO2. Figure 6 shows the Raman spectrum of as-prepared and annealed TiO2 NTs prepared under various anodization conditions. Six Raman peaks at 144, 200, 398.48, 519.54, 643.74, and 801 cm-1 of anatase are assigned to be Eg, Eg, B1g, A1g, Eg, and B1g, respectively.23 For the BF4-TiO2 annealed sample, a strong sharp band at 144 cm-1, three midintensity bands at 398, 519, and 643 cm-1, and a weak band at 200 cm-1 were observed. A superposition of two fundamental peaks occurred near 517 cm-1.24 These six peaks correspond to the six fundamental vibrational modes of anatase TiO2. The position and intensity of the five Raman active modes matched well with the anatase structure reference values. Miao et al.25 reported that a weak overtone scattering (B1g) at 801 cm-1 was observed in the anatase Raman spectrum. This was not observed for our BF4-TiO2 sample. This could be due to the large intensity ratio of fundamental peak to overtone, which might have made it difficult to be observed. Anatase belongs to the D4h point group.26 The irreducible representation for the optical mode was Γopt = A1g (R) + 2B1g (R) + 3Eg (R) + B2u (IR) + A2u (IR) + 2Eu (IR). Thus, there were six Raman active modes in anatase. For the BF4-TiO2 sample, a distinguishable weak and broad band was observed at around 300 cm-1. This peak was of a complex nature and under high resolution exhibited complex structure. It was suggested that second-order scattering and disorder effects were involved in the formation of the band at this range.27 The small peak observed at 446 cm-1 was characteristic of the rutile phase, which suggested that the sample was predominantly composed of anatase with a small amount of rutile phase.28 Raman spectra were sensitive to the trace amount of rutile and other impurities on the external surface of anatase TiO2. The broad Raman spectrum of BF4-TiO2 as anodized at 60 V was amorphous, which has no known distinct (22) Zhao, J.; Wang, X.; Sun, T.; Li, L. Nanotechnology 2005, 16, 2450. (23) Miao, L.; Tanemura, S.; Toh, S.; Kaneko, K.; Tanemura, M. J. Cryst. Growth 2004, 264, 246. (24) Zhang, Y.-H.; Chan, C. K.; Porter, J. F.; Guo, W. J. Mater. Res. 1998, 13, 9. (25) Miao, L.; Tanemura, S.; Toh, S.; Kaneko, K.; Tanemura, M. J. Cryst. Growth 2004, 264, 246. (26) Kudryavtseva, N. V.; Tikhonova, L. V. Russ. Phys. J. 1971, 14, 1534. (27) Gotic, M.; Ivanda, M.; Popovic, S.; Music, S.; Sekulic, A.; Turkovic, A.; Furic, K. J. Raman Spectrosc. 1997, 28, 555. (28) Wu, M.; Zhang, W.; Du, Z.; Huang, Y. Mod. Phys. Lett. B 1999, 13, 167.

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Figure 5. (A) XRD pattern and (B) HRTEM image of doublewall TiO2 NTs prepared at 60 V for 20 min using ethylene glycol and BMIM-BF4 solution and annealed under a N2 atmosphere for 3 h at 500 °C. The inset in panel B is the FFT pattern of the NTs.

Figure 6. Raman spectrum of the as-anodized BF4-TiO2, N2-annealed BF4-TiO2, and N2-annealed F-TiO2 NTs.

Raman peaks. This confirmed the results of XRD and TEM analyses. To evaluate the trap sites present within the band gap of TiO2 NTs, we conducted PL studies. The valence band of TiO2 was composed of oxygen 2p orbitals with the corresponding wave functions considerably localized on the O2- lattice sites. On the other hand, the conduction band of TiO2 consisted mostly of the 3d orbitals of Ti4+. The absorption of photons allowed the generation of free charge pairs (electron-hole) in the TiO2 matrix. The electron moved from the conduction band, and the hole stays in the valence band. The fate of the electron in recombining with the hole can be monitored by the PL studies. The electron might drop to the valence band directly to generate deep luminescence after combining with the valence band hole. In a second possible path, the electron may combine with the valence band hole indirectly through the sub-bands. These observations can be noticed in a PL spectrum through sharp peaks. The electron DOI: 10.1021/la900426j

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also sometimes recombined with the hole through irregular pathways created due to large bulk defects.29 A highly defective (nonstoichiometry) oxide generally fell into this category which can be detected by a broad band structure. To understand the phenomena described above, the PL spectra of TiO2 NT arrays were obtained by using an excitation wavelength of 325 nm in the range from 200 to 900 nm at room temperature (Figure 7). The original PL curves of BF4-TiO2 NTs (as anodized and annealed) and F-TiO2 NTs as annealed are illustrated in the figure. As expected, the as-anodized TiO2 NT exhibited a broad emission pattern. Deep luminescences are almost absent due to large bulk defects (presence of nonstoichiometry and impurities in the sample) present in the as-prepared sample.30 On the other hand, after the annealing treatment, several deep luminescence features emerged in the PL spectrum. The positions and relative intensity of all these bands were practically identical, irrespective of whether they were synthesized by using BMIM-BF4 or NH4F. This indicated that the PL mechanism of the BF4-TiO2 NTs might be the same as that of the F-TiO2 NTs. The PL emission peaks were located at 3.6, 3.39, 3.23, 3.03, 2.77, 2.62, 2.41, 2.17, 1.92, and 1.77 eV. The assignments of these peaks were made on the basis of the reported TiO2 electronic band structure and PL studies. Among the above bands, those at 3.4, 3.39, and 3.23 eV were possibly due to the lowest-energy indirect transition from Γ1 to X2/X1 (Γ being the center of the Brillouin zone and X the edge of the energy level diagram of TiO2).31 The peaks at 3.6 and 3.03 eV may be attributed to the highest-energy direct transition from X1 to X2/X1. Generally, the PL spectra of nanocrystalline materials might be due to three kinds of physical origins within the band gap: self-trapped excitons, oxygen vacancies (OVs), and surface states. The PL bands at 2.77 and 2.62 eV probably originated from the intrinsic states rather than the surface states. These PL bands could be considered as the self-trapped excitons localized on the TiO6 octahedra. These types of trap sites were found below the conduction band edge due to the conduction band splitting. The TiO6 octahedron in anatase was significantly distorted (symmetry lower than orthorhombic) which lifted degeneracies and created band splitting.32 The PL bands at 2.41, 2.17, and 1.92 eV can be assigned to the shallow trap levels related to OVs. In other words, it can be assigned as oxygen vacancies on the surface of TiO2 NT arrays.33 The PL emission identified with OVs might have occurred with photogenerated conduction band electrons trapped by ionized oxygen vacancy levels in TiO2 NT arrays, and subsequently recombined with the holes in the valence band. The peak positions and emission pattern were similar for both types of NTs. These results indicated that the double-wall TiO2 NT arrays behave like the 1D single-wall TiO2 NT arrays under 325 nm laser irradiation. The obtained emission intensities of double-wall TiO2 NTs were slightly higher than those of the single-wall TiO2 NTs. This was due to the better absorption properties of the double-wall NTs compared to the single-wall NTs. The (29) (a) Brus, L. E. J. Phys. Chem. 1986, 90, 2555. (b) Hagfeldt, A.; Gra¨tzel, M. Chem. Rev. 1995, 95, 49. (c) Linsebigler, A.; Lu, G.Jr.; Yates, J. T. Chem. Rev. 1995, 95, 735. (30) Sasaki, T.; Watanabe, M. J. Phys. Chem. B 1997, 101, 10159. (31) (a) Daude, N.; Gout, C.; Jouanin, C. Phys. Rev. B 1977, 15, 3229. (b) Redmond, G.; Fitzmaurice, D.; Graetzel, M. J. Phys. Chem. 1993, 97, 6951. (c) Serpone, N.; Lawless, D.; Khairutdinov, R. J. Phys. Chem. 1995, 99, 16646. (d) Lai, Y. K.; Sun, L.; Chen, C.; Nie, C. G.; Zuo, J.; Lin, C. J. Appl. Surf. Sci. 2005, 252, 1101. (32) Tang, H.; Berger, H.; Schmid, P. E.; Levy, F. Solid State Commun. 1993, 87, 847. (33) Hasimoto, K.; Hiramoto, M.; Sakata, T. In Proceedings of the Symposium on Photoelectrochemistry and Photoelectrosynthesis on Semiconducting Materials; Ginley, D. S., Honda, K., Nozik, A., Fujishima, A., Armstrong, N., Sakata, T., Kawai, T, Eds.; The Electrochemical Society: Pennington, NJ, 1988; p 395.

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Figure 7. Photoluminescence spectra of as-anodized BF4-TiO2, annealed BF4-TiO2, and annealed F-TiO2 NTs. The inset shows the enlarged wavelength range from 500 to 900 nm.

Figure 8. DRUV-vis spectrum of (a) P25 nanoparticles and (b) single-wall TiO2 NTs annealed under an N2 atmosphere and (c) double-wall TiO2 NT arrays prepared at 60 V for 20 min using ethylene glycol and a BMIM-BF4 solution and annealed under an N2 atmosphere for 3 h at 500 °C. The double-wall NTs absorbed significantly in the visible light region, whereas commercial TiO2 nanoparticles absorbed only in the UV region.

possibility of greater recombination losses in the double-wall NTs compared to the single-wall NTs also cannot be ruled out. The double-wall NTs with a large nanotube diameter can have better light absorption properties; at the same time, the concentric nature of these NTs might lead to more recombination losses.34 The potential importance of this phenomenon warrants further investigation. To understand the absorption properties of these NTs, DRUV-vis measurements were carried out. The DRUV-vis absorption spectrum of the N2-annealed BF4-TiO2 NT arrays showed absorption in both the UV and visible regions (Figure 8). An absorption tail was observed up to 600 nm, which was an indication of the modification of the intrinsic TiO2 structure. The visible light absorption might be due to a possible incorporation of heteroatoms (B and C substitution; from the BF4- and ethylene glycol) into the anatase crystal.15,35 A small absorption peak was also observed at ∼720 nm. This might be due to a possible complexation of BF4- ions into the TiO2 matrix during the anodization, which was stable during the annealing process.36 (34) Naito, K.; Tachikawa, T.; Fujitsuka, M.; Majima, T. J. Am. Chem. Soc. 2009, 131, 934. (35) Lu, N.; Zhao, H.; Li, J.; Quan, X.; Chen, S. Sep. Purif. Technol. 2008, 62, 670. (36) Yoo, K. S.; Lee, T. G.; Kim, J. Microporous Mesoporous Mater. 2005, 84, 211.

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The anodization process went through the diffusion of BF4- ions into the tubes and the effusion of [Ti(BF4)6]2- ions. Thus, there is a possibility of BF4- ions (Lewis base) bound to the TiO2 (Lewis acid) matrix. Similar absorption was not observed in the case of F-TiO2 NTs. For the sake of comparison, the UV-vis absorption spectrum of commercial P25 nanoparticles is also included. Pure TiO2 nanoparticles absorb only in the UV region (e400 nm). To gain insight into the chemical states of the NTs, we investigated the XPS Ti2p, C1s, N1s, and F1s core-level spectra for the F- and BF4- samples. XPS of various cores with and without Ar+ ion etching are shown in Figures 9, 10, and S3-S7. The spectra of Ti2p presented two obvious peaks at 458.7 and 464.3 eV corresponding to Ti2p3/2 and Ti2p1/2, respectively (Figure S3 of the Supporting Information). A separation of 5.6 eV between these two peaks confirmed the formation of TiO2. The broad shoulder around 457.8 eV corresponded to the lower oxidation state of the Ti ions (Ti3+).38 The C1s spectra of both F-TiO2 and BF4-TiO2 NTs showed two distinct peaks at 284.5 and 281.6 eV (Figure 9). The peak at 284.5 eV corresponded to the graphitized carbon with sp2 hybridization, and the peak at 281.6 eV corresponded to the C atom bonded directly to Ti atoms (Ti-C).37 The pattern and position of peaks in the C1s spectra were observed to be similar for both types of NTs. The carbon doping took place from the adsorbed ethylene glycol from the anodization solution. Figure 10 shows the B1s spectrum of BF4-TiO2 NTs annealed under nitrogen. A broad peak around 192.5 eV was observed which corresponded to B bonded to Ti.38 Masahashi and Oku38 observed an increase in Ti3+ concentration when boron was doped into the TiO2 matrix. In our sample, there was no significant increase in the shoulder intensity corresponding to Ti3+ observed in the BF4- samples compared to the fluoride sample (Figure S3 of the Supporting Information). This might be due to a lower concentration of boron in our sample. The F1s XPS spectra of F-TiO2 and BF4-TiO2 NTs are shown in Figures S4 and S5 of the Supporting Information. The as-anodized BF4-TiO2 NTs without surface cleaning showed two peaks at 684.5 and 688.7 eV (Figure S4 of the Supporting Information). On the other hand, after the surface had been cleaned for 300 s with Ar+ etching, the peak at the higher energy disappeared. The peak around 685 eV in the F1s spectrum is common in the literature, which could be assigned to the metal-fluoride bonding39 such as Ti-F. The peak around 688 eV was assigned to the doped F- atoms that would be in the TiO2 matrix.40 As this peak was not present in the bulk, it was not expected that any doped F- atoms in the TiO2 NTs during the anodization process. The F- atoms in BF4- anion were reported to peak at 685.9 eV41 which ruled out the presence of adsorbed BF4- species. Thus, this peak might be assigned to the complex anion formed by the reaction of Ti cation and BF4- anion. The annealed double-walled NTs exhibited a broad spectrum which can be attributed to various fluoride species. On the other hand, the F-TiO2 NTs showed a single peak at 685 eV corresponding to a F- atom bonded to Ti. At this stage, it is difficult to say if there was any F doped into the TiO2 matrix. This needs to be investigated further. The N1s XPS spectra of N2-annealed BF4-TiO2 NTs are shown in Figure S6 of the Supporting Information. The surface spectrum (without cleaning) showed the presence of a single peak at 400 eV, (37) (38) (39) (40) (41)

Cheng, Y.; Zheng, Y. F. Surf. Coat. Technol. 2007, 201, 4909. Masahashi, N.; Oku, M. Appl. Surf. Sci. 2008, 254, 7056. Suzuki, A.; Shinka, Y.; Masuko, M. Tribol. Lett. 2007, 27, 307. Ho, W.; Yu, C.; Lee, S. Chem. Commun. 2006, 1115. Fabre, B.; Kanoufi, F.; Simonet, J. J. Electroanal. Chem. 1997, 434, 225.

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Figure 9. C1s XPS spectra of F-TiO2 and BF4-TiO2 NTs.

Figure 10. B1s XPS spectrum of BF4-TiO2 NTs.

which was due to the adsorbed N2 (γ-N2) species. This peak was also assigned for the N atom from NOx;42 however, under these experimental conditions, the formation of NOx was not expected. After the surface had been cleaned with Ar+ ion sputtering for 300 s, a broad spectrum ranging from 403.6 to 393. 2 eV was noticed. This might be due to the presence of both γ-N2 and β-N2 (in TiN).43 This indicated that during N2 annealing most of the nitrogen stayed as the adsorbed species on the surface rather than doped nitrogen. The fraction of β-N2 was very small in these samples. The F-TiO2 NTs showed a single peak corresponding to adsorbed N2 (Figure S7 of the Supporting Information). The results described above indicated that the F-TiO2 NTs were doped with C atoms, whereas the BF4-TiO2 NTs were partially doped by C and B atoms. This gave rise to absorption properties for the former that were better than those of the former (Figure 8). Application for Water Splitting. The suitability of the TiO2 NT arrays for photoelectrochemical (PEC) applications was assessed through electrochemical characterization studies. The charge carrier density and flat band potential of the material were measured using Mott-Schottky analysis.44 Figure 11 shows the Mott-Schottky plot of N2-annealed BF4-TiO2 NTs in the dark and under illuminated conditions. From the linear portion of the (42) Varghese, O. K.; Paulose, M.; LaTempa, T. J.; Grimes, C. A. Nano Lett. 2009, 9, 731. (43) Ashai, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (44) Mohapatra, S. K.; Raja, K. S.; Mahajan, V. K.; Misra, M. J. Phys. Chem. C 2008, 112, 11007.

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John et al. Table 1. Comparison of Photocurrent Densities of TiO2 Nanocatalysts

catalyst double-wall TiO2NTs (BF4-) single-wall TiO2NTs (F-) P25/Ti a

photocurrent density (mA/cm2)a at 0.5 VAg/AgCl 1.65

visible light contribution (%)b 18

0.638

0.39

0.365

0.32

Under AM 1.5 illumination conditions. b Calculated using an UV filter.

Figure 11. Mott-Schottky plots showing the flat band potential and charge carrier density of dark and bright BF4-TiO2 samples.

Figure 13. Potentiostatic plot (at 0.2 VAg/AgCl) of BF4-TiO2 NT arrays under AM 1.5 illumination conditions.

Figure 12. Potentiodynamic plot of annealed single-wall (A) and double-wall (B) NT arrays under the illumination of (A) global AM 1.5 and (B) λ g 400 nm. The electrolyte used was a 1 M KOH solution.

Mott-Schottky plots, charge carrier densities (ND) were calculated using the relation (eq 1) ND ¼

2 eεε0 m

ð1Þ

where e is the elementary electron charge, ε is the dielectric constant, ε0 is the permittivity in vacuum, and m is the slope of the 1/C2 versus potential plot. The BF4-TiO2 NTs in the dark showed a charge carrier density of 2.35  1019 cm-3. While under illumination, the sample showed a charge carrier density of 5.49  1019 cm-3 which was higher than the carrier density in the dark. Results confirmed that additional charge carriers were produced upon illumination. The flat band potentials (UB) of the BF4-TiO2 NTs were found to be around -0.65 ( 0.2 V under dark and illumination conditions which were consistent with the values obtained in the PEC experiment. Since the TiO2 NTs showed n-type semiconductor behavior, the charge carriers were predominantly oxygen vacancies. Further, formation of oxygen vacancies was more 8246 DOI: 10.1021/la900426j

energetically favorable than formation of metal cation interstitials. The charge carriers were inferred as inherent oxygen vacancies and considered as point defects. The N2-annealed double-wall TiO2 NTs were tested for photoelectrolysis of water using global AM 1.5 and visible light sources. Figure 12 and Table 1 summarize the results of electrochemical H2 generated in terms of the photocurrent obtained from the PEC cell using various TiO2 NTs as the anode and Pt as the cathode. The as-prepared BF4-TiO2 NT arrays showed very low activity (∼4 μA/cm2), which was due to its amorphous characteristics. The N2-annealed sample exhibited a photocurrent density of 1.65 mA/cm2 at 0.5 VAg/AgCl, which was 2.5 times higher than that in the F-TiO2 NT arrays (0.638 mA/cm2). To find the contribution of the visible light components on the total activity of the NTs, experiments were conducted with UV filters (only λ g 400 nm is illuminated). We observed that unlike single-wall TiO2 NTs, double-wall NTs possessed good visible light photoactivity [18% compared to 0.39% using single-wall TiO2 NTs (Figure 12B)]. We also investigated the water splitting ability of a commercial product, TiO2 nanoparticles (Degussa P25 coated on Ti foil by the dip coating process). We found that double-wall NTs exhibited much better splitting ability than the commercial TiO2 nanoparticles (0.365 mA/cm2). The better performance could be attributed to higher porous structure, better visible light absorption, and 1D architecture. The photo response of the BF4-TiO2 electrode was captured by potentiostatic measurements under intermittent illumination (Figure 13). The photocurrent value was obtained at a potential of 0.2 VAg/AgCl. The current density value decreased to zero as soon as the illumination of light on the photoanode was stopped, and again returned to the original value as soon as the sample was illuminated with the light. The result confirmed that the current obtained for this experiment was entirely due to the photoactivity of the catalyst and the charge transport properties were very fast. From these discussions, we can summarize that NTs prepared using ionic liquids opened new opportunities for visible light-driven water splitting reactions (Table 1). Langmuir 2009, 25(14), 8240–8247

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4. Conclusions In summary, we have successfully synthesized double-wall, vertically oriented TiO2 NT arrays by a simple sonoelectrochemical anodization process using a room-temperature BF4- ionic liquid. The concentric NTs with external diameters of around 82 and 206 nm are formed by this process. In addition to the doublewall nature of these NTs, the external diameter of >200 nm is the largest among all the anodization processes. The N2-annealed double-wall NT arrays crystallized in the anatase phase and showed good visible light absorption (up to 600 nm) due to B and C doping. Compared to similar single-wall TiO2 NTs and commercial TiO2 nanoparticles, this new structure showed 2-6 times higher photocurrent density in the water splitting reaction. Unlike pure TiO2, double-wall TiO2 NTs show good visible light activity. Even though single-wall NT arrays have been well

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studied for the past few years, this process for synthesizing double-wall NT arrays will give a new direction to this field. Acknowledgment. We gratefully acknowledge the financial support by the U.S. Department of Energy through DOE Grant DE-FC36-06GO86066. We thank Dr. Mo Ahmadian (University of Nevada) for the TEM measurements. XPS, Raman, and PL studies were carried out at the WATLab and Department of Chemistry, University of Waterloo, Waterloo, ON. We are thankful to Dr. D. Pradhan and Prof. K. T. Leung for these measurements. Supporting Information Available: Figures S1-S7. This material is available free of charge via the Internet at http:// pubs.acs.org.

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