Synthesis of Y-branched TiO 2 nanotubes S.K. Mohapatra, M. Misra ⁎ , V.K. Mahajan, K.S. Raja Center for Materials Reliability, Chemical and Metallurgical Engineering, MS 388, University of Nevada, Reno, NV 89557, USA Received 16 August 2007; accepted 29 September 2007 Available online 5 October 2007 Abstract Self-organized, Y-branched TiO 2 nanotubes were synthesized using a multi-step sonoelectrochemical anodization method. A change in anodization temperature (ΔT =10 °C) at a constant applied potential leads to a Y-type branched TiO 2 nanotubes. The as-anodized titania nanotubes were annealed under hydrogen atmosphere at 500 °C to convert the amorphous titania nanotubes to crystalline with mostly anatase crystal structure. These nanotubes are found to possess higher photon absorption properties compared the 1D TiO 2 nanotubes. Various characterization techniques, viz., FESEM, GXRD, HRTEM, FFT, UV–VIS etc. are used to characterize the materials. © 2007 Elsevier B.V. All rights reserved. Keywords: Sonoelectrochemical; TiO 2 ; Y-branched; Nanomaterials 1. Introduction In 1999, Zwilling and coworkers successfully showed TiO 2 nanotube formation on an anodized Ti surface [1]. Since this report, there is a growing interest in the synthesis of titania nanotubes by anodization due to the simplicity in preparation and handling, and a more controllable synthesis. This anodization process was used to synthesize nanotube structures with variable tube diameter, tube length, tube surface, etc. are reported [2–5]. These titania nanotube arrays has drawn considerable interest due to their promise in various applications, viz., gas sensing, radiation sensors, water electrolysis, H 2 storage, and as a template to grow carbon nanotubes (CNTs), etc. [2,6–11]. Carbon nanotubes (CNTs) and boron nitride (BN) were reported to form Y-branched nanotubes with different electronic properties compared to the 1D nanotubes [12–16]. At present, 1D TiO 2 nanotubes can be routinely synthesized either by the anodization methods or sol-gel (hydrothermal) methods [2–5,17]. Similar to Y-branched CNTs, Y-shape TiO 2 nanotubes are also expected to acquire different electronic and photon absorption properties compared to its 1D nanotube structure. However, there is no report on the synthesis of branched TiO 2 nanotubes. In this communication, we report for the first time, the synthesis, and characterization of Y-branched TiO 2 nanotube structures by sonoelectrochemical anodization method. 2. Materials and methods 2.1. Synthesis procedure Y-branch ed nanotubu lar TiO 2 arrays were prepared by anodization of the Ti foils (ESPI) in 300 mL electrolytic solution using ultrasonic waves (100 W, 42 kHz, Branson 2510R-MT). Water (10 wt.%), ammonium fluoride (NH 4 F, 0.5 wt.%, Fischer) and ethylene glycol (Fischer) were mixed together thoroughly and used as the electrolytic solution (pH=6.6–6.7). A two- electrode configuration was used for anodization. A flag shaped platinum (Pt) electrode (thickness: 0.5 mm, area: 3.75 cm 2 ) serves as cathode. The distance between the two electrodes was kept at 4.5 cm in all the experiments. The anodization was carried out by applying potential using a rectifier (Agilent, E3640A). During anodization, ultrasonic waves were irradiated onto the solution continuously. The anodization was started using the above procedure at 20 V under ambient conditions. After 30 min of anodization the temperature of the solution was increased from 25 °C to 35 °C. The anodization was further continued for another 30 min at 20 V and 35 °C. A similar experiment was also carried out at 50 V to synthesize Y-branched nanotubes with larger nanotube diameter. A vailable online at www.sciencedirect.com Materials Letters 62 (2008) 1772– 1774 www.elsevier.com/locate/matlet ⁎ Corresponding author. E-mail address: misra@unr.edu (M. Misra). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.09.083 2.2. Characterization A field emission scanning electron microscope (FESEM; Hitachi, S-4700) was used to analyze the morphology of the synthesized TiO 2 nanotubes. Glancing angle X-ray diffraction (GXRD) was used to evaluate the crystal phases of Y-branched TiO 2 nanotubes using a Philips-12045 B/3 diffractometer. The target used in the diffractometer was copper (λ =1.54 Å). High resolution transmission electron microscopic studies (HRTEM, JEOL 2100F), fast Fourier transformation (FFT), and selected area electron diffraction (SAED) were carried out at 200 kV. Scanning transmission electron microscopy (STEM) was carried out in the bright field mode. Electron energy loss spectrum (EELS) was obtained from the samples using Gatam Enfina spectrophotometer. The sample was prepared by scratching the nanotube layer into the ethanol, which was already placed on the carbon coated copper grid. Diffuse reflectance ultraviolet and visible (DRUV–VIS) spectra of TiO 2 samples are measured from the optical absorption spectra using a UV–VI S spectrophotometer (UV-2401 PC, Shimadzu). 3. Results and discussion Fig. 1 shows the FESEM images of the Y-branched TiO 2 nanotube arrays. The top view (Fig. 1a) looks similar to the 1D, straight TiO 2 nanotubes, synthesized by sonoelectrochemical anodization method at constant temperature [8]. The average internal pore diameter of these nanotubes was found to be around 60 nm. The wall thickness was found to be in the range of 15 to 18 nm. Fig. 1 b shows the cross sectional view of these nanotube arrays. It can be seen from the figure that these nanotubes Fig. 1. FESEM images of: (a) front view and (b) cross sectional of the Y-branched TiO 2 nanotube arrays prepared at 20 V and two different temperatures. Fig. 2. TEM image of a Y-branched TiO 2 nanotube prepared at 20 V and two different temperatures. Fig. 3. HRTEM image of the wall of the TiO 2 nanotube arrays prepared by multi- step anodization method at 10 V. The lattice spacing of 0.35 nm shows the crystallization of the TiO 2 material into anatase phase. Inset shows the FFT pattern of a typical anatase TiO 2 . 1773S.K. Mohapatra et al. / Materials Letters 62 (2008) 1772–1774 after formed up to 500 nm, branches out to form Y-shape TiO 2 nanotubes. The total length of the nanotube arrays was found to be ∼ 1 μm. There was a decrease in tube diameter observed after the branching, which might be due the limited space available as these nanotube grow upside down in a titanium surface. Further, this process was also repeated at 50 V. FESEM image (Figure in Appendix A) of these nanotubes was found to be almost similar to the above discussed branched nanotubes prepared at 20 V except the diameters of these nanotubes were ∼ 100 nm. This might be due to the fact that the tube diameters of these nanotubes increase with the applied poten tial [8]. The pattern of the barrier layer (tube ends) of the branched nanotube arrays were also observed to be different from the nanotube arrays having 1D structure. The barrier layer of the latter consists of dome (spherical) shape ordered arrangement, where as the former shows a barrier layer with mostly depleted spherical tube ends (Figure in Appendix A). The distortion from a perfectly spherical shape was due to the strain environment generated due to branching. TEM analysis was carried out to get further insight of the branched TiO 2 nanotub es. Fig. 2 shows TEM image of Y-branched TiO 2 nanotubes. It confirms the formation of Y-branched TiO 2 nanotubes with similar nanotube parameters as obtained from FESEM measurements. The above results show that the position of the branching, tube diameter, length of the total nanotube arrays can be controlled by tuning the earlier discussed synthesis procedures. The GXRD pattern (figure not shown here) of the as-anodized TiO 2 layer was found to be amorphous in nature. However, after anneal at 500 °C using 10% H 2 in argon, the amorphous TiO 2 has changed to crystalline anatase phase [9,10]. This was further confirmed by the HRTEM analysis. A lattice spacing of 0.35 nm obtained from the HRTEM (Fig. 3; see inset) corresponds to the (101) plane of the anatase phase of TiO 2 [8]. The FFT pattern (Fig. 3) and SAED (Figure in Appendix A) also supported the above observation. EELS analysis showed a typical TiO 2 pattern (figure not shown here) [18].TheUV–VIS absorption (figure not shown here) studies showed that the Y-branched TiO 2 nanotubular layer absorbs 20–30% more visible light (λ max =580 nm; absorption onset 780 nm) than the 1D TiO 2 nanotubular layer of almost equal thickness. This is due to more dense growth of the branched nanotubes compared to the 1D nanotubes. These nanotubular branched TiO 2 nanotube arrays can be used in electro- catalysis, sensors, Li-ion batteries, solar cells, photocatalysis, photoelec- trolysis and hydrogen storage, etc. 4. Conclusions In conclusion, branched TiO 2 nanotubes have been success- fully synthesized using multi-step sonoelectrochemical anodiza- tion method. Various characterization studies, viz., FESEM, HRTEM, GXRD, FFTand SAED proved the successful synthesis of Y-type TiO 2 nanotubes with anatase crystal structure. These nanotubes absorb visible light more efficiently compared to the 1D titania nanotubes with comparable thickness. Since structural parameters affect the overall electronic and electrochemical properties of the TiO 2 , this study will give a new direction on the application of titania based materials. Acknowledgements This work has been sponsored by the U.S. Department of Energy through DOE Grant No: DE-FC36-06GO86066. The authors thank the financial support of DOE. The authors also gratefully acknowledge the financial support of National Science Foundation to establish a TEM facility at University of Nevada Reno through NSF Grant No: NSF-0321297. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.matlet.2007.09.083. References [1] V. Zwilling, E. Darque-Ceretti, A. Boutry-Forveille, D. David, M.Y. Perrin, M. Ancouturier, Surf. Interface Anal. 27 (1999) 629–637. [2] G.K. Mor, O.K. Varghese, M. Paulose, K. Shankar, C.A. Grimes, Sol. Energy Mater. Sol. Cells 90 (2006) 2011–2075 and references are cited therein. [3] J.M. Macak, H. Tsuchiya, L. Taveira, S. Aldabergerova, P. Schmuki, Angew. Chem. Int. Ed. 44 (2005) 7463–7465. [4] S.P. Albu, A. Ghicov, J.M. Macak, P. Schumuki, Phys. Status Solidi 1 (2007) R–65-R-67. [5] K.S. Raja, M. Misra, K. Paramguru, Electrochim. Acta, 51 (2005) 154–165. [6] P. Pillai, K.S. Raja, M. Misra, J. Power Sources 161 (2006) 524–530. [7] S.K. Mohapatra, M. Misra, V.K. Mahajan, K.S. Raja, J. Catal. 246 (2007) 362–369. [8] S.K. Mohapatra, M. Misra, V.K. Mahajan, K.S. Raja, J. Phys. Chem. C 111 (2007) 8677–8685. [9] S.K. Mohapatra, M. Misra, J. Phys. Chem. C 111 (2007) 11506–11510. [10] M. Misra, K. Paramguru, S.K. Mohapatra, J. Nanosci. Nanotech. 7 (2007) 2640–2646. [11] N.R. de Tacconi, C.R. Chenthamarakshan, G. Yogeeswaran, A. Watch- arenwong, R.S. de Zoysa, N.A. Basit, K. Rajeswar, J. Phys. Chem. B 110 (2006) 15347–15355. [12] J.M. Romo-Herrera, M. Terrones, H. Terrones, S. Dag, V. Meunier, Nano Lett. 7 (2007) 570–576. [13] G. Meng, Y.J. Jung, A. Cao, R. Vajtai, P.M. Ajayan, Proc. Natl. Acad. Sci. U.S. Am. 102 (2005) 7074–7078. [14] P.R. Bandaru, C. Daraio, S. Jin, A.M. Rao, Nature Materials 4 (2005) 663–666. [15] L.M. Cao, X.Y. Zhang, H. Tian, Z. Zhang, W.K. Wang, Nanotechnology 18 (2007) 155605. [16] J. Li, C. Papadopoulos, J. Xu, Nature (Lond.) 402 (1999) 253–254. [17] D.V. Bavykin, J.M. Friedrich, F.C. Walsh, Adv. Mater. 18 (2006) 2807–2824. [18] W.A. Bryant, J. Mater. Sci. 12 (1977) 1285–1306. 1774 S.K. Mohapatra et al. / Materials Letters 62 (2008) 1772–1774 . insight of the branched TiO 2 nanotub es. Fig. 2 shows TEM image of Y-branched TiO 2 nanotubes. It confirms the formation of Y-branched TiO 2 nanotubes. report on the synthesis of branched TiO 2 nanotubes. In this communication, we report for the first time, the synthesis, and characterization of Y-branched