APPLIED PHYSICS LETTERS 89, 163106 共2006兲
Photoelectrochemical water splitting at titanium dioxide nanotubes coated with tungsten trioxide Jong Hyeok Park and O Ok Parka兲 Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, South Korea
Sungwook Kim Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712
共Received 14 April 2006; accepted 16 August 2006; published online 16 October 2006兲 The photocatalytic splitting of water into hydrogen and oxygen using solar light is a potentially clean and renewable source for hydrogen fuel. Titanium oxide nanotubes coated with tungsten oxide were prepared to harvest more solar light for the first time and characterized their water splitting efficiency. The tungsten trioxide coatings significantly enhanced the visible spectrum absorption of the titanium dioxide nanotube array, as well as their solar-spectrum induced photocurrents. For the sample, upon white light illumination at 150 mW/ cm2, hydrogen gas generated at the overall conversion efficiency of 0.87%. © 2006 American Institute of Physics. 关DOI: 10.1063/1.2357878兴 The photocatalytic splitting of water using oxide semiconductors is initiated by the direct absorption of a photon, which creates separated electrons and holes in the energy band gap of the material.1–3 Titanium dioxide 共TiO2兲 has been considered one of the most promising photocatalytic materials due to its relatively low cost, chemical stability, and photostability.4,5 However, the catalytic property of TiO2 is limited with ultraviolet 共UV兲 regions, which accounts for only 4% of the incoming solar energy and thus renders the overall process impractical.6 Tungsten trioxide 共WO3兲 has been recently considered as a new photoanode material or mixture material with TiO2 for water splitting because the WO3 can offer relatively small band gap 共⬃2.5 eV兲 and corrosion stability in aqueous solution.7,8 Although WO3 has shown great potential such as photooxidation of water with visible light and high photocurrent with nanocrystals, the quantum yield is still low.9,10 The large band gap oxide semiconductors, such as n-type TiO2 or WO3 employed in photoelectrochemical devices, often have short exciton diffusion lengths, so it is mainly the carriers generated within the space charge layer that contribute to the photocurrent.11,12 Several efforts have been made to employ mixed WO3 / TiO2 system for enhancing the efficiency of electrochromic effects and the photocurrent in aqueous solution.13–15 We demonstrated previously the carbon doped TiO2 nanotube arrays with high aspect ratio to improve the photocurrent densities.16 In this study, we employ TiO2 nanotubes coated with WO3 as a photoanode for the first time. Here, we present photoelectrochemical data obtained using TiO2 nanotubes coated with WO3 in order, expecting the increase in solar harvesting efficiency. This nanocomposite material can be used as efficient and stable solar-driven photoanode for oxidation of water. TiO2 nanotube array with ⬃2 m length was fabricated by the methods which were reported elsewhere.16 Figure 1共a兲 shows scanning electron microscope 共SEM兲 images of the TiO2 nanotubes with ⬃2 m length. The pore diameter was about 80 nm and the wall thickness was about 20 nm. To a兲
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increase light harvesting, WO3 coated TiO2 nanotube array was prepared by electrochemical deposition of WO3 sol to TiO2 nanotube array. Peroxotungstic acid sol 共WO3−x · nH2O兲 was synthesized by dissolving the tungsten powder 共Aldrich兲 共1 g兲 in an ice-cooled beaker containing a 30% H2O2 aqueous solution 共Aldrich兲 共5 ml兲 and then diluting the solution using a water and 2-propanol mixture 共100 ml, volume ratio of 5:2兲. WO3 was deposited on the prepared Ti nanotube by electrophoresis 共−400 mV versus Ag/ AgCl reference electrode, Pt mesh counter electrode兲 from the solution for 5 min and then annealed at 450 ° C for 30 min in air. A SEM image of the WO3 / TiO2 nanotube array nanocomposite is shown in Fig. 1共b兲. After electrophoretic deposition of WO3 on TiO2 nanotubes array, the nanocomposite still has nanotubular structure 关inset of Fig. 1共b兲兴. However, the nanocomposite materials are fully filled with WO3 when the deposition time is longer than 20 min. X-ray photoelectron spectroscopy confirmed the coverage of WO3 on the TiO2 nanotubes. In addition, the color change of the nanotube array upon WO3 from gray to bright green demonstrates an effect on their optical response in the visible wavelength range. Typical cyclic voltammetry scan for WO3 / TiO2 nanotube composite electrode in acid solution 共1M H2SO4兲 was studied. The blue color was observed from the samples at potentials below 0 V. Also, the over-reduction current was observed around −250 mV. The blue color is related to the intercalation of protons in the solution and it is one of the strong evidences for successful electrophoretic deposition of WO3 on TiO2 nanotubes. Normalized UV-visible diffuse reflectance spectra of the TiO2 nanotubes and the WO3 / TiO2 nanotube nanocomposite were shown in the inset of Fig. 2. The absorption of visible light of the WO3 / TiO2 nanotube nanocomposite was enhanced compared with that of the TiO2 nanotube array. The photoelectrochemical measurements were carried out in a three electrode system by illuminating the xenon lamp 共150 mW/ cm2兲. The photoelectrochemical measurements were performed in a glass cell with a flat Pyrex glass window to facilitate the transmittance of light to the photoelectrode surface. The working electrode had a surface area of 1 cm2, and a platinum 共Pt兲 plate and an Ag/ AgCl elec-
0003-6951/2006/89共16兲/163106/3/$23.00 89, 163106-1 © 2006 American Institute of Physics Downloaded 13 Dec 2006 to 143.248.130.198. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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Appl. Phys. Lett. 89, 163106 共2006兲
FIG. 1. 共a兲 SEM images 共lateral view and top view兲 of TiO2 nanotubes. 共b兲 SEM images 共lateral view and top view兲 of WO3 / TiO2 nanotube nanocomposite electrodeposited for 5 min.
trode were used as counter and reference electrodes, respectively. The electrolyte solution used for the water splitting reaction was 1M HClO4. A scanning potentiostat was used for the measurement of photocurrent under an applied potential. Figure 2 shows photocurrent-potential plot for the WO3 / TiO2 nanotube nanocomposite compared to that of the TiO2 nanotubes without WO3 illuminated with 150 mW/ cm2 white light. The maximum photocurrent of the WO3 / TiO2 nanotube nanocomposite was increased compared with that of the TiO2 nanotubes due to additional light absorption as shown in the UV-visible absorption spectra. The saturation photocurrent was attained after only about 0.4 V difference from open circuit potential 共Fig. 2兲. A tandem cell incorporating an aqueous cell based on a WO3 thin film photoanode biased with a separated dye-sensitized solar cell has been focused on new hydrogen evolution system.17 So, it is important that the WO3 photoanode should have its maximum photocurrent within 0.7 V 共open circuit potential of dye-sensitized solar cell兲 difference from its open circuit potential. To compare photoelectrochemical behaviors of the WO3 / TiO2 nanotube nanocomposite with a WO3 / Ti planar
foil, we prepared electrophoretically deposited 5-m-thick WO3 nanocrystalline film on Ti foil. At 1.0 V versus Ag/ AgCl the photocurrent density of the WO3 / TiO2 nanotube nanocomposite was more than 30% greater than the value for the WO3 / Ti planar foil photoanode with a similar thickness. This suggests that the TiO2 nanotube structure can harvest solar light more effectively than photoanodes with a nanocrystalline structure under the same illumination. In addition, the TiO2 nanotubular structure also shows a steeper increase in the photocurrent with applied potential. In semiconductors, the orbitals are merged into a nearly filled valence band separated by the energy gap. When semiconductor is immersed in a solution under irradiation, charge transfer occurs at the interface because of the difference in the tendency of the two phases to gain or lose electrons 共that is, difference in electron affinity or electrochemical potential of the two phases兲.3 The net result is the formation of an electrical field at the surface of the semiconductor, inducing electron and hole separation. For the applications of TiO2 and WO3 as a photoanode in a photoelectrolysis cell under solar light illumination, it is particularly important to introduce a large interface area where photoinduced charge transfer by excitons into separated electrons and holes can efficiently occur. In this case, excitons are always generated within a diffusion length of a semiconductor/electrolyte interface, potentially leading to higher cell efficiency than a planar thick structure. However, even though two electrodes with different morphologies have the same surface area, the disordered structure has higher series resistance than an ordered structure. For example, some excitons formed in deep position will become extinct by recombination process, resulting to low quantum efficiency. The photoconversion efficiency 共兲 of light to hydrogen energy in the presence of an external applied potential is calculated as18
共%兲 = 关共total power output − electrical power output兲/light power input兴 ⫻ 100 = 关关j p共1.23 − Eapp兲 ⫻ 100兴/Io兴. 2
共1兲
FIG. 2. Photocurrent density vs applied potential for the WO3 / Ti foil, the The photocurrent density j p is in mA/ cm and the elecTiO2 nanotube arrays, and the WO3 / TiO2 nanotube nanocomposite illumitrical power input is j pEapp. Eapp = Emea − Eaoc, where Emea is nated by 150 mW/ cm2 xenon lamp. Open circuit potential of the WO3 / TiO2 the electrode potential 共versus Ag/ AgCl兲 of the working nanotube nanocomposite: 0.31 V. Inset: Normalized UV-visible diffuse reelectrode at which the photocurrent was measured under flectance spectra of the TiO2 nanotube arrays and the WO3 / TiO2 nanotube illumination and Eaoc is the electrode potential 共versus nanocomposite. Downloaded 13 Dec 2006 to 143.248.130.198. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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In conclusion, we have introduced water splitting characteristics of WO3 / TiO2 nanotube nanocomposite by using high-aspect-ratio TiO2 nanotube arrays as a starting material. The unique structures of the TiO2 nanotube arrays can be used as template for enhancement of photocurrent by using relatively lower band gap material compared with TiO2. Reported data strongly demonstrated such structures and materials could be used in WO3/dye-sensitized solar cell tandem structures for unassisted water splitting system. The authors thank C. R. Luman 共Department of Chemistry and Biochemistry, The University of Texas at Austin兲 for helpful advice. A. L. Linsebigler, G. Lu, and J. T. Yates, Jr., Chem. Rev. 共Washington, D.C.兲 95, 735 共1995兲. 2 A. J. Bard and M. A. Fox, Acc. Chem. Res. 28, 141 共1995兲. 3 A. J. Bard, Science 207, 139 共1980兲. 4 K. I. Hadjiivanov and D. K. Klissurski, Chem. Soc. Rev. 25, 61 共1996兲. 5 A. Heller, Acc. Chem. Res. 28, 503 共1995兲. 6 Z. Zou, J. Ye, K. Sayama, and H. Arakawa, Nature 共London兲 414, 625 共2001兲. 7 C. Santato, M. Ulmann, and J. Augustynski, J. Phys. Chem. B 105, 936 共2001兲. 8 H. Wang, T. Lindgren, J. He, A. Hagfeldt, and S. E. Lindquist, J. Phys. Chem. B 104, 5686 共2000兲. 9 W. Erbs, J. Desilvestro, E. Borgarello, and M. Gratzel, J. Phys. Chem. 88, 4401 共1984兲. 10 J. R. Darwent and A. Mills, J. Chem. Soc., Faraday Trans. 2 78, 359 共1982兲. 11 M. A. Butler, J. Appl. Phys. 48, 1914 共1977兲. 12 A. K. Ghosh and H. P. Maruska, J. Electrochem. Soc. 124, 1516 共1977兲. 13 S. Hasimoto and H. Matsuoka, J. Electrochem. Soc. 138, 2403 共1991兲. 14 S. Hasimoto, N. Kitahata, K. Mori, and M. Azuma, Catal. Lett. 101, 49 共2005兲. 15 I. Shiyanovakaya and M. Hepel, J. Electrochem. Soc. 146, 243 共1999兲. 16 J. H. Park, S. W. Kim, and A. J. Bard, Nano Lett. 6, 24 共2006兲. 17 M. Gratzel, Nature 共London兲 414, 338 共2001兲. 18 G. K. Mor, K. Shankar, M. Paulose, O. K. Varghese, and C. A. Grimes, Nano Lett. 5, 191 共2005兲. 1
FIG. 3. Photoconversion efficiency of WO3 / TiO2 nanotube nanocomposite with ⬃2 m length as a function of measured potential 共vs Ag/ AgCl兲. Inset: H2 and O2 evolution from the WO3 / TiO2 nanotube nanocomposite 共photoanode兲 and Pt electrode.
Ag/ AgCl兲 of the same working electrode at open circuit condition under same illumination and in the same electrolyte. Using Io = 150 mW/ cm2, photoconversion efficiency as a function of potential for the WO3 / TiO2 nanotube nanocomposite photoanode is shown in Fig. 3. A maximum conversion efficiency of 0.87% is obtained for WO3 / TiO2 nanotube nanocomposite. By collecting the gases at the WO3 / TiO2 nanotube nanocomposite photoanode and the Pt counter electrode during the photoreaction, we observed a 2.2:1 volume ratio of hydrogen and oxygen 共inset of Fig. 3兲. Argon was used as purge gas to remove dissolved gases and 2-nm-thin Pt layer was deposited on the WO3 / TiO2 nanotube nanocomposite before this experiment. Evolved gases collected in sealed glass tubes were confirmed by using gas chromatograph.
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