Published: May 13, 2011 r2011 American Chemical Society 2353 dx.doi.org/10.1021/nl2006802 | Nano Lett. 2011, 11, 2353–2357 LETTER pubs.acs.org/NanoLett Wafer-Level Photocatalytic Water Splitting on GaN Nanowire Arrays Grown by Molecular Beam Epitaxy Defa Wang, † Adrien Pierre, † Md Golam Kibria, † Kai Cui, † Xueguang Han, † Kirk H. Bevan, ‡,§ Hong Guo, || Suzanne Paradis, ^ Abou-Rachid Hakima, ^ and Zetian Mi* ,† † Department of Electrical and Computer Engineering, McGill University, 3480 University Street, Montreal, Quebec H3A 2A7, Canada ‡ Department of Mining and Materials Engineering, McGill University, 3600 University Street, Montreal, Quebec H3A 2T8, Canada § Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ) Centre for the Physics of Materials, Department of Physics, McGill University, 3610 University Street, Montreal, Quebec H3A 2T8, Canada ^ Defence Research and Development Canada Valcartier, 2459 Boulevard Pie XI North, Quebec, Quebec G3J 1X5, Canada b S Supporting Information S ince the discovery of the HondaÀFujishima effect in a TiO 2 / Pt photoelectrochemical cell in the early 1970s, 1À3 the use of semiconductors for photocatalytic water splitting has attracted tremendous interest: for it enables the generation of clean and renewable hydrogen fuel directly from solar irradiation without the consumption of electric power. Semiconductor photocata- lytic water splitting generally involves three fundamental pro- cesses: (i) band gap absorption of photons and excitation of electronÀhole pairs; (ii) separation and migration of photogen- erated charge carriers; (iii) surface redox reactions via photo- generated electrons and holes. Thermodynamically, if the conduction band minimum is more negative than the reduction potential of H þ /H 2 (0 V vs normal hydrogen electrode (NHE)) and the valence band maximum is more positive than the oxidation potential of O 2 /H 2 O (1.23 V vs NHE), then water molecules can be reduced by electrons to form H 2 and oxidized by holes to form O 2 to achieve overall water splitting. Over the past 4 decades, the development of photocatalysis has primarily focused upon large band gap metal oxides involving ions with filled or empty d-shell bonding configurations (i.e., Ti 4þ , Zr 4þ , Nb 5þ , Ta 5þ ,W 6þ , Ga 3þ , In 3þ , Ge 4þ , Sn 4þ , and Sb 5þ ) 4À6 and oxynitrides such as (Ga 1Àx Zn x )(N 1Ày O y ). 2,7 Recently, the use of group III nitride semiconductors for water splitting has attracted considerable attention. 8,9 Due to the more negative potential of the nitrogen 2p orbital, compared to that of the oxygen 2p orbital, metal nitrides often possess a narrow band gap and can poten- tially encompass nearly the entire solar spectrum. Moreover, the inherent chemical stability of nitrides also favors their use in the harsh photocatalysis reaction environment. 10 Indeed, recent first-principles calculations suggest that a single H 2 O molecule can be efficiently cleaved in an exothermic reaction to form H 2 under photoexcitation at Ga-terminated surface sites. 11 Ab initio molecular dynamic simulations further show that the overall water oxidation reaction at GaN surfaces can be energetically driven by photogenerated holes. 12 The size, morphology, surface chemistry, and crystal structure of photocatalysts often play a crucial role in determining their photophysical and photocatalytic properties. Conventional photocatalysts are typically employed in the form of powders. However, photocatalysts in the form of one-dimensional (1-D) nanostructures, such as nanowires, nanobelts, and nanotubes, are highly desired. 13À20 These nanomaterials exhibit extremely large Received: February 28, 2011 Revised: April 24, 2011 ABSTRACT: We report on the achievement of wafer-level photocatalytic overall water splitting on GaN nanowires grown by molecular beam epitaxy with the incorporation of Rh/Cr 2 O 3 coreÀshell nanostructures acting as cocatalysts, through which H 2 evolution is promoted by the noble metal core (Rh) while the water forming back reaction over Rh is effectively prevented by the Cr 2 O 3 shell O 2 diffusion barrier. The decomposition of pure water into H 2 and O 2 by GaN nanowires is confirmed to be a highly stable photocatalytic process, with the turnover number per unit time well exceeding the value of any previously reported GaN powder samples. KEYWORDS: GaN, nanowire, water splitting, hydrogen, photocatalytic, molecular beam epitaxy