© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com COMMUNICATION Yanbo Li, Tsuyoshi Takata, Dongkyu Cha, Kazuhiro Takanabe, Tsutomu Minegishi, Jun Kubota, and Kazunari Domen* Vertically Aligned Ta 3 N 5 Nanorod Arrays for Solar-Driven Photoelectrochemical Water Splitting Dr. Y. Li, Dr. T. Takata, Dr. T. Minegishi, Prof. J. Kubota, Prof. K. Domen Department of Chemical System Engineering The University of Tokyo 7-3-1 Hongo, Bunkyo-ku Tokyo 113-8656, Japan E-mail: domen@chemsys.t.u-tokyo.ac.jp Dr. D. Cha Advanced Nanofabrication Imaging and Characterization Laboratory King Abdullah University of Science and Technology (KAUST) 4700 KAUST, Thuwal 23955-6900, Saudi Arabia Prof. K. Takanabe Division of Chemical Life Sciences and Engineering KAUST Catalysis Center (KCC) 4700 KAUST, Thuwal 23955-6900, Saudi Arabia DOI: 10.1002/adma.201202582 Photoelectrochemical (PEC) water splitting is a promising approach to direct conversion of solar energy into storable hydrogen fuel that could act as a green energy carrier. [1–3] Since its first demonstration with TiO 2 photoelectrode, PEC cells made of different materials and configurations have been studied extensively to obtain a higher solar energy conversion efficiency. [4–8] Central to the device is the photoelectrode, whose material and structure both play critical roles in the device per- formance. An ideal semiconductor material for the photoelec- trode should have a band gap large enough ( >1.6 eV) to split water and, at the same time, small enough ( <2.2 eV) to absorb a wide range of the solar spectrum. Meanwhile, the conduction band and valence band of the semiconductor should straddle the water redox potentials in order to split water without external bias. Finally, the semiconductor material must be stable in aqueous solutions. Tantalum nitride (Ta 3 N 5 ) is one of the few semiconductor materials that satisfy these three requirements simultaneously. [9,10] With a band gap of 2.1 eV and suitable band positions, Ta 3 N 5 has the potential to utilize a large portion of the solar spectrum ( <600 nm) to split water even without external bias. In recent years, photoelectrodes made of Ta 3 N 5 thin films and particles have been demonstrated for PEC water splitting. [11–13] The rapid development of nanotechnology offers a wide selection of one-dimensional (1D) nanostructures (i.e., nanowires, nanorods, and nanotubes) as ideal building blocks for energy harvesting devices. [14,15] 1D semiconductor nano- structures, especially vertically aligned ones, have been widely used in photovoltaic solar cells and dye-sensitized solar cells and showed enhanced light absorption, reduced optical reflect- ance, and improved carrier collection efficiency. [16–18] PEC cells have also been assembled with a variety of 1D semiconductor nanostructures for water splitting. [19–24] Recently, photoelec- trodes made of Ta 3 N 5 nanotube arrays have been fabricated and showed enhanced visible light activity, [25] demonstrating the potential of 1D Ta 3 N 5 nanostructures as promising candidates for solar-driven PEC water splitting. In this work, we report on the fabrication of vertically aligned Ta 3 N 5 nanorod arrays using a scalable method for solar-driven PEC water splitting. Vertically aligned tantalum oxide (Ta 2 O 5 ) nanorod arrays were first grown via a through-mask anodiza- tion method and then transformed into Ta 3 N 5 nanorod arrays in a subsequent nitridation process. Porous anodic alumina (PAA) was employed as the mask and Ta 2 O 5 nanorods were embedded into the nanochannels of the PAA mask in a through-mask anodization process. As distinct from the anodization of Ta 2 O 5 nanotube arrays which required the use of fluorine-containing electrolyte, [25] the anodization of Ta 2 O 5 nanorod arrays was con- ducted in a mild electrolyte. The mild anodization yielded Ta 2 O 5 nanorod arrays with good film toughness, which enabled the nitridation to be carried out at a high temperature of 1000 °C. The resulting Ta 3 N 5 nanorod arrays exhibited high crystallinity, a highly conductive interlayer between the substrate, and high performance in PEC water splitting. Under irradiation of AM 1.5G 100 mW/cm 2 simulated sunlight, the Ta 3 N 5 nanorod pho- toelectrode yielded a photocurrent density of 3.8 mA/cm 2 at 1.23 V versus a reversible hydrogen electrode (V RHE ), which was ∼3.2 times higher than that of a planar Ta 3 N 5 photoelectrode. A maximum incident photon-to-current conversion efficiency (IPCE) of 41.3% was achieved at 440 nm under 1.23 V RHE . The PEC water splitting activity of the Ta 3 N 5 nanorods is one of the highest among all the photoanodes so far reported. [11–13,19–25] Furthermore, high and stable photocurrent was demonstrated by modifying the Ta 3 N 5 nanorods with cobalt phosphate (Co-Pi) co-catalyst. Vertically aligned Ta 3 N 5 nanorod arrays were fabricated by nitridation of Ta 2 O 5 nanorod arrays grown via a through-mask anodization method. The fabrication process is illustrated in Scheme 1. PAA mask was firstly formed on top of a tantalum (Ta) substrate by anodizing an evaporated aluminum (Al) layer. Ta 2 O 5 nanorods were then embedded into the nanochannels of the PAA mask by anodizing the Ta substrate through the PAA mask. Owing to the low solubility of Ta 5 + in the H 3 BO 3 solution and the volume expansion associated with the anodization of Ta into Ta 2 O 5 , the Ta 2 O 5 was filled into the nanochannels of the PAA mask under a high electric field. [26–28] Vertically aligned Ta 2 O 5 nanorod arrays were obtained with the through-mask anodization method (see Figure S1 in Supporting Information) and transformed into Ta 3 N 5 nanorod arrays in a consequent nitridation step. The fact that PAA masks can be fabricated with Adv. Mater. 2012, DOI: 10.1002/adma.201202582