Self-assembled hematite (a-Fe 2 O 3 ) nanotube arrays for photoelectrocatalytic degradation of azo dye under simulated solar light irradiation Zhonghai Zhang a , Md. Faruk Hossain b , Takakazu Takahashi a, * a Graduate School of Science and Engineering for Research, University of Toyama, 3190 Gofuku, Toyama 930-8555, Toyama, Japan b Graduate School of Science and Engineering for Education, University of Toyama, 3190 Gofuku, Toyama 930-8555, Toyama, Japan 1. Introduction In recent decades, photocatalytic (PC) technology has received significant attention for hydrogen generation by photoelectro- chemical water splitting and decomposition of harmful organic contaminants [1–6]. Since Fujishima and Honda first reported TiO 2 as photoanode for the photoelectrolysis of water [7], the TiO 2 materials have been widely researched for PC application. Whereas, the wide band gap (3.2 eV) of TiO 2 material limits its practical application because it only can be excited with light of wavelength near or shorter than 385 nm. The primary scientific and technological objective has been focused on exploring novel photocatalysts with high efficiency of utilization of solar irradia- tion, narrow band gap, operational stability, and preparation with relatively low cost [8]. Iron oxide (a-Fe 2 O 3 , hematite) is one of promising candidates for PC application due to its narrow band gap about 2.0–2.2 eV, which absorbs light up to 600 nm, collects up 40% of the solar spectrum energy, stability in most aqueous solution (pH > 3) and it maybe one of the cheapest semiconductor materials [9–11]. However, the drawbacks of a-Fe 2 O 3 materials, such as poor electron mobility, generally in the range of 0.01 cm 2 / Vs 4 to 0.1 cm 2 /V s 4 , which results the high electron–hole recombination rate, and short hole diffusion length (2–4 nm), are the major challenger for its practical application [12]. Recently, nanostructuring techniques have been proven useful in increasing the performance of a-Fe 2 O 3 for photoresponse [13]. The aligned nanotube has been considered the most suitable way to achieve larger enhancement of surface area without an increase of the geometric area, reduced the scattering of free electrons, and enhanced the electrons mobility, which could be expected to achieve higher PC activities. A variety of fabrication methods such as biomacromolecules template [14], AAO template [15,16], hydrothermal [17], surfac- tant-assisted solution method [18], and electrochemical anodiza- tion [19] have been used for the preparation of a-Fe 2 O 3 nanotubes (a-Fe 2 O 3 NTs). Among them, electrochemical anodization method shows advantages of easily controllable pore size, good uniformity, and conformability over large areas at low coat. Because of significant photoelectrochemical performance, the formation of a-Fe 2 O 3 NTs with high surface area has been paid much interest by a number of researchers. Misra and co-workers [20] have first fabricated ultrathin a-Fe 2 O 3 nanotubes by sonoelectrochemical anodization method for water photooxida- tion. Grimes and co-workers [21] have discussed temperature dependent growth of a-Fe 2 O 3 NTs by anodization method and prepared this nanomaterial at high temperature. However, to our best knowledge, there has been no report regarding application of Applied Catalysis B: Environmental 95 (2010) 423–429 ARTICLE INFO Article history: Received 8 November 2009 Received in revised form 14 January 2010 Accepted 20 January 2010 Available online 25 January 2010 Keywords: Hematite (a-Fe 2 O 3 ) Anodization Nanotubes Photoelectrochemical Photoelectrocatalytic ABSTRACT Self-assembly aligned hematite (a-Fe 2 O 3 ) nanotube arrays (a-Fe 2 O 3 NTs) were successfully prepared on the Fe foils by a simple two-step electrochemical anodization method in NH 4 F organic electrolyte. The a- Fe 2 O 3 NTs electrodes were characterized by field-emission scanning electron microscopy, energy dispersive X-ray spectroscopy, grazing incidence X-ray diffraction, UV–vis absorbance spectra, and X-ray photoelectron spectroscopy. The resulting a-Fe 2 O 3 NTs showed a pore diameter of 40 nm, thickness of 2 mm, and a minimum wall thickness of 10 nm. The systematic photoelectrochemical responses on the a-Fe 2 O 3 NTs electrodes were presented. The maximum photoconversion efficiencies of 0.51% and 0.60% were collected at 0.3 V under illumination of visible light and simulated solar light (AM 1.5G), respectively. The photoelectrocatalytic (PEC) and photocatalytic (PC) activities of the a-Fe 2 O 3 NTs electrodes were evaluated by degradation of azo dye. The significant PEC and PC performance indicated that the a-Fe 2 O 3 NTs electrodes were an effective photoelectrode under visible light and simulated solar light illumination. ß 2010 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +81 76 445 6716; fax: +81 76 445 6716. E-mail addresses: zhonghaizhangwill@gmail.com (Z. Zhang), takahash@eng.u-toyama.ac.jp (T. Takahashi). Contents lists available at ScienceDirect Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb 0926-3373/$ – see front matter ß 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2010.01.022