Journal of Hazardous Materials 183 (2010) 754–758 Contents lists available at ScienceDirect Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat Microwave-assisted hydrothermal synthesis of N-doped titanate nanotubes for visible-light-responsive photocatalysis Yen-Ping Peng a , Shang-Lien Lo a,∗ , Hsin-Hung Ou a,b , Shiau-Wu Lai c a Research Center for Environmental Pollution Prevention and Control Technology, Graduate Institute of Environmental Engineering, National Taiwan University, 71 Chou-Shan Rd., Taipei 106, Taiwan b W.M. Keck Laboratory, California Institute of Technology, Pasadena, CA 91125, USA c Department of Chemical Engineering and Materials Science, Yuan-Ze University, Taiwan article info Article history: Received 31 May 2010 Received in revised form 20 July 2010 Accepted 20 July 2010 Available online 30 July 2010 Keywords: Titanate nanotubes Nitrogen doped Visible-light Photocatalysis abstract This study employs a rapid, energy frugal and environmental friendly method to synthesize nitro- gen doped titanate nanotubes (NTNTs), and uses TEM, XRD, Raman, nitrogen adsorption–desorption isotherms analysis, and UV–vis spectroscopy to characterize the obtained NTNTs. TEM results demon- strate that the current research successfully synthesized one-dimensional NTNTs via the microwave hydrothermal (M-H) method, and show that NTNTs retain a tubular structure after sintering at a tem- perature of 350 ◦ C. XRD results agree well with Raman spectrum findings. Both show that the intensity of anatase crystallization increases with an increase in sintering temperature. After sintering at high temperature, above 250 ◦ C, the UV–vis absorbance edges of NTNTs significantly shift to the visible-light region, which illustrates N atom doping into nanotubes. Photocatalytic tests conclude that the NTNTs-350 shows good efficiency with visible-light response. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Research has intensively investigated titanate nanotubes (TNTs) due to one-dimensional nanostructure, large internal and exter- nal surfaces, ion exchangeability, and photocatalytic activity. These advantages lead to various applications such as solar cells, photo- catalysis, and electroluminescent devices [1–5]. However, the TNTs high band gap between 3.3 eV and 3.87 eV obstructs application [6]. In other words, only UV light, which occupies less than 5% of the solar spectrum, excites TNTs. Therefore, studies have made great attempts to extend the absorption of TNTs to visible-light- sensitization. Particle size [7,8], manipulation of oxygen vacancy [9,10], and doping are the major ways to engineer the TiO 2 band gap. Some of the methods have used various transition metals [11–13] and anions such as F, S, C, and N [14–17] as dopants for increasing the photocatalytic efficiency of TiO 2 under visible- light. Only a few studies have focused on TNTs band gap reduction [18–23]. In 2004, Tokudome and Miyauchi [23] first examined the visible-light activity of N-doped TNTs. Geng et al. [24] proposed that instead of substituting O 2 - ions in the TNTs lattice, the dop- ing N is likely located at the interstitial sites. Jiang et al. [18], Wu et al. [20], and Qamar et al. [19] successfully synthesized N-doped ∗ Corresponding author. Tel.: +886 2 23625373; fax: +886 2 23928830. E-mail address: sllo@ntu.edu.tw (S.-L. Lo). TNTs, C-doped TNTs, and Ni-TNTs, to reduce TNTs band gap and shift absorbance to the visible-light region. Since the first discovery of TNTs [25,26], many methods have attempted to synthesize TNTs including the sol–gel process [27], electrochemical anodic oxidation [28–30], and hydrothermal treat- ment [5,31–34]. Among these methods, hydrothermal treatment has attracted much attention owing to cost-effectiveness and con- venience to synthesize TNTs with excellent morphology. However, the typical hydrothermal procedure, synthesized at 110–130 ◦ C in autoclave for 20–120 h, is relatively time- and energy-consuming. A previous study used a novel method, modified from the tradi- tional hydrothermal method, with microwave assistance, namely microwave hydrothermal treatment (M-H treatment), to synthe- size TNTs [2]. M-H treatment has several advantages such as shorter reaction time, lower energy usage, and enhancing the wall- structure intensity of TNTs. Previous study successfully synthesized TNTs under 400W irradiation at 130 ◦ C for only 1.5 h with S BET val- ues of 256 m 2 g -1 and found the TNTs preferentially assigned for Na x H 2-x Ti 3 O 7 , with a vague rutile phase and no clear anatase phase. Unfortunately, similar to the traditional hydrothermal method, the TNTs prepared by the M-H method showed weak photocatalytic activity. This study dopes nitrogen into TNTs via M-H treatment for the first time to seek an expeditious, energy saving way to synthesize nitrogen doped tatanate nanotubes (NTNTs), excited under visible- light. 0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2010.07.090