Double-nucleation hydrothermal growth of dense and large-scale ZnO nanorod arrays with high aspect ratio on zinc substrate for stable photocatalytic property Jing Zhang, Yanjie Su, Hao Wei, Jian Wang, Chao Zhang, Jiang Zhao, Zhi Yang, Maojie Xu, Liling Zhang, Yafei Zhang n Key Laboratory for Thin Film and MicrofabricationTechnology of the Ministry of Education, Institute of Micro/Nanometer Science and Technology, Shanghai Jiao Tong University, Shanghai 200240, China article info Article history: Received 27 April 2013 Accepted 29 May 2013 Available online 6 June 2013 Keywords: ZnO nanorod Nanocrystalline materials Nucleation Crystal growth Hydrothermal synthesis Photocatalytic property abstract We have successfully fabricated dense and large-scale ZnO nanorod (NR) arrays by hydrothermal growth in Zn-NH 3 Á H 2 O system by the addition of zinc nitrate. The characterizations demonstrate that the as- prepared sample has a large dimension of ZnO NR arrays with a density of ∼105/μm 2 and an aspect ratio of ∼100. It is supposed that the high density is attributed to the double-nucleation phenomenon related to the zinc salt solution and oxidation of substrate, and the high aspect ratio relies on sufficient zinc source for c-axis growth. A possible growth mechanism was proposed. The photocatalytic studies of the ZnO NR arrays displayed high photocatalytic efficiency and stability. This facile strategy for the preparation of discretionary dimensions of ZnO NR arrays can be applicable to the photocatalytic field for manifold cycles. & 2013 Elsevier B.V. All rights reserved. 1. Introduction As a well-known photocatalyst, ZnO such as commercial ZnO or nanostructured ZnO powder has attracted great attention in the complete degradation of environmental organic pollutants, due to which is one of the significant functional oxide semiconductors for its direct wide band gap (3.37 eV) and large exciton binding energy (60 meV). High surface area may be one of the crucial roles for the enhancement of photocatalytic activities [1]. Never- theless, it makes no sense if the conventional ZnO photocatalyst cannot be reused or separated from the reacted aqueous suspen- sion unless the recyclability and separation ability are taken into account. Preparation of large-scale well-aligned ZnO nanostruc- ture films with high surface area (high aspect ratio for 1D nanomaterials), high stability and convenient reuse will make it possible to overcome this shortcoming and to scale up the industrial applications in environmental manipulation. Among numerous methods for the growth of large-scale ZnO nanoarrays, hydrothermal synthetic methods are more attractive and exten- sively employed owing to their simplicity, easy regulation, low temperature and low cost [2–4]. However the regular ideas for hydrothermal synthesis of ZnO nanoarrays rely on either direct oxidation of zinc foil in amino aqueous solution in which slow oxidation rate limits the length as well as aspect ratio of the products [5], or the aqueous thermal decomposition of Zn 2+ amino complex depositing onto a crystal seed layer in which the seed layer is expected to be of high quality [6,7]. It is still a great challenge to directly fabricate large-scale ZnO nanoarrays with high aspect ratio via a convenient hydrothermal process. Herein, we report a simple hydrothermal process for preparing dense and large-scale ZnO NR arrays with high aspect ratio in a Zn-NH 3 Á H 2 O system with the addition of Zn(NO 3 ) 2 Á H 2 O. The photocatalytic activities and stability are also studied by degrading methyl blue (MB) aqueous solution under UV light irradiation. 2. Experimental section The flowchart of ZnO nanoarray preparation was shown in Fig. 1a. One cleaned zinc foil substrate (7 Â 30 Â 0.015 cm 3 ) was curled into a roll and immersed in 150 mL of 0.02 mmol Zn (NO 3 ) 2 Á 6H 2 O and 10% NH 3 Á H 2 O (v/v) aqueous solution in a 200 mL Teflon-lined autoclave followed by 12 h heating at a constant temperature of 95 1C. They were then rinsed with deionized water and dried with N 2 for further characterization. The surface morphology of the products was determined by a FESEM (Zeiss Ultra55, Germany). The crystal structure identifica- tion was performed by XRD (D8 ADVANCE, Germany) with Cu Ka Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters 0167-577X/$- see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.05.134 n Corresponding author. Tel./fax: +86 21 3420 5665. E-mail address: yfzhang@sjtu.edu.cn (Y. Zhang). Materials Letters 107 (2013) 251–254