Fabrication of 3D structured ZnO nanorod/reduced graphene oxide hydrogels and their use for photo-enhanced organic dye removal Van Hoang Luan, Huynh Ngoc Tien, Seung Hyun Hur ⇑ School of Chemical Engineering, University of Ulsan, Daehak-ro 93, Nam-gu, Ulsan 680-749, South Korea article info Article history: Received 11 June 2014 Accepted 30 August 2014 Available online 16 September 2014 Keywords: ZnO nanorods Reduced graphene oxide hydrogel Methylene blue abstract Hybrid 3-dimensional (3D) structures composed of zinc oxide (ZnO) nanorods and reduced graphene oxide hydrogel (rGOH) were fabricated by chemical reaction between Zn ions and GO followed by in-situ lateral growth of ZnO nanorods using Zn ions as seed points. The 3D networked ZnO nanorod–rGOH (ZNR–rGOH) fabricated in this study exhibited excellent methylene blue (MB) removal efficiency due to efficient physical adsorption of dye molecules because of electrostatic attractive forces and enhanced photocatalytic activity by the laterally grown ZnO nanorods. The Langmuir–Hinshelwood rate constant of ZNR–rGOH was 4-fold higher than that of pristine rGO due to the enhanced photocatalytic effects obtained by incorporating laterally grown ZnO nanorods inside the rGOH network. Ó 2014 Elsevier Inc. All rights reserved. 1. Introduction Zinc oxide (ZnO) nanostructures are used in gas sensors, ultra- violet (UV) photodiodes, UV lasers, solar cells, and photocatalysts because of their high surface to volume ratio and their unique physical and optical properties such as a high excitation binding energy, a wide direct band gap, high piezoelectricity, and high pho- tocatalytic efficiency [1–7]. Graphene nanosheets, which contain a single layer of carbon atoms with a two-dimensional (2D) honeycomb structure, have been extensively studied in photocatalysis as supporting materials for metal oxide photocatalysts due to their excellent electronic conductivity, superior chemical stability, and high specific surface area [8–11]. Recent studies have shown that when one-dimen- sional (1D) ZnO nanorods are hybridized with other materials such as graphene, metal nanoparticles, and semiconducting metal oxi- des, the electrical and electrochemical performances of sensors or photocatalysts can be enhanced due to effective charge transfer between ZnO and other materials [12]. Under UV illumination with a phonon energy higher energy than that of the band gap of ZnO nanorods, excitation of electrons from the valence band (VB) to the conduction band (CB) leaves a positive hole in the VB and results in electron–hole pairs, which can remove organic dyes such as methylene blue (MB) by redox reaction. Generally, the photocat- alytic activity of pure ZnO is very low due to fast recombination of electron–hole pairs. However, when ZnO nanorods are in contact with a conductive material such as graphene, which has a lower work function than the conduction band of ZnO, effective charge transfer from ZnO to graphene hinders the recombination of elec- tron–hole pairs, which results in enhanced degradation of MB [12]. Removal of dye-containing wastewater is challenging, because organic dyes are resistant to simple UV and chemical treatments [13]. Hybrid structures composed of ZnO nanorods and chemically crosslinked three-dimensional (3D) graphene oxide (GO) may be candidates for effective removal of organic dye due to (i) effective adsorption afforded by a large surface area and strong interactions between the GO and dye molecules, (ii) enhanced photocatalytic activity resulting from effective charge transfer from ZnO nanorods to 3D GO [14]. In this report, we fabricated hybrid structures composed of ZnO nanorods and 3D reduced GO hydrogel (rGOH) by a simple in-situ hydrothermal process. Initially, a high surface area reduced GO hydrogel (rGOH) was fabricated by crosslinking Zn 2+ ions and the functional groups of GO, then ZnO nanorods were grown in-situ inside the 3D rGOH network by hydrothermal synthesis using Zn 2+ ions as seeds for ZnO nanorods. GOH fabricated by Zn 2+ ions in this study had a highly developed 3D network structure. The MB removal efficiency of rGOH containing laterally grown ZnO nanorods was much higher under both UV and visible light illumination than that of the original GOH, which also contained a small amount of ZnO nanoparticles, because of the better crystal structures and enhanced light absorption of the former hybrid structure. This type of multi- functional hybrid material can be effectively used in many applica- tions such as solar cells and reactive chemical adsorption under visible light due to its enhanced electrical and optical properties [15–19]. http://dx.doi.org/10.1016/j.jcis.2014.08.071 0021-9797/Ó 2014 Elsevier Inc. All rights reserved. ⇑ Corresponding author. E-mail address: shhur@ulsan.ac.kr (S.H. Hur). Journal of Colloid and Interface Science 437 (2015) 181–186 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis