Fabrication of erbium-doped spherical-like ZnO hierarchical nanostructures with enhanced visible light-driven photocatalytic activity Jin-Chung Sin, Sze-Mun Lam, Keat-Teong Lee, Abdul Rahman Mohamed n School of Chemical Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Pulau Pinang, Malaysia article info Article history: Received 3 September 2012 Accepted 16 September 2012 Available online 26 September 2012 Keywords: Semiconductors Nanocrystalline materials Microstructure Optical materials and properties abstract Erbium-doped spherical-like ZnO hierarchical nanostructures (Er/ZnO) were newly synthesized via a facile and surfactant-free chemical solution route. Field emission scanning electron microscopy and transmission electron microscopy observations showed that the assemblies were composed of large amounts of interleaving nanosheets with the thickness of about 15 nm. X-ray diffraction and energy dispersive X-ray results revealed that Er ion was successfully doped into ZnO. The Er doping increased the visible light absorption and a red shift appeared in UV–vis diffuse reflection spectra for Er/ZnO. Photoluminescence spectra showed that the improvement of visible emission in the Er/ZnO was a result of oxygen vacancy of defect increased by addition of Er in ZnO. The photocatalytic tests revealed that Er/ZnO was a new promising photocatalyst in remediation of water contaminated by organic pollutants under visible light irradiation. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Nowadays, ‘‘green life’’ has been inspiring people to pay more and more attention to the removal of hazardous substances in the wastewater, especially organic pollutants. One of the best ways to eliminate the organic pollutants is the photocatalytic treatment. ZnO is a wide band gap (  3.3 eV) semiconductor which has garnered considerable interest because of its catalytic and photo- chemical properties along with its low cost. In nanostructure forms, self-assembly of ZnO nanoscaled building blocks including nanor- ods, nanowires and nanosheets into three-dimensional (3D) hier- archical architectures has been a research hotspot [1,2]. Owing to their outstanding optical, electronic and catalytic properties, for example, flower-like ZnO hierarchical architectures showed an enhanced photocatalytic performance compared with the mono- morphological nanostructures [1]. Hitherto, most of 3D ZnO nanos- tructures were developed via the surfactants or structure directing reagents assisted assembly mechanism, and self-assembly of nanos- caled ZnO building blocks into the 3D structured morphologies without any surfactants still remains an intricate challenge. Usually, the properties of ZnO nanostructures depend on their crystal structure, morphology, size and surface defects. It has been extensively proven that modifications of ZnO nanostructures such as doping of transition metals or rare earth metals could improve their properties. The photocatalytic properties of ZnO were sig- nificantly improved when modified with the incorporation of dopant ions [3]. The doping of metal ions in ZnO nanostructures can lead to effects such as enhancement/decrease in fluorescence and controlling concentration of surface defects. Moreover, ZnO-doped samples with a high concentration of oxygen defects will exhibit excellent photodegradation of organic pollutants [3]. Pure ZnO nanostructures doped with metal ions like Co, Pr, Sm, Tb, Ho and Tm have been reported in the literatures [2,3]. In fact, doping of ZnO nanostructures is a subject of broad interest for the past few years. To our knowledge, the application of Er 3 þ -doped ZnO nanostructure as a photocatalyst has not been reported yet. On the basis of the above considerations, this work reports the synthesis of Er-doped spherical-like ZnO hierarchical nanostruc- tures (Er/ZnO) by a simple chemical solution route without any organic solvent or surfactant. A possible explanation of the forma- tion of the hierarchical nanostructure was also presented. Further- more, the photocatalytic activities of the products were studied by degrading the phenol in water under visible light irradiation. 2. Results and discussion Fig. 1(a) is the XRD patterns of the as-synthesized products. All the diffraction peaks can be indexed as the hexagonal wurtzite ZnO structure and no peaks were detected from any other impurities. The sharp and narrow peaks showed that the products obtained to be well in a crystallized form. The diffraction peaks of Er/ZnO shifted slightly toward the lower angle, inferring that the lattice parameter was a little larger than those of pure ZnO. The lattice parameter (c ¼ 5.22030 nm) of the Er/ZnO increased com- pared with the pure ZnO (c ¼ 5.20540), indicating the substitution Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters 0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.09.049 n Corresponding author. Tel.: þ60 4599 6410; fax: þ60 459 41013. E-mail address: chrahman@eng.usm.my (A.R. Mohamed). Materials Letters 91 (2013) 1–4