Regular article Growth of ZnO nanorods on the surface and edges of a multilayer graphene sheet Faramarz Hossein-Babaei ⁎, Mehdi Akbari-Saatlu Electronic Materials Laboratory, Electrical Engineering Department, K. N. Toosi University of Technology, Tehran 16317-14191, Iran abstract article info Article history: Received 5 May 2017 Accepted 14 June 2017 Available online xxxx The spray pyrolysis of ZnCl 2 solutions on the highly oriented pyrolytic graphite (HOPG) substrates is investigated, and the growth of symmetrically faceted ZnO nanorods on the edges of multilayer graphene sheets is demon- strated. Profound lattice mismatch between the basal plane of wurtzite ZnO and graphene and the selective ox- ygen adsorption on HOPG render the graphene edges the preferred nucleation sites. A multilayer graphene sheet integrates with the edge-nucleated ZnO crystallite growing on both of its sides, resulting in an excellent graphene/ZnO electrical contact. Substrate temperature, within the 200–450 °C range, is the most influential pa- rameter in determining the nanorods morphology. © 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Two-dimensional materials HOPG Graphene ZnO nanorods Spray pyrolysis The controlled growth of metal oxide semiconductor nanorods on two-dimensional electronic materials would form three-dimensional structures suitable for various considerations [1,2]. Of particular interest are the structures shaped by ZnO nanorods grown on graphene-based substrates, whose formation with different growth techniques has been the subject of independent investigations [3,4,5]. In its polycrystal- line form, zinc oxide is widely used for various applications in the fields of chemical sensing [6], energy harvesting [7], and optoelectronics [8]. More recently, ZnO nanorods have been considered for the fabrication of highly sensitive environmental monitoring devices which allow harmful gas detection in air at ppb concentration levels [6,9]. Such de- vices are made by depositing nanostructured ZnO layers on insulating substrates and providing metallic contacts to the ZnO nanostructure for response measurements [9]. The structure of the substrate in atomic scale and the utilized growth technique are both influential in determining the morphology of the nanocrystallites and the nanostructure of the grown ZnO layer. Seeded growth is possible on amorphous substrates [10], but wurtzite zinc oxide nanorods can readily grow on substrates with hexagonal crystal structure of close to matching basal plane structures [11]. Graphene- based two-dimensional transparent conductors have been shown to provide electrical contacts of ohmic quality to ZnO [12] and, hence, a graphene-coated substrate would simultaneously facilitate the seedless growth of nanorods as well as a good quality transparent back contact to the grown crystallites. The envisaged structure would be advantageous for many optoelectronic device fabrications. ZnO nanorods have been grown on graphene-coated or graphite- structured substrates by different growth techniques: hydrothermal growth has resulted in nanorods of about 45 nm diameter [13]; nano- rods in a wide diameter range have been electrochemically deposited from different aqueous zinc nitrate electrolytes [14]; and ZnO layers consisting of 20 nm diameter nanowires have been produced using cat- alyst-free metal–organic chemical vapor deposition [15]. Wet methods, however, are generally slow and prone to impurity-caused instabilities which would hinder reproducible device production in industrial scale. The ultrasonic spray pyrolysis (USP) has been utilized for growing different metal oxide semiconductor layers with growth rates as high as 100 nm/min [16]. Moreover, USP facilitates some degrees of control on the microstructure and morphology of the grown layers [16,17]. The obvious advantages of this deposition technique have been demon- strated for ZnO and SnO 2 deposition on silica and alumina substrates [16–19]. USP and a number of other chemical vapor deposition-based methods have also been utilized for ZnO deposition on the graphene monolayer-coated oxidized silicon substrates, resulting in nanowires [20] and nanoneedles [21] with ~20 nm diameters [20–22]. The method has not yet been applied for metal oxide deposition on graphite sub- strates. Here, we report growing hexagonally facetted ZnO nanorods on the substrates made of highly oriented pyrolytic graphite (HOPG) and the self-supporting multilayer graphene sheets cleaved from the HOPG slabs, and demonstrate that the layer edges are the preferred ZnO crystallite nucleation sites. Some degree of control on the morphol- ogy and population of the crystallites are exerted by the accurate mon- itoring of the growth conditions. The USP system utilized is homemade; the structural details of the system are given in reference [16]. The precursor is ZnCl 2 (108816, Merck, Germany) solution in ethanol (99.7% v/v pure) at 0.2 mol/L. Scripta Materialia 139 (2017) 77–82 ⁎ Corresponding author. E-mail address: fhbabaei@kntu.ac.ir (F. Hossein-Babaei). http://dx.doi.org/10.1016/j.scriptamat.2017.06.025 1359-6462/© 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat