nanomaterials Article Influence of Colloidal Au on the Growth of ZnO Nanostructures Frank Güell 1 , Andreu Cabot 2,3 , Sergi Claramunt 1 , Ahmad Ostovari Moghaddam 4 and Paulina R. Martínez-Alanis 1, *   Citation: Güell, F.; Cabot, A.; Claramunt, S.; Moghaddam, A.O.; Martínez-Alanis, P.R. Influence of Colloidal Au on the Growth of ZnO Nanostructures. Nanomaterials 2021, 11, 870. https://doi.org/10.3390/ nano11040870 Academic Editor: Sotirios Baskoutas Received: 26 February 2021 Accepted: 20 March 2021 Published: 29 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 ENFOCAT-IN2UB, Universitat de Barcelona (UB), C/Martí i Franquès 1, 08028 Barcelona, Catalunya, Spain; frank.guell@ub.edu (F.G.); sclaramunt@el.ub.edu (S.C.) 2 Catalonia Institute for Energy Research (IREC), Sant Adrià de Besòs, 08930 Barcelona, Catalunya, Spain; acabot@irec.cat 3 ICREA, Pg. Lluis Companys 23, 08010 Barcelona, Catalunya, Spain 4 Department of Materials Science, Physical and Chemical Properties of Materials, South Ural State University, Lenin Ave. 76, 454080 Chelyabinsk, Russia; ostovary@aut.ac.ir * Correspondence: paulina.martinez@ub.edu Abstract: Vapor-liquid-solid processes allow growing high-quality nanowires from a catalyst. An al- ternative to the conventional use of catalyst thin films, colloidal nanoparticles offer advantages not only in terms of cost, but also in terms of controlling the location, size, density, and morphology of the grown nanowires. In this work, we report on the influence of different parameters of a colloidal Au nanoparticle suspension on the catalyst-assisted growth of ZnO nanostructures by a vapor-transport method. Modifying colloid parameters such as solvent and concentration, and growth parame- ters such as temperature, pressure, and Ar gas flow, ZnO nanowires, nanosheets, nanotubes and branched-nanowires can be grown over silica on silicon and alumina substrates. High-resolution transmission electron microscopy reveals the high-crystal quality of the ZnO nanostructures obtained. The photoluminescence results show a predominant emission in the ultraviolet range corresponding to the exciton peak, and a very broad emission band in the visible range related to different defect recombination processes. The growth parameters and mechanisms that control the shape of the ZnO nanostructures are here analyzed and discussed. The ZnO-branched nanowires were grown spontaneously through catalyst migration. Furthermore, the substrate is shown to play a significant role in determining the diameters of the ZnO nanowires by affecting the surface mobility of the metal nanoparticles. Keywords: ZnO nanostructures; Au nanoparticles; VLS; solvents; thermal dewetting 1. Introduction ZnO is a direct band-gap (3.37 eV) semiconductor having a large exciton binding energy (60 meV). It is widely used in numerous applications due to the abundance and low cost of its constituent elements and its unique semiconducting, optoelectronic and piezoelectric properties [1]. These functional properties of ZnO can be enhanced at the nanoscale through the growth of nanostructures such as nanotubes (NTs), nanosheets (NSs), and nanowires (NWs) [2]. ZnO NWs show efficient third harmonic UV generation [3] and are valuable materials not only for future nanoscale sensors [4], but also for optoelectronic devices such as solar cells [5]. For all these applications, the availability of ZnO NWs with high-crystal quality is mandatory [6]. ZnO NWs can be synthesized using different approaches such as solution growth [7], wet chemical methods [8], template-induced growth [9] and vapor transport [10]. The vapor- transport method allows growing high-quality single-crystal ZnO NWs by the vapor-liquid- solid (VLS) process using Au as a catalyst. In this process, an Au thin layer deposited over a substrate is annealed to generate Au clusters that act as a catalyst for the ZnO NW growth. The control over the ZnO NW morphology comes from the layer thickness that determines the Au cluster size after the thermal annealing [11]. Nanomaterials 2021, 11, 870. https://doi.org/10.3390/nano11040870 https://www.mdpi.com/journal/nanomaterials