Improving Morphological Quality and Uniformity of Hydrothermally Grown ZnO Nanowires by Surface Activation of Catalyst Layer

Murillo et al. Nanoscale Research Letters (2017) 12:51 DOI 10.1186/s11671-017-1838-x NANO EXPRESS Open Access Improving Morphological Quality and Uniformity of Hydrothermally Grown ZnO Nanowires by Surface Activation of Catalyst Layer Gonzalo Murillo1*, Helena Lozano1, Joana Cases-Utrera1, Minbaek Lee2 and Jaume Esteve1 Abstract This paper presents a study about the dependence of the hydrothermal growth of ZnO nanowires (NWs) with the passivation level of the active surface of the Au catalyst layer. The hydrothermal method has many potential applications because of its low processing temperature, feasibility, and low cost. However, when a gold thin film is utilized as the seed material, the grown NWs often lack morphological homogeneity; their distribution is not uniform and the reproducibility of the growth is low. We hypothesize that the state or condition of the active surface of the Au catalyst layer has a critical effect on the uniformity of the NWs. Inspired by traditional electrochemistry experiments, in which Au electrodes are typically activated before the measurements, we demonstrate that such activation is a simple way to effectively assist and enhance NW growth. In addition, several cleaning processes are examined to find one that yields NWs with optimal quality, density, and vertical alignment. We find cyclic voltammetry measurements to be a reliable indicator of the seed-layer quality for subsequent NW growth. Therefore, we propose the use of this technique as a standard procedure prior to the hydrothermal synthesis of ZnO NWs to control the growth reproducibility and to allow high-yield wafer-level processing. Keywords: ZnO nanowires, Gold catalyst layers, Hydrothermal growth, Cyclic voltammetry, Electrochemistry Background In most environments regardless of human endeavors, it is not difficult to find sources of residual energy which are being usually wasted, e.g., light, heat, or motions. The conversion of such ambient energy into electricity is of great interest, so that energy harvesting research field has been drawing special attentions in past years [1, 2]. Aside from other energy sources, mechanical energy present in various forms such as vibrations, random motions, human movements, or noise. The most common transduction methods to convert this mechanical energy into electricity are electrostatic, electromagnetic, triboelectric, and piezoelectric. In particular, a piezoelectric material has the ability of creating an inherent electric field when strained (i.e., direct piezoelectric effect). * Correspondence: 1 Department of Nano and Microsystems, Instituto de Microelectrónica de Barcelona (IMB-CNM, CSIC), 08193 Bellaterra, Spain Full list of author information is available at the end of the article Nowadays, there are several examples of commonly used piezoelectric materials such as AlN, PZT, ZnO, PVDF, or quartz. Recently, ZnO has become very popular due to the extensive diversity of nanostructures that can exhibit. In addition, this material has the property of being a semiconductor, piezoelectric, and direct bandgap material which makes possible a huge range of applications [3, 4]. One of the most useful nanostructures that can be utilized to generate energy is the nanowire (NW) because of the simple alignment of its crystal axis during the growth process [5, 6]. Power generators based on these nanostructures are commonly called nanogenerators, and they have the advantages of being more flexible and less sensitive to fracture than generators based on thin films [7]. It has already been demonstrated that a single ZnO NW can generate a piezoelectric potential along its length in response to a 5-nN force applied by the tip of an atomic force microscope (AFM) [8]. The energy output generated by one NW in one discharge © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Murillo et al. Nanoscale Research Letters (2017) 12:51 event was calculated to be 0.05 fJ. The ZnO NWs can be grown via different methods and on various substrates, but a crystalline substrate with a similar lattice constant is the best choice to obtain aligned NWs with high quality [9, 10]. Currently, there are numerous bottom-up approaches to grow ZnO nanostructures, such as vapor–solid and vapor–liquid–solid processes or electrochemical deposition [6, 11, 12]. However, these methods require high temperatures and pressures, conductive substrates, or acid-resistant environments that make them difficult to be integrated with standard fabrication processes and future flexible electronics. At the beginning, organometallic synthesis of ZnO nanoparticles in an alcoholic medium received wider acceptance and more attention because it offered faster nucleation and growth than in a water-based medium. However, there are several reports of hydrothermal synthesis in an aqueous medium in the literature. Baruwati et al. reported the aqueous synthesis of ZnO nanoparticles using zinc nitrate hexahydrate in an autoclave at 120 °C after adjusting the pH to 7.5 with ammonium hydroxide [13]. They obtained ZnO nanoparticles in powder form after washing and drying. Lu et al. successfully prepared crystalline ZnO powder through a hydrothermal process, using ammonia as the base source and Zn(NO3)2 as the Zn2+ ion source [14]. The effects of growth temperature and pH were studied, and their results showed that all of the zinc hydroxide precursors dissolved and formed a clear solution at pH > 11 to nucleate ZnO powder in a homogeneous solution. On the other hand, the zinc hydroxide precursors were partially dissolved at pH < 11, and the ZnO powder nucleated in a heterogeneous system with higher probability of nucleation. In this work, we utilized a hydrothermal method to grow c-axis-aligned NWs due to the combination of being inexpensive, simple, and with a mild processing temperature. This method is based on a chemical reaction that takes place directly on a silicon substrate covered by an Au catalyst layer in presence of an aqueous nutrient solution of zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and hexamethylenetetramine (HMTA) at mild temperatures (<80 °C) [15]. The growth time, processing temperature, and nutrient concentration have a direct influence on the resulting height and diameter of the growth NWs [16]. In the aqueous solution, Zn(NO3)2·6H2O provides Zn2+ ions required for the ZnO NWs, water provides O2− ions, and the thermal degradation of HMTA allows the release of hydroxyl ions to form ZnO by reacting with Zn2+ ions. This process can be summarized by Eqs. 1, 2, and 3 as follows: Page 2 of 8 ðCH2 Þ6 N4 þ 6H2 O↔6HC (...truncated)