Contents lists available at ScienceDirect Materials Science & Engineering B journal homepage: www.elsevier.com/locate/mseb High aspect-ratio semiconducting ZnO nanowires formed by anodic oxidation of Zn foil and thermal treatment Leszek Zaraska a, ⁎ , Krystyna Mika a , Katarzyna E. Hnida b , Marta Gajewska b , Tomasz Lojewski c , Marian Jaskula a , Grzegorz D. Sulka a a Department of Physical Chemistry and Electrochemistry, Faculty of Chemistry, Jagiellonian University in Krakow, Gronostajowa 2, 30-387 Kraków, Poland b AGH University of Science and Technology, Academic Centre for Materials and Nanotechnology, A. Mickiewicza 30, 30-059 Krakow, Poland c AGH University of Science and Technology, Faculty of Materials Science and Ceramics, A. Mickiewicza 30, 30-059 Krakow, Poland ARTICLE INFO Keywords: Zinc oxide Nanowires Anodization Bicarbonates Annealing ABSTRACT Arrays of zinc oxide (ZnO) nanowires with diameter of about 200 nm and length of > 20 μm were successfully obtained by simple anodic oxidation of Zn foil in sodium bicarbonate electrolyte and thermal post-treatment. The as formed anodic film consists of overlapping nanowire bundles forming a flower-like or grass-like structure. Thermal annealing in air at temperatures higher than 150 °C results in the conversion of initially formed hy- droxycarbonate precursor to crystalline wurtzite ZnO. The crystallinity of ZnO nanowires increases with in- creasing annealing temperature. On the other hand, optical band-gap energy of obtained semiconducting na- nowires was found to be independent of the temperature applied during the thermal treatment. It is expected that the described method can be further scaled up and offers a great potential even for technological appli- cations. 1. Introduction Due to many promising properties such as high exciton binding energy (60 meV) and high electron mobility, zinc oxide (ZnO) a wide band gap (3.37 eV) n-type semiconductor has received a considerable attention as a promising material for various applications including photovoltaics [1], photocatalysis [2], light emitting diodes [3], sensing devices [4] and many others [5]. Moreover, one-dimensional (1D) na- nostructures such as nanowires or nanotubes seem to be the most promising morphologies offering enhanced electron transport efficiency and an extremely high surface-to-volume ratio [6]. Therefore, many scientificefforts are focused on the development of simple and cost- effective methods for the large-scale fabrication of ZnO nanowires. Among many physical and chemical strategies that have been already reported [7], electrochemical methods have received some attention as they are relatively cheap, fast and do not require any sophisticated equipment and complicated procedures. Recently, anodic oxidation of metallic Zn has been also proposed as an efficient method for the rapid synthesis of ZnO nanowires [8–12] or even nanotubes [11,12]. Here, we present the fabrication of ZnO nanowire arrays by simple anodization of Zn foil in sodium bicarbonate electrolyte followed by thermal treatment in air. Despite some recent papers provide a com- prehensive analysis of the effect of anodizing conditions on the anodic formation of nanowires on Zn in similar electrolytes [9,13–16] a de- tailed and systematic inspection of the effect of post-treatment tem- perature on the morphology and properties of anodic films is still missing. Therefore, a special emphasis is put on characterization of the composition, crystallinity and semiconducting character of synthesized materials as a function of the annealing temperature. It is believed that obtained nanostructures will be promising candidates for many prac- tical applications including photocatalysis, photovoltaics, and sensing devices. 2. Experimental Zn foil (purity 99.95%, Goodfellow) was used as a starting material. Prior to anodization specimens with dimensions of about 2.5 cm × 1.5 cm were electrochemically polished in a vigorously stirred mixture of 85 wt% phosphoric acid and ethanol (1:2 vol) at the constant potential of 10 V for 10 min. The temperature of the electro- lyte was 5 °C. Then, as prepared samples were anodized in 5 mM NaHCO 3 electrolyte at the potential of 10 V. All anodizations were carried out at room temperature in a home-made Teflon cell (for details see Ref. [17]) with the Zn substrate placed directly on a conductive support and a Pt grid working as a cathode. After anodization, samples were rinsed several times with deionized water and dried in the stream http://dx.doi.org/10.1016/j.mseb.2017.09.003 Received 5 June 2017; Received in revised form 31 August 2017; Accepted 4 September 2017 ⁎ Corresponding author. E-mail address: zaraska@chemia.uj.edu.pl (L. Zaraska). Materials Science & Engineering B 226 (2017) 94–98 0921-5107/ © 2017 Elsevier B.V. All rights reserved. MARK