Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat The geometry of free-standing titania nanotubes as a critical factor controlling their optical and photoelectrochemical performance Jakub Wawrzyniak a , Katarzyna Grochowska a, ⁎ , Jakub Karczewski b , Piotr Kupracz a , Jacek Ryl c , Anna Dołęga d , Katarzyna Siuzdak a a Centre for Plasma and Laser Engineering, The Szewalski Institute of Fluid-Flow Machinery, Polish Academy of Sciences, Fiszera 14 St., 80-231 Gdańsk, Poland b Faculty of Applied Physics and Mathematics, Gdańsk University of Technology, Narutowicza 11/12 St., 80-233 Gdańsk, Poland c Department of Electrochemistry, Corrosion and Materials Engineering, Faculty of Chemistry, Gdańsk Univeristy of Technology, Narutowicza 11/12 St., 80-233 Gdańsk, Poland d Department of Inorganic Chemistry, Faculty of Chemistry, Gdańsk University of Technology, Narutowicza 11/12 St., 80-233 Gdańsk, Poland ARTICLE INFO Keywords: Spaced titania nanotubes Photoactivity Tauc plot Flat band position Surface trapped holes ABSTRACT Titanium dioxide nanotubes are regarded as one of the most important functional materials and due to their unique electronic properties, chemical stability and photocorrosion resistance, they fnd applications in, for example, highly efcient photocatalysis or perovskite solar cells. Nevertheless, modifcation of TiO 2 nanotubes is required to overcome their main drawback, i.e. large energy bandgap (> 3.2 eV) limiting their ability to capture solar light. In this work, we report the changes in optical and photoelectrochemical properties of well-separated TiO 2 nanotubes that are tuned by varying the geometry of the material. The ordered tubular titania is formed via anodization in the presence of fuoride ions in diethylene glycol at elevated temperature. Length, inner diameter, wall thickness, and separation distance are described in function of synthesis parameters such as applied voltage and duration. The morphology and optical properties are characterized by means of scanning electron micro- scopy and UV–Vis spectroscopy techniques, respectively, while cyclic voltammetry, linear voltammetry and chronoamperometry are used to determine electrochemical/photoelectrochemical activity in diferent light conditions. The obtained results suggest a link between specifc surface area, the width of the band-gap, and photoactivity, each of which could be individually optimised via anodization conditions. Moreover, the beha- viour of the Mott-Schottky plot before and after 3 min of irradiation is studied indicating the positive shift of the fat band position and an increase in donor density values for all the obtained materials. The Mott-Schottky analysis was correlated with the linear voltammetry scans suggesting the important role of surface trapped holes. Presented in here results signifcantly supplement the current state-of-art regarding separated TiO 2 nanotubes that are considered as not fully investigated and unappreciated class of titania materials which due to the exposure of inner and outer wall can be used for further modifcations. 1. Introduction Today, titanium dioxide nanotubes (TiO 2 NTs) can be placed among the most researched nanomaterials and substrates used for further modifcation. So far, the attention has been put on studies regarding environmental protection [1], gas sensing [2,3], photocatalysis [4–6], and solar cells [7–10]. Due to their remarkable corrosion resistance [11], chemical stability, and developed surface area [12], the number of papers dedicated to them grows year by year. TiO 2 NTs can be syn- thesized hydrothermally [13], via atomic layer deposition of titania over the porous alumina template [14], or anodization of the titanium foil under particular conditions [15]. However, the electrochemical procedure stands out among other routes, because of its simplicity and quite low equipment requirements. To fabricate titania NTs, the ar- rangement of two electrodes placed opposite of each other and im- mersion in the electrolyte containing fuoride ions is used while the voltage applied between them stays as a driving force of the whole process. Such a synthesis enables the formation of the nanostructured material directly onto the conducting substrate and thus no further immobilization method is required to reach the form adequate for further characterization, especially for the electrochemical measure- ments. Particularly distinctive in comparison to other listed preparation routes, is that by adjusting electrolyte composition and temperature as well as process parameters - for example, applied voltage or the https://doi.org/10.1016/j.surfcoat.2020.125628 Received 29 December 2019; Received in revised form 18 February 2020; Accepted 11 March 2020 ⁎ Corresponding author. E-mail address: kgrochowska@imp.gda.pl (K. Grochowska). Surface & Coatings Technology 389 (2020) 125628 Available online 12 March 2020 0257-8972/ © 2020 Elsevier B.V. All rights reserved. T