Contents lists available at ScienceDirect Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat A comparative study of photocatalysis on highly active columnar TiO 2 nanostructures in-air and in-solution Muhammad Zubair Ghori, Salih Veziroglu, Bodo Henkel, Alexander Vahl, Oleksandr Polonskyi, Thomas Strunskus, Franz Faupel ⁎ , Oral Cenk Aktas ⁎ Institute for Materials Science – Multicomponent Materials, Faculty of Engineering, Christian-Albrechts-University of Kiel, Kaiserstraße 2, 24143 Kiel, Germany ARTICLE INFO Keywords: Photocatalysis TiO2 Methylene blue Leuco methylene blue Reversible Irreversible ABSTRACT While there is continuous progress in development of new photocatalytic thin films and coatings, the lack of a reliable and standard procedure for measuring the photocatalytic performance of such active surfaces makes it difficult to compare results between research groups and different measurement setups. Here, a comparative study was carried out to demonstrate the high photocatalytic activity of sputter-deposited TiO 2 film with self- organized nanocrack networks by using two different analytical approaches: (i) bleaching of a thin Methylene Blue (MB) solid layer on photocatalytic TiO 2 thin film (in-air) and (ii) the decolorization of a MB aqueous solution in the presence of TiO 2 thin film (in-solution). While the decolorization of aqueous MB solution provides an indirect observation of the photocatalytic effect imposed by the TiO 2 film, the use of a solid MB layer as an indicator allows monitoring of photocatalytic reactions at the solid-air interface directly. We showed the ap- plicability of this approach as a complementary and a fast analysis method to reveal the photocatalytic efficiency of thin films by comparing it with the state of the art inks (based on MB and other similar organic dyes) used as photocatalysis indicator. 1. Introduction The interest in using heterogeneous photocatalysis as an alternative for environmental clean-up is continuously increasing as the water re- sources diminish enormously and the air pollution threatens the human health seriously [1]. TiO 2 is one of the most widely used photocatalytic materials for environmental remediation applications due to its low cost, chemically inertness, non-toxicity, high photocatalytic activity and recyclability [2]. The band gap of TiO 2 is around 3 eV (3.0 eV for rutile and 3.2 eV for anatase), thus making TiO 2 photoactive under the ultra-violet (UV) radiation [3]. Beside other phases of TiO 2 (brookite and rutile), the anatase is gen- erally proposed as the most efficient polymorph for photocatalytic appli- cations [4]. Thermodynamic and structured-based analysis showed that anatase is the most stable TiO 2 phase at nanoscale due to its relatively lower surface energy [5]. Anatase nanoparticles have been shown to de- grade various organic pollutants in water with a high efficiency, thanks to their stability and high surface area [1]. On the other hand, it is a chal- lenge to separate such extremely tiny particles from the water after the photocatalytic clean-up. The difficulty in handling suspended TiO 2 parti- cles and the interest in expanding the use of TiO 2 in air cleaning appli- cations make it obvious that there is a need for robust TiO 2 thin films. In general, suspended TiO 2 nanoparticles exhibit much higher photocatalytic activity thanks to their high active surface area compared to immobilized TiO 2 nanoparticles or thin films [6]. On the other hand, TiO 2 thin films with highly porous morphology have been shown to exhibit very high photocatalytic performance [7,8]. The morphology and the porosity play a major role on the photocatalytic activity of such surfaces [9]. In addition, both density of surface states and energetic location (energy levels) are very dependent on geometric characteristics of TiO 2 surface [10,11]. Be- sides geometric factors the incorporation of dopants is an effective ap- proach for the modification of energy states of TiO 2 to get higher photo- catalytic efficiency [12]. Especially Au-TiO 2 and Ag-TiO 2 heterostructures have been shown to extend the photocatalytic activity to the visible region and to improve the charge separation and transfer properties which en- hance the photocatalytic efficiency [13,14]. Basically, larger surface area of TiO 2 nanoparticles gains them a higher photocatalytic activity by offering more active sites for catalytic reactions and accommodating higher number of defect states (oxygen vacancies) for the adsorption of water and oxygen. The significant difference between the surface areas of suspended and immobilized photocatalytic nanoparticles (or thin films) must be considered when comparing their performance. There are various standardized methods established by the International Union of Pure and Applied Chemistry https://doi.org/10.1016/j.solmat.2018.01.019 Received 7 December 2017; Received in revised form 5 January 2018; Accepted 14 January 2018 ⁎ Corresponding authors. E-mail addresses: ff@tf.uni-kiel.de (F. Faupel), oca@tf.uni-kiel.de (O.C. Aktas). Solar Energy Materials and Solar Cells 178 (2018) 170–178 0927-0248/ © 2018 Elsevier B.V. All rights reserved. T