The effect of electrolyte re-utilization in the growth rate and morphology of TiO 2 nanotubes José D. Costa a,1 , Paula Quitério a,1 , Arlete Apolinário a,b,n , Célia T. Sousa a , João Azevedo a,b , João Ventura a , Luísa Andrade b , Adélio Mendes b , João P. Araújo a a IFIMUP and IN – Institute of Nanosciences and Nanotechnology, Dep. de Física e Astronomia, Faculdade de Ciências da Universidade do Porto, Rua do Campo Alegre, 678, 4169-007 Porto, Portugal b LEPABE-Laboratory for Process Engineering, Environment, Biotechnology and Energy, Dep. Engenharia Química-Faculdade de Engenharia da Universidade do Porto, R. Dr. Roberto Frias, 4200 À465 Porto, Portugal article info Article history: Received 19 January 2016 Accepted 20 February 2016 Available online 22 February 2016 Keywords: TiO 2 nanotubes Anodization Growth rate Electrolyte Re-utilization abstract In an industrial context, the re-utilization of resources is very important to monetize processes, reducing the production costs without overlooking a proper performance of the devices. For specific applications such as dye-sensitized solar cells (DSC), a high-aspect-ratio configuration of the produced TiO 2 nano- tubes (NTs) is essential to achieve higher conversion efficiencies. In this work, we report the effect of electrolyte re-utilization in the diameter, length and morphology of TiO 2 nanotubes (NTs) produced by electrochemical anodization. A detailed study of the bias voltage effect on the anodic NTs for new and re- utilized electrolytes was performed. Also, the possibility of obtaining NT arrays with ribs morphology just by re-using the electrolyte was unveiled. & 2016 Published by Elsevier B.V. 1. Introduction Vertically oriented titanium dioxide (TiO 2 ) nanotubes (NTs) have unique physico-chemical properties, making them attractive for potential applications in emerging fields, such as dye-sensi- tized solar cells (DSC) [1–3]. Since they provide a more efficient charge transport, the NTs geometry ensures a faster electron transfer towards the counter-electrode, minimizing the chance for charge trapping and electron-hole recombination [3,4]. Among the various synthesis methods, electrochemical anodi- zation is an easy and low-cost approach to fabricate NT arrays. Since the NT properties are closely related with the geometric surface areas, it is important to control NT parameters such as length, pore size and wall thickness. Such tailoring can be easily achieved with electrochemical anodization by simply changing the anodizing parameters [3,5–7]. The fabrication of self-assembled TiO 2 NTs was introduced by Zwilling et al. in 1999 [8]. However, this first generation of NTs was produced in aqueous hydrofluoric acid based electrolytes which has a high TiO 2 chemical dissolution rate, limiting the NTs growth to lengths of up to 500 nm [9,10]. Subsequently, the NT length was increased (up to 4.4 mm) by controlling the anodization electrolyte pH and thus reducing the chemical dissolution effect [11,12]. To suppress local concentration fluctuations and pH bursts during anodization, viscous organic electrolytes were introduced in the third generation of NTs, leading to smoother NTs with several micrometers or even millimeters [3,13,14]. A common feature to all this anodic approaches is that the outer walls of the tubes show considerable thickness variation ribs, also known as bamboo morphology [6,15,16]. The growth of ribs around the nanotubes has been related with periodical pH burst at the pore bottom (in fluoride-based electrolytes) [13,17], with the dehydration process of the hydroxide layer (double layer wall of NTs), by the fluoride- rich layer in the outer wall of the NTs [16,18] or, more recently, as a consequence of the chemical dissolution of the surface layer (at the NTs tops) [19,20]. This structure was also obtained using al- ternating voltage cycling instead of potentiostatic anodizations, proving to be useful in the optimization of DSC structures [6,15,21,22]. Ti anodizations are usually performed using fresh electrolytes and the effect of electrolyte re-utilization in the TiO 2 NTs mor- phology is still not understood. The re-use of the electrolyte (electrolyte aging) was earlier reported to be critical in electrolytes with low water contents ( r1.0 wt%) [22,23]. The increase in the water amounts (moisture absorption) during the anodization process lead to an increase of the NTs organization and growth rate. However, it is with fresh ethylene glycol electrolyte solutions Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters http://dx.doi.org/10.1016/j.matlet.2016.02.085 0167-577X/& 2016 Published by Elsevier B.V. n Corresponding author at: IFIMUP and IN – Institute of Nanosciences and Na- notechnology, Dep. de Física e Astronomia, Faculdade de Ciências da Universidade do Porto, Rua do Campo Alegre, 678, 4169-007 Porto, Portugal. E-mail address: arletteapolinario@gmail.com (A. Apolinário). 1 These authors have contributed equally to this work and both should be considered as first authors. Materials Letters 171 (2016) 224–227