Electrochimica Acta 82 (2012) 98–102 Contents lists available at SciVerse ScienceDirect Electrochimica Acta j ourna l ho me pag e: www.elsevier.com/locate/electacta Water annealing and other low temperature treatments of anodic TiO 2 nanotubes: A comparison of properties and efficiencies in dye sensitized solar cells and for water splitting Ning Liu, Sergiu P. Albu, Kiyoung Lee, Seulgi So, Patrik Schmuki ∗ Department of Material Science WW-4, LKO, University of Erlangen-Nuremberg, Martensstrasse 7, 91058 Erlangen, Germany a r t i c l e i n f o Article history: Received 15 December 2011 Received in revised form 1 June 2012 Accepted 4 June 2012 Available online 12 June 2012 Keywords: TiO2 nanotubes Anodization Annealing Photoresponse a b s t r a c t The present work compares different annealing treatments for TiO 2 nanotubes in terms of their photoelec- trochemical performance. First, self-organized TiO 2 nanotubes were grown in a most typical electrolyte of 0.5 wt% NH 4 F + 2 wt% H 2 O in ethylene glycol to length of ≈15 m. These “as-formed” tubes are amor- phous. Then the layers were either thermally annealed, thermally and hydrothermally annealed or “water annealed”. All these treatments show conversation of the tubes to anatase but with a considerably different level of crystallinity. Water annealing leads to strong tube wall roughening with correspond- ing area increase. In all investigated cases, the photocurrent properties (including dye sensitized solar cells (DSSCs)) and photocatalysis (decomposition of organics and water splitting), either in a two elec- trode configuration or under OCP, thermal annealing results in by far the best performance, followed by hydrothermal approaches. Water annealing turns out to be only of a minor improvement over using “as-formed” amorphous tubes. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction Over the past 10 years, self-organized TiO 2 nanotube arrays pro- duced by a simple but optimized anodization process on Ti metal substrates, have attracted wide spread interest (for an overview on formation properties and applications see e.g. [1]). A main driving force for progress is the prospect of beneficially using these nan- otube layers in classic titania applications, such as dye-sensitized solar cells (DSSCs), biomedical devices, photocatalysis, and for effi- cient water splitting. Tube arrays are usually formed in water-based or organic dilute fluoride electrolytes. After anodization and dry- ing, these (as-formed) tubes are generally found to be amorphous in nature. However, for all electrochemical or photoelectrochemical appli- cations, the performance is drastically increased if the tubes are crystallized to either an anatase or mixed anatase/rutile structure [2–6]. Most frequently this conversion is carried out by annealing the tubes in a furnace or by a rapid thermal annealer in air, or an O 2 containing environment. At temperatures around 300 ◦ C, typi- cally conversion of the amorphous material to anatase occurs and above 550 ◦ C the tubes are converted to anatase/rutile mixtures [6,7]. Nevertheless, there are a number of interesting reports that ∗ Corresponding author. Tel.: +49 9131 85 275 75; fax: +49 9131 85 275 82. E-mail address: schmuki@ww.uni-erlangen.de (P. Schmuki). find for H 2 O prepared tubes [8,9], hydrothermally treated tubes [10,11], or tubes exposed to water for extended periods of time (days) [12], either traces or significant amounts of conversion to anatase, and in some cases a drastic increase of the surface area. Particularly, low temperature water treatments would indeed rep- resent an elegant way of annealing tubes and the reported increase in surface area would be highly beneficial for applications such as DSSCs [13–15] (where tube based solar cells still suffer from a much lower dye loading than nanoparticle based cells), or for nanotube membranes that exhibit a high temperature annealing problem [12,16–18]. Therefore, in the present work we compare “water annealing” and hydrothermal annealing processes with the classic thermal annealing, and assess the performance of the differ- ent treatments in most relevant photoelectrochemical applications of TiO 2 nanotubes (namely photocurrent response, photocatalysis, water splitting, and use in DSSCs). 2. Experimental As substrates for TiO 2 nanotube growth we used titanium foils (99.6% purity, Goodfellow) with a thickness of 0.1 mm. Prior to tube formation the foils were cleaned by sonication in acetone and ethanol followed by rinsing with deionized (DI) water and drying in a nitrogen stream. To perform the electrochemical TiO 2 nanotube formation, the foils were anodized using a power supply (Voltcraft VLP 2403 pro) in a two electrode configuration with a counter 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.06.006