Adsorption kinetics of alkanes on TiO 2 nanotubes array – structure–activity relationship B. Hokkanen a , S. Funk a , U. Burghaus a, * , A. Ghicov b , P. Schmuki b a Department of Chemistry, North Dakota State University, Fargo, USA b University of Erlangen-Nuremberg, Department of Materials Science, Erlangen, Germany Received 29 May 2007; accepted for publication 30 July 2007 Available online 3 August 2007 Abstract Presented are thermal desorption spectroscopy (TDS) measurements of iso-/n-butane adsorption on a variety of TiO 2 nanotubes (TiNTs) samples which are characterized by different crystal structures. The results are compared with a prior study on anatase(0 0 1) thin films grown on SrTiO 3 (0 0 1). A distinct kinetic structure–activity relationship was present, i.e., the binding energies of the alkanes depend on the polymorph (anatase vs. mixed anatase/rutile) of TiO 2 . A direct-fitting procedure of the TDS data has been applied to extract the kinetics parameters. The binding energies in the limit of zero coverage decrease as anatase thin film > amorphous-TiNTs  polycrystalline anatase TiNTs > polycrystalline mixed anatase/rutile TiNTs. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Ultra-high vacuum surface chemistry; Inorganic nanotubes; TiO 2 nanotubes; Anatase; Rutile; Thermal desorption spectroscopy 1. Introduction The economic potential of nanotubes (NT) [1–6] was ex- plored shortly after the discovery of carbon nanotubes (CNTs) in 1991 [7]. The currently discussed approaches may be labeled mostly as material science applications such as additives for composite materials [8], molecular sensors [9,10], the capture of toxic compounds [11], and hydrogen [12]/Li [13,14] storage in NT. Furthermore, the benefits of nano-electronics by the electrical tuneability [15] (metal- lic/semiconducting) of aligned [16] CNTs as molecular wires for connecting components, such as nano-field-effect transistors [17] or diodes [18], are extensively studied. How- ever, rather little attention has been paid so far to applica- tions in heterogeneous catalysis; in particular in ultra-high vacuum (UHV) surface chemistry model studies [11,19–23] concerning inorganic NT [1–3]. The understanding of gas–surface interactions is mostly unexplored for NT [1–6,24,25]. However, improvements in a large variety of applications require detailed knowledge about the gas- nanotube interaction in order to enable a functionalization and tuning of NT for catalysis and material science (gas sensors). Very promising are TiO 2 nanotubes (TiNTs) which are, in contrast to CNTs, intrinsically open-ended; a prerequi- site for taking advantage of the enhancement in surface area. Already quite divers applications of TiNTs are re- ported in materials science [26], medicine [27], and catalysis [28]. Most importantly, TiNTs can be synthesized as pow- ders or thin films with different crystal structure; polycrys- talline (anatase and mixed anatase/rutile) or amorphous systems can be obtained. This possibility allows one to di- rectly address the question of whether the anatase or rutile polymorph of TiO 2 is catalytically more active, i.e., if a structure–activity relationship (SAR) is present which would allow for catalyst tailoring. Although TiO 2 exists in different crystallographic phases, most surface science studies have focused on rutile TiO 2 (1 1 0) single crystals. However, industrial TiO 2 powder catalysts consist of rutile and anatase crystallites. Moreover, it has been proposed 0039-6028/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2007.07.021 * Corresponding author. Fax: +1 701 231 8831. E-mail address: uwe.burghaus@ndsu.edu (U. Burghaus). URL: www.chem.ndsu.nodak.edu (U. Burghaus). www.elsevier.com/locate/susc Surface Science 601 (2007) 4620–4628