Published: November 01, 2011 r2011 American Chemical Society 45 dx.doi.org/10.1021/cs200392m | ACS Catal. 2012, 2, 45–49 LETTER pubs.acs.org/acscatalysis Effect of Reactor Materials on the Properties of Titanium Oxide Nanotubes Alon Danon, † Kaustava Bhattacharyya, † Baiju K. Vijayan, ‡,^ Junling Lu, § Dana J. Sauter, †,|| Kimberly A. Gray, ‡ Peter C. Stair, †,§ and Eric Weitz † † Institute for Catalysis in Energy Processes and Department of Chemistry, ‡ Institute for Catalysis in Energy Processes and Department of Civil and Environmental Engineering, Northwestern University, Evanston, Illinois 60208, United States § Energy System Division, Argonne National Laboratory, Argonne, Illinois 60439, United States M etal oxide-based nanotubes (NTs) have generated signifi- cant scientific interest as a result of their intriguing struc- tureÀactivity relationship (SAR), which results from their 1D morphologies. 1 Of particular interest are NTs based on titanium oxides in view of the fact that they generally have greater morphological stability than other metal oxide NTs. 1 In addition, titanium oxide NTs exhibit an unusual SAR, which has led to their proposed utilization for a number of applications, including dye-sensitized solar cells, 4 photocatalysis, 2,3 sensors, 5,6 and bat- teries. 7 For both sensor and photocatalytic applications, NTs based on titanium oxides share the important common attributes of high surface area, tubular structures, highly active surface sites on the tube walls, and unique morphology. 6 For sensor applica- tions, titanium oxide-based NTs synthesized by anodizing tita- nium foil produce a nanotube array architecture that displays remarkable sensitivity toward hydrogen. 5 These NTs, when exposed to alternating atmospheres of nitrogen containing 1000 ppm of hydrogen and air, can exhibit variations in elec- trical resistance, at RT, of up to 8.7 orders of magnitude. 6 This effect has been explained by direct chemisorption of hydrogen on the NTs leading to a tremendous reduction in resistance of the NTs. The activation of hydrogen is proposed to occur on the walls of the undoped nanotubes at highly active surface sites located at nanoscale surface defects. 6 This result also suggests possible low-energy reactive pathways for the formation of oxygen vacancies with the production of water as the driv- ing force. The use of titanium oxide-based NTs in photocatalysis has been limited mainly to the UV region, since the electronic absorption of the native NTs begins at ∼380 nm. 2,3 Conse- quently, for this material, only 2À3% of the solar spectrum can be utilized to drive a photocatalytic reaction. Motivated by a desire to improve the efficiency of solar driven reactions, NTs synthe- sized by both electrochemical anodization 8,9 and hydrothermal synthesis 10,11 techniques have been doped by nitrogen and metals in an attempt to alter the band gap to produce an increase in photon absorption in the visible region of the spectrum. Recent studies performed on crystalline TiO 2 anatase nano- particles report that an enhanced solar absorption can be achieved through the introduction of disorder in the surface layers of the nanophase TiO 2 through hydrogenation. 12 It was reported that exposure to hydrogen led to a remarkable change in color, from white to black, as midband gap states are proposed to form with energy distributions that differ from that of a single crystal defect. Remarkably, the change in color was reported to persist for over a year’s time. The synthesis of these TiO 2 black anatase nanoparticles involves a high-pressure hydrogen treat- ment in which the nanoparticles are “hydrogenated” for a significant period of time under relatively harsh conditions: 20.0 bar H 2 atmosphere at approximately 200 °C for 5 days in a stainless steel container. In this Letter, we report the synthesis and characterization of black titanium oxide NTs and their activity toward acetaldehyde Received: August 3, 2011 Revised: October 31, 2011 ABSTRACT: Subtleties in the synthesis of materials can have a profound effect on the catalytic and photocatalytic properties of materials. Black TiO 2 nanotubes, demonstrating remarkable solar absorption, were synthesized using a stainless steel reactor. Using UVÀvis diffuse reflectance spectroscopy, XPS, EDS, ICP, and TEM, the change in electronic absorption of the TiO 2 nanotubes is explained by the discrete introduction of Cr concentrated particles from the stainless steel reactor. The black TiO 2 nano- tubes displayed significant solar-driven photocatalytic activity with the photo-oxidation of acetaldehyde under visible light (λ > 450 nm). KEYWORDS: titanium oxide, titantia, nanotubes, photocataly- sis, chromium