Photocatalytic Carbon-Nanotube–TiO 2 Composites By Karran Woan, Georgios Pyrgiotakis, and Wolfgang Sigmund* 1. Introduction Carbon nanotube-anatase titanium dioxide (CNT-TiO 2 ) compo- site systems are currently being considered for many applications including their potential use to address environmental problems. TiO 2 has always been one of the best candidate materials due to its photocatalytic properties, its relative nontoxicity, and long-term thermodynamic stability. Several groups managed to enhance the photocatalytic properties and optimized titania’s use to degrade various organic and inorganic pollutants. Two TiO 2 polymorphs are typically being used—anatase and rutile, with the anatase phase exhibiting a significantly higher photocatalytic activity than the rutile phase. Although the reasons are not yet fully understood, it is speculated that the higher charge-carrier mobility of 80 cm 2 V 1 s 1 in anatase, [1] which is 89 times faster than in rutile, [2] causes their superior photocatalytic properties. However, the mixture of the two polymorphs yields even higher photocatalytic activity at specific ratios, as exhibited by the commercially available AEROXIDE TiO 2 P25. The enhancement is attributed to the special electronic states of the two crystal structures, which allow for a semiconductor–semiconductor junction. The details of such mechanisms will be explained later. The mixture of two types of semiconductor particles, semiconductor particles with metal particles, and recently carbon particles with anatase, showed photocatalytic enhancements in many cases to be described later. This concept can then be extended to defined carbon structures with tailored electronic properties. Fullerenes and CNTs are excellent candidates to allow deeper insight into the semiconductor junction of titania with metallic or semi- conducting carbons. Furthermore, CNTs have excellent mechanical properties and a large specific surface area (>150 m 2 g 1 ). [3] They also allow for surface chemical modifications to control the type of bonds that can be formed with titania, be it chemically bonded or van der Waals bonded. The mixture of titania and CNT also has a large area where pollutants (organic or inorganic reactants) can adsorb. Adsorption is a key process in the photo- catalytic destruction of pollutants, as will be discussed later. Thus, CNT-TiO 2 mixtures and composites are able to achieve photo- catalytic activities well beyond the anatase/rutile composites. This article highlights the literature on the synthesis of CNT- titania composite structures, and addresses the enhancement mechanism of the anatase–rutile composite and the CNT-TiO 2 systems. The flexibility to tailor the CNT electronic properties are discussed next, followed by proposed mechanisms for photo- catalysis. Furthermore, the recent publications for CNT-TiO 2 composites in photocatalytic degradation studies are examined with focus toward understanding the activity enhancement. Finally, a discussion of the various applications and the materials science challenges are presented. 2. Background 2.1. Anatase–Rutile Composite System One of the commercially available forms of TiO 2 often used as a standard for photocatalytic studies is AEROXIDE TiO 2 P25. This is a mixed particle system of approximately 75–80% anatase and 25–20% rutile with a specific surface area of about 50 m 2 g 1 and synthesized by the Aerosil (flame pyrolysis) process. Anatase has a higher photocatalytic activity than rutile for several reasons. One of them is the size of the band-gap, where anatase has a larger band-gap than rutile, 3.2 eV versus 3.0 eV, respectively. This provides anatase with a higher redox potential. Another reason is that anatase has a higher area density of surface hydroxyls, which slows the recombination of photogenerated electron–hole pairs. In 1991, Bickley initially proposed that the enhancement of the photocatalytic properties in P25 stemed from a space-charge region formed at the rutile–anatase interface where band bending occured due to the difference in the band-gap of the two phases. The charge carriers generated were thought to form in the anatase particles, and then the holes migrate into the rutile particles. RESEARCH NEWS www.advmat.de [*] Prof. W. Sigmund, K. Woan, G. Pyrgiotakis Department of Materials Science and Engineering University of Florida Rhines Hall, PO Box 116400 Gainesville, FL 32611-6400 (USA) E-mail: wsigm@mse.ufl.edu DOI: 10.1002/adma.200802738 The literature and advances in photocatalysis based on the combination of titania (TiO 2 ) and carbon nanotubes is presented. The semiconductor basis for photocatalysis is introduced for anatase and rutile. Furthermore, the proposed mechanisms of catalytic enhancement resulting from the pairing of the titania semiconductor with either metallic, semiconducting, or defect-rich carbon nanotubes (CNT) is discussed. Differences are apparent for the mixtures and chemically bonded CNT–TiO 2 composites. The article then highlights the recent advances in the synthesis techniques for these com- posites and their photocatalytic reactions with organic, inorganic, and biological agents. Finally, various applications and challenges for these composite materials are reported. Adv. Mater. 2009, 21, 2233–2239 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2233