Core–shell Zn-doped TiO 2 –ZnO nanofibers fabricated via a combination of electrospinning and metal–organic chemical vapour deposition M. E. Fragal a, * a I. Cacciotti, b Y. Aleeva, a R. Lo Nigro, c A. Bianco, b G. Malandrino, a C. Spinella, c G. Pezzotti d and G. Gusmano b Received 10th March 2010, Accepted 11th May 2010 DOI: 10.1039/c004157b Zn-doped TiO 2 nanofibers shelled with ZnO hierarchical nanoarchitectures have been fabricated combining electrospinning of TiO 2 (anatase) nanofibers and metal–organic chemical vapor deposition (MOCVD) of ZnO. The proposed hybrid approach has proven suitable for tailoring both the morphology of the ZnO external shell as well as the crystal structure of the Zn-doped TiO 2 core. It has been found that the Zn dopant is incorporated in calcined electrospun nanofibers without any evidence of ZnO aggregates. Effects of different Zn doping levels of Zn-doped TiO 2 fibers have been scrutinized and morphological, structural, physico-chemical and optical properties evaluated before and after the hierarchical growth of the external ZnO shell over the electrospun nanofibers. Moreover, doping promotes the incipient transition from the anatase to rutile phase in the core–shell Zn-doped TiO 2 –ZnO nanostructures at lower temperature than that observed for pure TiO 2 . Finally, the present core–shell hierarchical nanofibers possess a very large surface to volume ratio and exhibit a marked cathodoluminescence with a strong UV and visible emission. Introduction Hierarchical hetero-nanostructures 1 find applications in a variety of fields such as field emission 2 photovoltaics, 3 supercapacitors, 4 fuel cells 5 and multifunctional nanocomposites 6 that require high surface areas. Synthetic routes suited for enhancing robustness and controlling sizes, shapes, and compositions have therefore received increasing attention in recent years. In this scenario, metal oxide semiconductors having dual or multiple morphol- ogies and structures, become of interest since they represent a versatile solution for performance enhancement and applica- tions in multifunctional devices. 7 In particular, great attention has been recently devoted to the fabrication of nanocomposites containing ZnO and TiO 2 . 8–11 In fact, implementation of TiO 2 materials with ZnO has often proved to improve the photocatalytic properties of TiO 2 8,12 and to promote some anatase-to-rutile phase transition. 13 Actually, the co-existence of these two phases due to the presence of dopant heteroatoms 13–15 appears particularly promising since it modifies the spatial charge separation and reduces the recombi- nation efficiency, hence improving the photocatalytic perfor- mances. 16 In addition, ZnO hierarchical nanostructures are excellent sensors (i.e. gas, 17–19 humidity, 20 UV 21 ) due to the enhanced surface interactions depending on a more favorable surface to volume ratio than ZnO bulk and continuous nanostructured films. The recent interest toward electrospun TiO 2 nanofibers as a structural alternative to nanoparticle-based electrodes in dye sensitive solar cells (DSSCs) 22 is another motivation to study ZnO–TiO 2 composite nanofiber materials. 23 Finally, electrospun nanofibers have been recently used as nanoplatforms for gas sensors 23,24 and several approaches have been adopted for func- tional activation of their surfaces. 25,26 Therefore, any effort to enhance the working surface area of the multifunctional devices can successfully take advantages of three-dimensional (3-D) nanostructured materials and hierar- chical single-crystal branches of smaller size represent an effec- tive approach for surface activation. 27 In this context, we report here on the successful fabrication and full characterization of Zn-doped TiO 2 electrospun nano- fibers (herein d:TiO 2 ) shelled via MOCVD deposition with hierarchical ZnO nanostructures. The present approach combines the flexibility of electrospinning to large-scale production of ceramic nanofibers 28 with all the MOCVD benefits associated with high reproducibility, easily controlled growth (largely proven on large scale), short process times, quality grade crystallinity and elemental purity. Presently, this strategy has been successfully tested for the reproducible fabrication of standing alone d:TiO 2 as well as of core nanofibers shelled with ZnO hierarchical nanostructures (herein d:TiO 2 –ZnO). The particularly favorable surface-to-volume ratio, due in turn to the large density of ZnO single crystalline nanorods/nanoneedles on the shell surface, promotes a remarkable cathodoluminescence emission in the UV and visible regions. The obtained nano- architectures are quite complex and therefore a careful charac- terization of the Zn-doped TiO 2 core, before and after the ZnO external shell deposition, is herein presented. a Dipartimento di Scienze Chimiche Universit  a di Catania and INSTM UdR Catania, Viale Andrea Doria, 6, 95125 Catania, Italy. E-mail: me. fragala@unict.it b University of Rome ‘‘Tor Vergata’’, Dipartimento di Scienze e Tecnologie Chimiche, INSTM UdR, Roma Tor Vergata, Via della Ricerca Scientifica, 00133 Rome, Italy c Istituto per la Microelettronica e Microsistemi, IMM CNR, Strada VII Zona industriale n 5, 95121 Catania, Italy d Ceramic Physics Laboratory & Research Institute for Nanoscience, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto, 606-8585, Japan 3858 | CrystEngComm, 2010, 12, 3858–3865 This journal is ª The Royal Society of Chemistry 2010 PAPER www.rsc.org/crystengcomm | CrystEngComm Downloaded by CNR Bologna on 08 March 2011 Published on 12 July 2010 on http://pubs.rsc.org | doi:10.1039/C004157B View Online