Micro-Raman analysis of titanium oxide/carbon nanotubes-based nanocomposites for hydrogen sensing applications S. Santangelo a,n , G. Messina a , G. Faggio a , A. Donato a , L. De Luca a , N. Donato b , A. Bonavita c , G. Neri c a Department of Mechanics and Materials, ‘‘Mediterranea’’ University of Reggio Calabria, I-89122 Reggio Calabria, Italy b Department of Matter Physics and Electronic Engineering, University of Messina, I-98166 Messina, Italy c Department of Industrial Chemistry and Materials Engineering, University of Messina, I-98166 Messina, Italy article info Article history: Received 14 June 2010 Received in revised form 9 August 2010 Accepted 10 August 2010 Available online 13 August 2010 Keywords: Micro-Raman spectroscopy Anatase Carbon nanotube composites Hydrogen sensors abstract Titanium oxide/carbon nanotubes-based nanocomposites (TiO 2 /CNTs, prepared by sol–gel method, and 2%Pt/TiO 2 /CNTs, obtained by wetness impregnation of the TiO 2 /CNTs base material with a solution of platinum acetylacetonate) have been recently used as active layer in hydrogen sensing devices at near room temperature, obtaining quite different responsiveness. The microstructure of these hybrid materials is here systematically investigated by micro-Raman spectroscopy at 2.41 eV. The results show that regardless of the nominal C/Ti molar ratio (3.6 or 17.0) only the anatase phase of titania is formed. Theoretical calculations demonstrate that phonon confinement is fully responsible for the large blue- shift ( 10 cm 1 ) and broadening ( 20 cm 1 ) of the lowest-frequency Raman mode with respect to bulk anatase. The average size (4.3–5.0 nm) of TiO 2 crystallites, resulting from Raman spectra fitting, is in excellent agreement with those inferred from transmission electron microscopy and X-ray diffraction measurements. & 2010 Elsevier Inc. All rights reserved. 1. Introduction Growing interdisciplinary efforts are nowadays devoted to the development of hydrogen-fueled systems, whose safe use demands efficient hydrogen sensor devices. Several hybrid nanocomposites have been proposed as active materials for the fabrication of resistive sensors for detection of this gas down to very low concentration. A good response towards H 2 has been obtained by the use of nanostructured Pt (or Pd) on surface chemically oxidized CNTs [1,2]. Efficient RT hydrogen sensing devices have been prepared also by using Pt on TiO 2 -nanotubes [3]. There is great interest also in detecting high concentration of H 2 in inert atmosphere, by means of devices operating at temperature as low as possible, preferably room temperature (RT). Recently, prototypes of sensors for monitoring high hydro- gen concentration in inert environment at near RT have been realized employing Pt/TiO 2 /CNTs as sensing layers [4]. Preliminary sensing tests have shown that, different from Pt/CNTs and TiO 2 /CNTs, Pt/TiO 2 /CNTs allow detection of H 2 . This suggests the possibility that a synergic action among noble metal, titanium dioxide and nanotubes must be responsible for their sensing properties. In addition, the nominal C/Ti molar ratio is found to have a strong influence on responsiveness of the Pt/TiO 2 / CNT-based sensors, while the thermal pretreatment in reducing ambient has positive effects on the sensor performance [4]. This paper deals with TiO 2 /CNTs (prepared by sol–gel method), 2%Pt/CNTs and 2%Pt/TiO 2 /CNTs (obtained by wetness impregna- tion). Aiming at evidencing the effects of platinum addition and thermal treatment on their microstructure, as well as of the nominal C/Ti molar ratio (3.6 or 17.0) on the crystallization phase of titania, a systematic micro-Raman spectroscopy (MRS) inves- tigation is carried out on these hybrid materials. From the MRS study indications about the occurrence of relevant grain size effects emerge, which are discussed in the light of the comple- mentary results obtained by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The existence of any correla- tion between the results of spectroscopic analysis and sensing tests is further investigated. 2. Experimental 2.1. Nanocomposite preparation In order to prepare CNT-based composites, CNTs are firstly functionalized by a treatment with concentrated nitric acid (15.8 N) at 110 1C for 18 h. TiO 2 /CNTs composites with different nominal C/Ti molar ratios are prepared, by a sol–gel method, dispersing the functionalized CNTs in a solution of titanium isopropoxide in isopropanol previously held under reflux at 80 1C Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jssc Journal of Solid State Chemistry 0022-4596/$ - see front matter & 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jssc.2010.08.018 n Corresponding author. Fax: + 39 0965 875201. E-mail address: saveria.santangelo@unirc.it (S. Santangelo). Journal of Solid State Chemistry 183 (2010) 2451–2455