On the role of individual metal oxide nanowires in the scaling down of chemical sensorswz Francisco Hernandez-Ramirez,* abc J. Daniel Prades, ab Roman Jimenez-Diaz, a Thomas Fischer, d Albert Romano-Rodriguez,* a Sanjay Mathur d and Joan R. Morante* ab Received 13th March 2009, Accepted 26th May 2009 First published as an Advance Article on the web 24th June 2009 DOI: 10.1039/b905234h Single-crystalline semiconductor metal oxide nanowires exhibit novel structural and electrical properties attributed to their reduced dimensions, well-defined geometry and the negligible presence of grain boundaries and dislocations in their inside. This favours direct chemical transduction mechanisms at their surfaces upon exposure to gas molecules, making them promising active device elements for a new generation of chemical sensors. Furthermore, metal oxide nanowires can be heated up to the optimal operating temperature for gas sensing applications with extremely low power consumption due to their small mass, giving rise to devices more efficient than their nanoparticle-based counterparts. Here, the current status of development of sensors based on individual metal oxide nanowires is surveyed, and the main technological challenges which act as bottleneck to their potential use in real applications are presented. 1. Introduction Representative metal oxide semiconductors have unique properties perfect for several applications such as sensors, photocatalyzers, and varistors. 1–5 The basic working principle of these materials as gas sensors is based on the chemico- electrical transduction reactions which take place at the inter- face between molecular adsorbents and the metal oxide’s surfaces. 4–6 However, their final performance is highly deter- mined by manifold experimental parameters such as the con- centration of oxygen vacancies, which are originated during the synthesis of metal oxides; and the intrinsic carrier concen- tration in their inside. 5–8 Commercial sensors are commonly based on thin layers of metal oxide semiconductors deposited onto hotplates which are used to fix the temperature at the optimal values and thus, activate the surface transduction mechanisms necessary to detect the gas species 4,9 (Fig. 1a). Although this design has become a success story from a commercial point of view, 9,10 it presents two major drawbacks: (1) high power consumption which hinders its use in portable Fig. 1 Schematic diagrams of different types of conductometric gas sensors based on metal oxides. (a) Commercial thin-film sensor formed by a layer of nanoparticles. Here, electrons must go through a network of nanocrystals with different size and shape. From an energy point of view, electrons are to overcome potential barriers [(i) metal–semiconductor barriers (eV C ) and (ii) intergrain boundary barriers (eV B )]. The overall influence of gas on the height of the barriers determines the final response of the sensor. This is equivalent to a network of resistors [(i) metal– semiconductor contacts (R C ), (ii) grain boundary interfaces (R B ) and (iii) metal-oxide grains (R G )]. (b) Multi-nanowire sensor. The above mentioned discussion is valid here as well. (c) Single-nanowire sensor. If a nanowire is measured in 4-probe DC configuration, the conductometric response is basically determined by changes of the conduction channel along the nanowire (R NW ). On the contrary, contacts effects are overcome. a EME/XaRMAE/IN 2 UB, Departament d’Electro `nica, Universitat de Barcelona, C/ Martı´ i Franque `s 1, Planta 2, E-08028 Barcelona, Spain. E-mail: aromano@el.ub.es b Institut de Recerca en Energia de Catalunya (IREC), C/ Josep Pla 2, B3, Ground Floor, E-08019 Barcelona, Spain. E-mail: jrmorante@irec.cat c Electronic Nanosystems S. L., C/Baldiri Reixac 4-, 6, E-08028 Barcelona, Spain. E-mail: fhernandezra@gmail.com d Department of Inorganic Chemistry, University of Cologne, Cologne, Germany w This article was submitted as part of a Themed Issue on metal oxide nanostructures. Other papers on this topic can be found in issue 19 of Vol. 11 (2009). This issue can be found from the PCCP hopepage [http://www.rsc.org/PCCP]. z Electronic supplementary information (ESI) available: The dynamic response of individual nanowires; interval between gas–surface collisions at the nanowires. See DOI: 10.1039/b905234h This journal is  c the Owner Societies 2009 Phys. Chem. Chem. Phys., 2009, 11, 7105–7110 | 7105 PERSPECTIVE www.rsc.org/pccp | Physical Chemistry Chemical Physics Published on 24 June 2009. Downloaded on 05/05/2014 13:50:26. View Article Online / Journal Homepage / Table of Contents for this issue