J. Phys. Chem. C 2007, 111, 13967-13971
13967
Nanocrystals as Very Active Interfaces: Ultrasensitive Room-Temperature Ozone Sensors with In2O3 Nanocrystals Prepared by a Low-Temperature Sol-Gel Process in a Coordinating Environment Mauro Epifani,*,† Elisabetta Comini,‡ Jordi Arbiol,§,| Eva Pellicer,§ Pietro Siciliano,† Guido Faglia,‡ and Joan R. Morante§ Consiglio Nazionale delle Ricerche - Istituto per la Microelettronica ed i Microsistemi (C.N.R.-I.M.M.), Via Monteroni, I-73100 Lecce, Italy, CNR-INFM and Dip. Chimica e Fisica per l’Ingegneria e i Materiali, SENSOR, UniVersita` di Brescia, Via Valotti 9, Brescia, Italy, EME/CeRMAE/IN2UB, Departament d’Electro` nica, UniVersitat de Barcelona, C. Martı´ i Franque` s 1, 08028 Barcelona, CAT, Spain, and TEM-MAT, SerVeis Cientificote` cnics, UniVersitat de Barcelona, C. Lluı`s Sole` i Sabaris 1, 08028 Barcelona, CAT, Spain ReceiVed: June 8, 2007; In Final Form: July 13, 2007
In2O3 nanocrystals were prepared by injection of In2O3 sol in a hot (160 °C) solution of tetradecene and an amine. By properly controlling the amine chain length, the processing of the starting sol and the heat-treatment temperature, In2O3 nanocrystals with a size ranging from 4 to 10 nm were obtained. The nanocrystals suspensions could be easily processed by drop-coating onto alumina substrates to obtain gas-sensing devices. The devices displayed an enormous response to ozone gas, with a response of several orders of magnitude even to 60 ppb of ozone. In particular, the monitoring of low ozone concentrations was possible even by operating at room temperature, where the ozone concentration profile could be rapidly and reversibly monitored. The enhanced gas-sensing properties were explained as the cooperative effect of the nanocrystal size and surface chemistry, characterized by high densities of oxygen vacancies.
Introduction Semiconducting oxides are widely employed in the processing of commercial chemoresistive gas sensors,1 in which the chemical interaction between the sensing oxide and the gaseous analyte is transduced in a variation of the electrical resistance of the oxide itself. Common drawbacks of chemoresistive sensors are the lack of selectivity and time stability of their performances. The use of oxide nanocrystals was shown to result in a remarkable improvement of the performances of chemical sensors2 as concerns the magnitude of the response to the gaseous analyte. The response is defined as the relative change of the electrical resistance of the sensing layer upon exposure to the gas with respect to the resistance of the layer in pure air. The high response of nanocrystal-based sensors is now commonly attributed to the larger and faster modulation of the charge depletion layer induced by the interaction with gases in the surrounding atmosphere,3 and is a reason for the increasing interest toward sensing devices based on nanomaterials: high responses allow faster detection of smaller gas concentrations, which is of particular interest in the detection of toxic or explosive gases. Another reason for interest in oxide-nanocrystal-based gas-sensors is the need to substitute expensive or nonuser-friendly sensing devices with easily operated, almost automatic, and cheap detection systems. An important example is the sensing of ozone. Ozone is a toxic gas widely used in industrial processes such as food sterilization and water * Author to whom correspondence may be addressed. E-mail:
[email protected]. † Consiglio Nazionale delle Ricerche - Istituto per la Microelettronica ed i Microsistemi. ‡ Universita ` di Brescia. § Departament d’Electro ` nica, Universitat de Barcelona. | Serveis Cientificote ` cnics, Universitat de Barcelona.
disinfection. Moreover, environmental concerns require its monitoring in the upper layers of the atmosphere. The currently available sensing devices rely on expensive or not easily operated instruments such as optical sensors in the UV region or electrochemical cells. Indium oxide (In2O3) has been shown to be a selective material for chemoresistive sensing of ozone,4 but the sensor performances have still to be improved before they can be compared with the currently available devices. For this reason, and in view of the previous general considerations, the interest for In2O3 nanocrystals-based ozone sensors is immediate. In this work we present the application of a general synthetic procedure5 of oxide nanocrystals to the preparation of differently sized In2O3 nanocrystals. Selected samples were used for the processing of ozone-sensing devices, and their testing to ozone demonstrated the potential of In2O3 nanocrystals in the preparation of highly performing ozone sensors, obtaining in particular (a) an enormous response to ozone at operating temperatures as low as 200 °C, indicating the possibility of easily sensing ultralow (
