In2O3 and Pt-In2O3 nanopowders for low temperature oxygen sensors

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Abstract

In2O3 and Pt-doped In2O3 nanopowders were investigated for oxygen monitoring at low temperature (from RT to 350 °C). In the first step, In2O3 nano-sized powders were synthesized by a non-aqueous sol–gel method, followed by deposition of 1 or 3 wt% Pt on the surface of the semiconducting metal oxide by wetness impregnation.

In comparison to undoped In2O3, the Pt-doped sensors showed better performances in terms of sensitivity, response and recovery times, presumably due to the increased number of active sites for oxygen adsorption and dissociation, which favours the exchange kinetics of gaseous oxygen with adsorbed oxygen species. Depending on the working temperature, surface and/or bulk effects contribute to the oxygen sensing mechanism, strongly influencing the response and recovery times, as well as, the shape of the transient response. The high response combined with fast response and recovery times makes the Pt-In2O3 sensors a promising candidate for applications in oxygen monitoring at low temperature.

Introduction

The main application field of oxygen sensors lies in the control of combustion processes such as in the evaluation of air to fuel ratio (λ) for improving engine efficiency. In this particular case, the lambda-probes work at temperatures higher than 600 °C for functional and environmental reasons [1], [2], [3]. However, for other applications in the medical field, food processing and waste management industries, oxygen monitoring at lower temperatures, for example, in the range from room temperature (RT) to 350 °C, is more important. Therefore, different types of oxygen sensors have been developed for this purpose. Electrochemical-type sensors (Clark electrodes) are able to measure oxygen in liquid media, paramagnetic sensors are accurate but expensive, and optical sensors make use of polymeric sensing layers [4].

Unfortunately, the use of resistive devices, based on ceramic materials for low temperature oxygen sensors, is limited due to the low sensitivity and conductivity of the currently used oxides.

Recently, a nanostructured p-type oxide, SrTiO3, was reported as an oxygen sensor operating near room temperature (40 °C). However, the very low conductivity of the sensing layer prevents its integration into commercial devices [5], [6]. Regarding n-type semiconducting oxides, only few reports can be found in the literature [7], [8]. As oxygen is an oxidizing gas, its interaction with the metal oxide surface further decreases the conductivity of the sensing layer. This feature makes n-type oxides less suitable for O2 sensing application in the low temperature range.

Among n-type metal oxides, indium oxide, In2O3 is a good candidate for oxygen sensing at low temperature because of its high intrinsic conductivity. Up to now, indium oxide was widely investigated as a sensing material for NOx, CO, ozone, CH4 and ethanol at low temperature [9], [10]. However, almost no data on oxygen sensing was reported, except of our previous work [11]. In fact, n-type In2O3 nanocrystals doped with Pt showed a high sensitivity towards oxygen at RT. An efficient way to increase the sensitivity is the reduction of the particle size down to the nanometer range. Many studies reported an increasing sensitivity with decreasing particle size. However, rather controversial explanations were given for this effect [12], [13], [14]. One of the consequences of the grain size reduction is the maximization of the surface available for the adsorption and reaction of the gas to be probed. On the other hand, when the particle size is smaller than 4–5 nm the nanocrystal is fully depleted. Furthermore, a high number of grain–grain interfaces and potential barriers increase the resistance of the sensing layer with decreasing particle sizes [15].

Also in the case of In2O3 doping with Pt was necessary to improve the sensitivity. In general, doping with noble metals is widely reported to promote the response of different metal oxides towards many reducing gases. This is due to the sensitizing effects of the noble metal on the metal oxide through chemical and/or electronic interactions [16], [17].

The response and recovery times observed in our preliminary study at RT were too slow for practical applications [11]. A higher working temperature would lead to an increase of the diffusivity of oxygen vacancies and surface reactivity and thus to shorter response and recovery times, finally also improving the sensitivity. Therefore, in order to develop a technologically relevant oxygen sensor, here we study the effect of working temperature on In2O3 and Pt-In2O3 thick films-based sensors. Moreover, a thorough investigation of the influence of temperature on the oxygen sensing mechanism is presented.

Section snippets

Nanopowders synthesis and characterization

Nanopowders of In2O3 were synthesized by a non-aqueous sol–gel method [18]. In a glove box, indium isopropoxide In(OiPr)3 was mixed with benzyl alcohol and reacted in an autoclave for two days at 220 °C. Pt-doped In2O3 powders were prepared by wet impregnation of indium oxide nanopowders with a solution of H2PtCl6 (Pt 37.5% min Alfa Aesar) [11]. Two doped samples were prepared having a nominal platinum loading of 1 and 3 wt%, respectively.

XRD analysis was carried out on an italstructure

Nanopowders characterization

The synthesis procedure for preparation of In2O3 nanopowders by the non-aqueous sol–gel method was described in details elsewhere [18], [19]. The complete absence of water, halide precursors and surfactants in the synthesis procedure results in pure and highly crystalline In2O3 powders. Doping with Pt was achieved by the incipient wetness impregnation method [11].

The TEM micrograph of the 3 wt% Pt-doped sample in Fig. 1 shows the cube-like morphology of the In2O3 grains with a side length

Conclusions

Nanostructured In2O3 powders doped with 1 wt% of platinum have been tested as O2 sensors at low temperature (from RT up to 350 °C). The presence of Pt promotes the response and decreases the operating temperature. Pt provides more active sites for the oxygen adsorption and dissociation on the indium oxide surface, thus favouring the kinetic of exchange of gaseous O2 with adsorbed oxygen species. This observation nicely agrees with the lower activation energy values of the Pt-In2O3 sensor for the

G. Neri received his degree in chemistry from the University of Messina in 1980. He is full professor of chemistry and director of the Department of Industrial Chemistry and Materials Engineering of the University of Messina. His research activity covers many aspects of the synthesis, characterization and chemical-physics of solids, with particular emphasis to catalytic and sensing properties. In the latter research area, his work has been focused on the preparation of metal oxide thick and

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    G. Neri received his degree in chemistry from the University of Messina in 1980. He is full professor of chemistry and director of the Department of Industrial Chemistry and Materials Engineering of the University of Messina. His research activity covers many aspects of the synthesis, characterization and chemical-physics of solids, with particular emphasis to catalytic and sensing properties. In the latter research area, his work has been focused on the preparation of metal oxide thick and thin films and their application in gas sensors.

    A. Bonavita received her degree in materials engineering from the University of Messina in 1997. At present time, she is at the Department of Industrial Chemistry and Materials Engineering of the University of Messina. Her research activity concerns with the preparation, characterization and development of semiconductor films for gas sensing applications.

    G. Micali received his degree in electronic engineering from the University of Messina in 2003. At present time, he is at the Department of Industrial Chemistry and Materials Engineering of the University of Messina. His research activity concerns with the implementation of software procedures for automated instrumentation control and the electrical characterization of gas sensing devices.

    G. Rizzo received his degree in chemistry from the University of Messina in 1999. Actually, he works at the Department of Industrial Chemistry and Materials Engineering of the University of Messina. His research activity is focused on the synthesis and characterization of materials by sol–gel method both for catalytic and optical applications.

    Nicola Pinna, chemist, senior researcher, received his degree in chemistry in 1998 and his PhD in 2001 from the University Pierre et Marie Curie (Paris, France). In 2002, he went to the Fritz Haber Institute of the Max Planck Society (Berlin, Germany). In 2003, he joined the Max Planck Institute of Colloids and Interfaces (Potsdam, Germany). In 2005, he joined the Martin Luther University, Halle-Wittenberg (Halle, Germany), as an associate professor of inorganic chemistry. In 2006, he joined the associated laboratory CICECO at the University of Aveiro in Portugal as senior researcher. His research activity is focused on the synthesis of nanomaterials by non-aqueous sol–gel routes, their characterization and the study of their physical properties.

    Markus Niederberger studied chemistry at the Swiss Federal Institute of Technology (ETH) Zurich, where he also received his PhD degree in the year 2000. After a postdoctoral stay at the University of California at Santa Barbara (USA), he became group leader at the Max Planck Institute of Colloids and Interfaces at Potsdam (Germany). Since 2007, he is assistant professor in the Department of Materials at the ETH Zurich. His research is directed towards the development of solution routes to inorganic nanoparticles, their surface functionalization and their assembly into ordered nano- and mesostructures.

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