Al-doped ZnO for highly sensitive CO gas sensors

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Highlights

  • Preparation of Al-doped ZnO (AZO) nanoparticles.

  • AZO nanoparticles can be used in resistive sensors for detecting carbon monoxide (CO).

  • AZO-based sensors exhibit excellent sensing properties for CO detection.

  • Relationships between Al loading and sensing properties were established.

Abstract

Al-doped ZnO (AZO) nanoparticles have been prepared using a modified sol–gel technique. The as-prepared AZO nanoparticles were annealed at 400 °C, and their morphologies and microstructural characteristics were investigated using transmission electron microscopy (TEM) and x-ray powder diffraction (XRD) analyses. Crystallites with an average size of approximately 60–70 nm and ZnO as a primary phase were observed in all samples. In addition, smaller nanoparticles (less than 5 nm) with an Al-rich structure covering the surface of larger ZnO crystallites were also noted on the Al-doped samples.

Chemoresistive devices consisting of a thick layer of AZO nanoparticles on interdigitated alumina substrates have been fabricated, and their electrical and sensing characteristics for carbon monoxide were investigated. Al-doping provided a remarkable decrease in the electrical resistance of the sensing layer at the working temperature of the sensors (250–300 °C). The sensors based on Al-doped ZnO exhibited a higher response than the pure ZnO sample, allowing the detection of CO at sub-ppm concentrations in air. The enhancement in sensing properties was discussed in terms of the characterization and electrical data.

Graphical abstract

Morphology of the synthesized Al-doped ZnO nanoparticles and response of developed sensor to different concentrations of CO in air.

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Introduction

Semiconducting metal oxides, such as SnO2, ZnO, and In2O3, are commonly employed in resistive gas sensors, exploiting the changes in the electrical conductivity of these materials upon exposure to gaseous mixtures [1], [2], [3], [4], [5], [6]. Among these metal oxides, the utilization of ZnO in gas sensor applications has a long history; it was used in chemo-resistive sensors to detect several gases, such as H2, NH3, CH4, O2, ethanol and CO [7], [8], [9], [10], [11], [12], [13]. The recent demonstration of gas sensors based on nanostructured metal oxides has further stimulated substantial efforts to explore novel ZnO nanostructures for high gas sensing performance. Nanostructured zinc oxide with different morphologies (such as nanoparticles, nanowires, nanorods) has been extensively studied for gas sensor applications due to its favorable morphological, microstructural and electric properties [14], [15], [16].

Doping is another important and effective method for improving the sensing properties of metal oxide semiconductors [17]. In fact, doping with noble metals (Pt, Pd) or other additives (such as Al and Cu) enhances the sensitivity and selectivity of gas sensors [18], [19], [20], [21]. These dopants and additives enhance the gas-sensing properties by changing the energy-band structure and morphology, increasing the surface-to-volume ratio and creating more centers for gas interaction on the metal oxide semiconductor surface.

In this study, Al-doped ZnO (AZO) nanoparticles were synthesized, characterized and tested for the monitoring of low concentrations of CO in air. Carbon monoxide (CO) is one of the most dangerous gases in air pollution and for human life. CO is produced by the incomplete combustion of fuels and is commonly found in automobile exhaust, the burning of domestic fuels, and so on. It is highly toxic and extremely dangerous because it is colorless and odorless. The health effects of CO depend on the CO concentration and length of exposure. The U.S. Environmental Protection Agency (EPA) recommends an ambient air quality of 9 ppm CO or lower averaged over 8 h and 35 ppm or lower over 1 h [22]. At CO levels above 70 ppm, symptoms can include headache, fatigue and nausea. At sustained CO concentrations above 150–200 ppm, disorientation, unconsciousness, and death are possible. CO sensors are, therefore, required in various situations, including the detection of smoldering fires and air quality in urban and closed environments [23].

Al improves the electrical conductivity due to the small size of the Al3+ ion compared to that of Zn2+ (rAl3+ = 0.054 nm and rZn2+ = 0.074 nm) [24]. Aluminum plays a very important role in ZnO gas sensors, but few reports address the synthesis and characterization of AZO nanopowders for specific sensing applications [25], [26], [27]. Here, we present a study on the electrical and sensing properties of AZO materials with different Al loadings to optimize the formulation of the sensing layer and to determine the best conditions for CO sensing.

Section snippets

Sample preparation

ZnO nanopowders were prepared using a sol–gel route with 16 g of zinc acetate dihydrate [Zn(CH3COO)2·2H2O; 99%] as a precursor in 112 ml of methanol. After 10 min of magnetic stirring at room temperature, an adequate quantity of aluminum nitrate-9-hydrate corresponding to [Al/Zn] ratios of 0, 0.01, 0.03 and 0.05 were added. After 15 min of magnetic stirring, the solution was placed in an autoclave and dried in supercritical ethyl alcohol (Tc = 243 °C; Pc = 63.6 bar), according to protocol reported in

Morphological and microstructural characterization

AZO nanoparticles with different Al loadings were prepared using a modified sol–gel route, as reported in detail in a previous paper [28]. The TEM images in Fig. 1, Fig. 2 show the shapes and particle size distributions of the as-prepared AZO samples from the sol–gel route. The pure ZnO sample shows particles with irregular shapes that have a broad particle size distribution; the majority of the particles have a size of approximately 20–40 nm (Fig. 1). In addition to these large particles, we

Conclusion

Al-doped ZnO powders with different Al loadings were synthesized using a sol–gel process. After annealing at 400 °C, they were investigated as the sensing layer for CO sensing. The morphologies, sizes and microstructures, investigated as a function of the Al loading, showed the presence of ZnO as the primary phase in all samples, whereas a secondary Al-rich structure in the form of very small nanoparticles (less than 5 nm) that cover the surface of the larger ZnO grains was found in the Al-doped

M. Hjiri received a Master's Degree in Materials and Nanomaterials in 2010 from the University of Monastir. Currently, he is a PhD student at the Laboratory of Physics of Materials and Nanomaterials Applied at Environment, Gabes, Tunisia. His research activities are focused on the development of chemical sensors based on metal oxide nanomaterials.

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M. Hjiri received a Master's Degree in Materials and Nanomaterials in 2010 from the University of Monastir. Currently, he is a PhD student at the Laboratory of Physics of Materials and Nanomaterials Applied at Environment, Gabes, Tunisia. His research activities are focused on the development of chemical sensors based on metal oxide nanomaterials.

L. El Mir completed his PhD in 1995 from College of Sciences in Tunis in collaboration with the University of Paris VI in the field of electronic transport in semiconductors and his “HDR” (Ability of Direction of Research) in 2007 from College of Sciences in Sfax, Tunisia in the field of nanotechnology. Between 1992 and 2010, he spent several years of research at the University of Paris VI and the Institute of NanoSciences in Paris (France). He became a Full Professor of Physics in 2012. From 1997 to 2002, he was the Head of the Physics Department at the College of Sciences in Gabes University. His main research interests are the synthesis and characterization of nanoparticles, thin films and nanocomposites for a variety of applications, such as solar cells, transparent electrodes, advanced catalyst supports, water treatment, energy storage and gas sensors. Now, he is a visiting Professor in the College of Sciences at the Al-Imam Muhammad Ibn Saud Islamic University, Saudi Arabia.

S.G. Leonardi received a degree in Materials Engineering from the University of Messina in 2011. He is currently a PhD student in the Engineering and Chemistry of Materials Department at the University of Messina. His research activity is focused on the development of chemical and biochemical sensors based on nanostructured materials.

A. Pistone received a degree in Industrial Chemistry in 1996 and his PhD degree in 2000. He is an assistant professor in the Department of Electronic Engineering, Chemistry and Industrial Engineering, University of Messina, where he holds courses in Applied Chemistry and Materials Technology that address the topics of materials science, recycling, alternative energy and nanomaterials. His main research interest has been focused for several years on the nanotechnology field and, in particular, the synthesis and functionalization of carbon nanotubes for applications in various fields of materials technology, including polymer composites, drug delivery, catalysis and gas sensors.

L. Mavilia holds a degree in Industrial Chemistry and is a Materials Science and Technology academic assistant professor. Currently, he is affiliated with the Department of Heritage, Architecture and Urban Planning. Here, inside the laboratory Materials Analysis for Restoration, he performs studies on the evaluation of chemical and physical properties of ancient and modern building materials. His current research topics are alteration mechanisms of construction materials, the development of new sustainable inorganic building materials, the characterization methodologies and conservation practices of archeological finds and many other topics.

G. Neri received a degree in Chemistry in 1980. From 1987 to 1998, he was a researcher at the University of Reggio Calabria. From 1991 to 1996, he spent several years conducting research at the University of Michigan (USA). In 1998, he moved to the University of Messina as an Associate Professor. Since 2001, he has been a Full Professor of Chemistry. From 2004 to 2007, he was the Director of the Department of Industrial Chemistry and Materials Engineering at the University of Messina. His research activities cover many aspects of the synthesis and characterization of materials and the study of their catalytic and sensing properties. His recent research has been focused on the application of gas sensors composed of nanostructured metal oxides and novel organic–inorganic hybrid nanocomposites.

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