Investigation of CdO nanostructures synthesized by microwave assisted irradiation technique for NO₂ gas detection

Abstract

Cadmium oxide nanostructures were prepared with simple and cost-effective chemical route assisted by microwave irradiation. The synthesized samples were characterized by X-ray Diffraction (XRD), Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), Selected Area Electron Diffraction (SAED), Energy Dispersive X-ray (EDX) analysis and Fourier Transform Infrared spectroscopy (FT-IR) techniques. Highly crystalline CdO nanostructures with different morphology (spherical and rod-like) have been obtained by the microwave-assisted procedure adopted without any post-synthesis annealing treatment. Furthermore, the formation of these CdO nanostructures can be easily addressed within short times (5–15 min) by a fine tuning of the microwave irradiation. The CdO nanostructures have been investigated as sensing layer in resistive sensors and tested for NO₂ sensing at mild temperature. The sensors have shown high response to NO₂ down to 0.5 ppm at low operating temperature of 100 °C and high selectivity against CO.

Introduction

It is well recognized that quantum mechanical effects become dominant when the nanometer size range is reached, typically at distances of 100 nm or less. Additionally, a number of physical (thermal, electrical, optical, etc.) and mechanical properties change when compared to macroscopic systems. One example is the increase in surface area to volume ratio altering thermal and catalytic properties of materials. Furthermore, it is known that as the particle diameter, a, of semiconductor metal oxides decreases down to critical size ∼80 – 100 nm, the redistribution of electrons from bulk donors to the surface vacancies (“surface electron traps”) takes place. This leads to a significant (by three orders of magnitude) drop in the bulk conduction electron density. As a result, conductance of the material becomes very sensitive to the concentration of gas molecules adsorbed on the surface [1].

CdO is considered one of the most promising metal oxide materials. It is an n-type semiconductor with band gap of ∼2.5 eV, showing low resistivity due to defects of oxygen vacancies and cadmium interstitials. Then, due to its high electrical conductivity and optical transmittance in the visible region of solar spectrum, it has great potential for advanced applications (flat panel display, organic light emitting diodes, gas sensors, etc.).

Different techniques such as sol–gel [2], DC magnetron sputtering [3], radio-frequency sputtering [4], spray pyrolysis [5], pulsed laser deposition [6], chemical vapor deposition [7], chemical bath deposition [8], hydrothermal [9] and solvothermal [10] have been used to prepare CdO nanostructure. Microwave-assisted synthesis has more advantages with respect to above conventional methods such as very short reaction times, production of small particles with a narrow particle size distribution, and high purity [11], [12], [13], [14]. Further, it allows the controlled synthesis of nanostructures, a theme of outmost importance due to recent interests toward the search of nanomaterials with peculiar size and shape dependent properties having superior performances [2], [15].

In the present work, we report the preparation of CdO nanostructures with a simple and cost-effective chemical route assisted by microwave irradiation without any post-synthesis annealing treatment. Only few articles reported the preparation of crystalline CdO nanostructures without high temperature treatments [2]. Compared to previous reports, we have successfully prepared crystalline CdO nanostructures within 5–15 min.

Despite its interesting electrical properties, very little literature is concerning devices which employ CdO for gas sensors. So far, CdO has been used for detecting liquefied petroleum gas [16], carbon dioxide [17] and other reducing gases. To our knowledge, there are instead only limited reports on gas sensing studies towards oxidizing gases, such as NO₂ [18]. The present work aims therefore investigating the NO₂ sensing properties at mild temperature of CdO samples prepared by the chemical route assisted by microwave irradiation without any post-synthesis annealing treatment.