Metal Oxides: Nanostructured Metal Oxides for Gas Sensing Applications

Abstract

Semiconductor metal oxide gas sensors have undergone extensive development during the last decades following the invention of the original tin oxide sensor by Taguchi.1,2 Their operating principle is based on the measurement of changes in the electrical conductance of a metal oxide film, resulting from physicochemical reactions with gas molecules adsorbed on its surface, which can be directly correlated to the gas concentration.3 While tin dioxide is the most widely employed material for gas sensors, because it is sensitive to practically all the toxic and inflammable gases of interest, various other metal oxides may be used to obtain better sensitivity and selectivity to specific gases. Furthermore, the effects of interferences due to the presence of different gases can be largely suppressed by an appropriate choice of the sensor operating temperature. An overview will be given here of the most commonly used methods for the production of thin and thick films and the application of different metal oxides for the detection of low concentrations of various gases. The techniques employed for the synthesis of metal oxide layers include physical vapor deposition (PVD), chemical vapor deposition (CVD), spray pyrolysis, sol–gel processes, and screen printing of metal oxide powders.4 Gas sensors can be fabricated using either thick-film or thin-film technologies. Although thick-film devices have the advantage of more robust construction, thinfilm technology may be more compatible with conventional microelectronic manufacturing methods. The use of standard semiconductor processing steps in conjunction with micromachining techniques permits device miniaturization, enabling fabrication of monolithic integrated solid-state sensor arrays on a single silicon chip.5 The deposition conditions significantly influence the microstructure, electrical properties, and gas sensitivity of metal oxide films. Major advances in solid-state gas sensor performance have been possible due to recognition that the electrochemical properties of metal oxide films are closely related to the grain size. Reduction of the crystallite diameter to dimensions comparable to twice the width of the depletion layer, formed as a result of charge exchanges with adsorbed oxygen species, greatly increases sensitivity.6 Improvements in both sensitivity and selectivity can be achieved by the use of noble metal dopants, either deposited on or embedded in the metal oxide layer. Further progress in gas sensor technology may be feasible by employing low-dimensional or hierarchical metal oxide architectures, such as nanowires, nanorods, and nanobelts.7,8 These novel structures provide increased surface area on which gas adsorption and reactions can take place and may induce quantum confinement effects that lead to the modification of the electronic band structure of semiconductor materials.