Nanosensors: towards morphological control of gas sensing activity. SnO₂, In₂O₃, ZnO and WO₃case studies

Abstract

Anisotropy is a basic property of single crystals. Dissimilar facets/surfaces have different geometric and electronic structure that results in dissimilar functional properties. Several case studies unambiguously demonstrated that the gas sensing activity of metal oxides is determined by the nature of surfaces exposed to ambient gas. Accordingly, a control over crystal morphology, i.e. over the angular relationships, size and shape of faces in a crystal, is required for the development of better sensors with increased selectivity and sensitivity in the chemical determination of gases. The first step toward this nanomorphological control of the gas sensing properties is the design and synthesis of well-defined nanocrystals which are uniform in size, shape and surface structure. These materials possess the planes of the symmetrical set {hkl} and must therefore behave identically in chemical reactions and adsorption processes. Because of these characteristics, the form-controlled nanocrystals are ideal candidates for fundamental studies of mechanisms of gas sensing which should involve (i) gas sensing measurements on specific surfaces, (ii) their atomistic/quantum chemical modelling and (ii) spectroscopic information obtained on same surfaces under operation conditions of sensors.

In-Depth Understanding of the Relation between CuAlO2 Particle Size and Morphology for Ozone Gas Sensor Detection at a Nanoscale Level

“…The BET specific surface areas of CuAlO 2 flowers and hexagonal structures were calculated to be 59. 8 , respectively. The results suggest that the surface area distribution of the CuAlO 2 hexagonal structure is larger than that of flowers, due to its dimension formed by the aggregation of the CuAlO 2 nanoparticles, and hence could show good sensing behavior.…”

“…3−7 Ozone (O 3 ) is a powerful oxidizing reagent and a very strong disinfectant whose appearance in the atmosphere is very harmful for human health. 8 As per the World Health Organization (WHO), the air quality guidelines for ozone exposure of 1 ppm cause burning eyes, headaches, and irritation to the respiratory passages. An individual, if exposed to 0.2 ppm or 200 ppb of O 3 for 2 h, will sustain a loss of 20% in breathing capacity and, if exposed to 1 ppm of O 3 for 6 h, will suffer an attack of bronchitis.…”

Surface interaction of WO3 nanocrystals with NH3. Role of the exposed crystal surfaces and porous structure in enhancing the electrical response

“…21 Recently, several studies have demonstrated that the electrical response and the selectivity of nanostructured metaloxide gas sensors are signicantly affected by the shape of the nanocrystals and specically by their exposed crystal surfaces. 10,20,25,26 In this perspective, efforts have been devoted to the development of morphology controlled semiconductor oxides with specic exposed crystal surfaces. 20,[25][26][27] For instance, Han et al 26 demonstrated that shape controlled WO 3 nanocrystals with highly exposed {020} facets exhibit higher sensitivity towards 1-butylamine compared to nanoparticles with other prominent facets.…”

“…10,20,25,26 In this perspective, efforts have been devoted to the development of morphology controlled semiconductor oxides with specic exposed crystal surfaces. 20,[25][26][27] For instance, Han et al 26 demonstrated that shape controlled WO 3 nanocrystals with highly exposed {020} facets exhibit higher sensitivity towards 1-butylamine compared to nanoparticles with other prominent facets. The differences in the gas sensing capability were attributed to the presence of coordinatively unsaturated cations on the {020} surfaces, i.e.…”

Tuning of the Humidity-Interference in Gas Sensitive Columnar ZnO Structures

“…generally dominated by nonpolar or polar facets that provide a specific electronic structure at the surface. Thus, the morphological control of ZnO as well as other metal oxides has found to be particularly important in surface-dependent applications such as gas sensing, as it allows the tuning of the properties of gas sensitive materials [1]. The first stage toward the morphological control of structures includes the use of synthetic methods (e.g., bottom-up) capable of producing well-defined crystals, with uniform size, shape, and surface structure.…”