Nanostructured Semiconducting Metal Oxides for Ammonia Sensors. A Novel HSAB Sensing Paradigm

Bogdan-Catalin Serban,Cornel Cobianu,Marius Bumbac,Cristina Mihaela Nicolescu,Mihai Brezeanu,Octavian Buiu

doi:10.17344/acsi.2018.4564

Abstract

The paper demonstrates how the Hard-Soft Acid Base (HSAB) theory can be used as a valuable criterion in the selection process of semiconducting metal oxides (MOX) suitable as sensing layers for ammonia detection. Six different cases of ammonia detection performed by chemiresistive sensors employing MOX and related nanocomposites as sensing layers are identified and discussed. The role of HSAB as an efficient selection tool for appropriate sensing layer (any type of gas), is further reinforced by analyzing and discussing literature results on MOX-based trimethylamine sensing layers. By analyzing the operation of a fiber-optic ammonia sensor, we demonstrate that the HSAB principle can be also successfully applied to the selection of sensing layers for detectors employing other sensing principles, different than the chemiresistive one. Changing the sensing paradigm (i.e., the amino groups-based compounds are part of the sensing layer, rather than part of the analyte), the paper shows that these types of molecules (polymers, carbon nanotubes, ionic liquids) are appropriate constituents of a CO2 sensing layer, in full accordance to the HSAB criteria.

Highlights

Ammonia (NH3), a natural gas that is present throughout the atmosphere, is a highly toxic compound, with low odor threshold (20 ppm), corrosive to the skin, eyes, throat and lungs
To support the conjecture that the Hard-Soft Acid Base (HSAB) principle can be used when selecting the appropriate gas sensing layer, we will bring into the discussion facts and conclusions related to the following three issues: 1) the results obtained for the metal oxides (MOX) based – trimethylamine sensing; 2) evaluation of fiber-optic ammonia sensor using MOX
By investigating the literature, we found that most of the MOX exhibiting good performances to ammonia detection are classified as hard acids according to HSAB theory

Summary

Introduction

Ammonia (NH3), a natural gas that is present throughout the atmosphere, is a highly toxic compound, with low odor threshold (20 ppm), corrosive to the skin, eyes, throat and lungs. MOXbased sensors are by far the least expensive type of ammonia sensors and are not damaged or consumed by prolonged exposure to the analyte Because their output signal is non-linear, they need to be calibrated and adjusted for being reliably used in the desired detection range.[13] Another drawback of MOX-based sensors is the significant cross-sensitivity, with relative humidity. To support the conjecture that the HSAB principle can be used when selecting the appropriate gas sensing layer (in particular for NH3), we will bring into the discussion facts and conclusions related to the following three issues: 1) the results obtained for the MOX based – trimethylamine sensing; 2) evaluation of fiber-optic ammonia sensor using MOX and their nanocomposites, showing that, despite a completely different detection principle, HSAB interpretation works and explains sensing mechanism; 3) changing the sensing paradigm and showing that, using the same principle, amino group – based molecules (polymers, carbon nanotubes, ionic liquids) can be evaluated as appropriate candidates for carbon dioxide detection

MOX-based Gas Sensing Layers

MOX-Based Gas Sensing Layers

MOX-Based Sensing Layers for Ammonia Chemiresistive Detection

MOX-Based Sensing Layers for Ammonia Optical Detection

HSAB Theory Employed as a Sensing Layer Selection Tool

Conclusions