Insights in the Application of Stoichiometric and Non-Stoichiometric Titanium Oxides for the Design of Sensors for the Determination of Gases and VOCs (TiO2-x and TinO2n-1 vs. TiO2).

Simonas Ramanavicius,Arunas Ramanavicius

doi:10.3390/s20236833

Abstract

In this review article, attention is paid towards the formation of various nanostructured stoichiometric titanium dioxide (TiO2), non-stoichiometric titanium oxide (TiO2−x) and Magnéli phase (TinO2n−1)-based layers, which are suitable for the application in gas and volatile organic compound (VOC) sensors. Some aspects related to variation of sensitivity and selectivity of titanium oxide-based sensors are critically overviewed and discussed. The most promising titanium oxide-based hetero- and nano-structures are outlined. Recent research and many recently available reviews on TiO2-based sensors and some TiO2 synthesis methods are discussed. Some promising directions for the development of TiO2-based sensors, especially those that are capable to operate at relatively low temperatures, are outlined. The applicability of non-stoichiometric titanium oxides in the development of gas and VOC sensors is foreseen and transitions between various titanium oxide states are discussed. The presence of non-stoichiometric titanium oxide and Magnéli phase (TinO2n−1)-based layers in ‘self-heating’ sensors is predicted, and the advantages and limitations of ‘self-heating’ gas and VOC sensors, based on TiO2 and TiO2−x/TiO2 heterostructures, are discussed.

Highlights

Gas and volatile organic compound (VOC) sensors are important for various safety and environmental control issues
Rather simple gas and VOC sensors based on the measurement of the electrical resistance of the semiconducting layer are developed [1,2,3,4,5,6]
Semiconducting materials are applied in the design of sensing layer, which is responsible for the formation of analytical signal towards target gases and/or VOCs

Summary

Introduction

Gas and volatile organic compound (VOC) sensors are important for various safety and environmental control issues. Rather simple gas and VOC sensors based on the measurement of the electrical resistance of the semiconducting layer are developed [1,2,3,4,5,6]. Metal oxide-based semiconductors [7], many different forms of stoichiometric TiO2 and various nonstoichiometric titanium oxides (TiOn) are applied in the design of gas and VOC sensors [8,9], because these titanium-based compounds are rather cheap, nontoxic, biocompatible, chemically stable and insoluble at neutral pHs. Stoichiometric TiO2 is n-type semiconducting materials and exist in three main solid phases (anatase, rutile and brookite); during the formation of sensing layer phase transformations are very often exploited to modify sensitivity and selectivity of formed TiO2 layers. In addition to stoichiometric TiO2 , recently, Sensors 2020, 20, 6833; doi:10.3390/s20236833 www.mdpi.com/journal/sensors

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