Selective trimethylamine sensors using Cr₂O₃-decorated SnO₂ nanowires

Abstract

Pristine SnO₂ nanowires (NWs), Cr₂O₃-decorated SnO₂ NWs, and SnO₂–Cr₂O₃ core-shell nanocables (NCs) were prepared by thermal evaporation, and their gas-sensing characteristics were investigated. The decoration of discrete p-type Cr₂O₃ nanoclusters on SnO₂ NWs increased their response to trimethylamine (TMA). Highly sensitive and selective detection of TMA was attributed to the chemical affinity and catalytic activity of Cr₂O₃ toward TMA. In contrast, the SnO₂–Cr₂O₃ NCs, formed by the coating of a continuous p-type Cr₂O₃ overlayer on SnO₂ NWs, showed negligibly low responses to all analyte gases used. The variations in gas-sensing characteristics due to configuration changes in one-dimensional SnO₂–Cr₂O₃ hetero-nanostructures are explained and discussed in relation to the gas-sensing mechanisms of n- and p-type oxide semiconductors.

Introduction

Single crystalline nanowires (NWs) of SnO₂ and ZnO are attractive platforms for the fabrication of highly sensitive and reliable chemiresistors because of their high surface-area-to-volume ratio and excellent thermal stability at high sensor temperatures [1], [2], [3], [4]. Moreover, a simple sensing algorithm based on changes in charge carrier concentration due to the interaction between an analyte gas and oxide surface provides various benefits such as simple structures, cost effectiveness, and integration into small devices. Moreover, gas sensing characteristics of oxide NW sensors can be enhanced or manipulated by loading/doping catalytic additives [5], [6] or controlling the composition [7].

The gas response of n-type oxide semiconductors is known to increase dramatically when the size of nanostructures is comparable or equal to twice the thickness of the electron depletion layer (5–10 nm) [8], [9], [10], [11]. The diameter/thickness of SnO₂ and ZnO NWs reported in the literature ranges from 50 to 200 nm [12]. Considering that the preparation of single crystalline NWs with a diameter/thickness of <20 nm is very difficult, the enhancement of the gas response of oxide NWs by realizing full electron depletion remains challenging. Moreover, pure oxide NWs often react with a range of different gases owing to their simple sensing algorithm, and this hampers the selective detection of a specific gas.

In previous works, the present authors have suggested that the gas response of n-type ZnO NWs can be significantly increased by decorating them with a discrete configuration of p-type nanoclusters such as those of NiO [13], Cr₂O₃ [14], and Mn₃O₄ [15]. The extension of electron depletion layer in n-type oxide NWs beneath p-type nanoclusters due to the formation of a nanoscale p–n junction was suggested as the reason for the high gas response of oxide NWs thicker than 80 nm. Moreover, it has been proved that the catalytic activity of p-type nanoclusters promotes the selective detection of an analyte gas [14], [16]. Nevertheless, the approach of designing highly sensitive and selective gas sensors by using one-dimensional hetero-nanostructures between n- and p-type oxide semiconductors is still in a nascent stage and should be investigated further to obtain a more general design rule for gas sensors based on oxide hetero-nanostructures. For example, it remains uncertain whether the decoration of representative chemiresistive SnO₂ NWs with catalytic p-type oxide nanoclusters can enhance their gas response and selectivity. Moreover, gas sensing characteristics of SnO₂ NWs coated with continuous configuration of p-type oxide overlayer should be investigated to understand the gas sensing mechanism and the key parameters to determine the gas response.

In the present study, p-type Cr₂O₃-decorated n-type SnO₂ NWs and SnO₂–Cr₂O₃ core–shell nanocables (NCs) are prepared, for the first time, by a two-step thermal evaporation method, and their gas-sensing characteristics are investigated. The decoration of Cr₂O₃ nanoclusters on SnO₂ NWs greatly enhanced both their response and selectivity to trimethylamine (TMA), while no specific gas could be detected in a sensitive and selective manner in the SnO₂–Cr₂O₃ core–shell NCs with continuous Cr₂O₃ overlayer. This study is mainly focused at understanding why a discrete but not continuous configuration of a p-type Cr₂O₃ overlayer on SnO₂ NWs is advantageous for high-performance gas sensor design.