Room-temperature ammonia sensing via Nb2CTₓ/SnS2 nanocomposite sensor with enhanced selectivity

Abstract

The detection of ammonia (NH3) is essential for environmental monitoring, industrial safety, and medical diagnosis. However, fluctuating environmental conditions and the limited stability of conventional sensing materials make it difficult to achieve highly selective, highly sensitive, and reliable NH3 sensing at room temperature. To improve NH3 selectivity, we investigated a dual-functionalized Nb2CTx/SnS2 composite containing both amine and carboxyl groups. Combining the superior NH3 adsorption capability of SnS2 with the outstanding electrical conductivity and surface reactivity of the Nb2CTx MXene produces a composite that can be used as a highly sensitive and selective chemiresistive sensor. Experimental results revealed that this sensor had a low detection threshold of 10 ppm, along with fast response (32 s) and recovery (78 s) times at 100 ppm of NH3 under ambient conditions. Moreover, under harsh environmental conditions such as exposure to interfering gases, high humidity, and temperature fluctuations, the incorporation of the amine and carboxyl groups significantly enhanced the structural integrity and selectivity of the sensor. In real-world applications, this sensor could exhibit exceptional selectivity for NH3 against common interfering gases like formaldehyde, acetone, ethanol, trimethylamine, and CO2. Overall, these results highlight that this material could be used to develop a high-performance NH3 sensor with promising sensing characteristics under a wide range of environmental conditions.

1. Introduction

With the rapid growth of modern society, toxic and harmful gases are being released into the atmosphere, posing serious health and environmental threats. Consequently, the environmental pollution caused by toxic gases has become a major global concern [1]. Among these gases, ammonia (NH3) is a pungent, hazardous compound. Although it plays vital roles in natural and industrial processes, excessive emissions, mainly from industrial activities, raise health concerns. In addition to eye irritation and respiratory issues, exposure to NH3 at concentrations above 25 ppm can result in death or damage to the central nervous system [2–3]. Therefore, the ability to detect even small amounts of ammonia gas has gained significant attention in the biomedical field. NH3 gas sensors that operate at room temperature, with low detection limits, rapid responses, and high selectivity can detect harmful gases in the atmosphere and aid in early disease detection [4–6]. However, high humidity, high temperature, and poor selectivity pose major challenges for reliable gas detection. These issues arise in complex environments where the relative humidity (RH) exceeds 80% and dozens of interfering gases are present.

Chemiresistive sensors are widely used to detect harmful gases owing to their sensitivity, quick response time, and ease of use. A large operating temperature above 200 °C is typically required for NH3 detection because of the properties of the sensing materials (e.g., metallic oxides). MoS2/MoO3-based and MoO3/MoS2/rGO-based composite materials have been developed [7] (rGO—reduced graphene oxide), which require operations at 400 °C and 200 °C, respectively. Although these temperatures are necessary for energy activation, they pose safety risks and do not satisfy the power consumption or convenience demands of many applications. Therefore, the development of sensing materials that can detect NH3 at room temperature is crucial to meet the market needs and reduce the impact on society.

Recent research into two-dimensional (2D) materials has expanded to include graphene [8], MXenes [9], and transition metal dichalcogenides (TMDs) [10]. MXenes are 2D layered transition metal carbides or nitrides with the general formula Mn+1XnTx (n = 1, 2, or 3), where M is a transition metal, X is carbon or nitrogen, and Tx represents the surface functional groups (OH, O, and F) [11–12]. Recently, 2D layered MXenes have been studied for gas-sensing applications owing to their tunable physical and electronic properties. Density functional theory (DFT) calculations have also confirmed that MXenes such as Ti3C2Tx could be used to sense NH3 at room temperature [13–14]. Pure MXenes have low sensitivities and high detection limits for ammonia gas. However, when combined with other materials, such as TMDs, the composite exhibits a much stronger response and lower limit of detection, making it a promising candidate for high-performance room-temperature NH3 sensing.