FABRICATION OF LOW-DIMENSIONAL NANOMATERIALS BASED SENSORS FOR HUMIDITY AND TOXIC GAS DETECTION

Abstract

Sensors acting as an automated device to make a correlation between the digital and physical worlds having the unique performance which can create new applications and ways for better operator interaction. There are several types of sensors used for explaining and solving the numerous problems of electronic devices which are temperature sensor, gas sensor, touch sensor, light sensor, humidity sensor etc. Portable, reliable and low-cost humidity sensors play an important role in our day to day life, including industry, agriculture, environmental fields and medical devices. For keeping the required surrounding environment, it is essential to have a suitable humidity sensor which can detect and control the ambient environment under different conditions including the rise and fall of temperature, or the mixtures of various gases precise and providently. So far, many efforts have been made to develop high-performance humidity sensors using various transduction techniques, such as capacitance, resistance, optical fiber, and various electronic devices. Furthermore, several kinds of sensing materials have been employed in humidity sensors, such as polymers, metal oxide, carbon nanotubes, and composites, but they have their own advantages and specific conditions of application. Also, there are various kinds of natural, chemical and artificial species available in our surrounding environment, some of them are very essential while others are more harmful or less. The essential gases/species like oxygen (O2), nitrogen (N2) and humidity should be maintained at sufficient level in global atmosphere while excess emission of hazardous or toxic gases (CO2, N2O, NO, NO2, NH3, and SO2 etc.) can harm living atmosphere. In related to various chemical compounds, volatile organic compounds (VOCs) like, ethanol, methanol, acetone, chloroform, isopropanol etc. also attract much more interest from the last few decades in abundant significant fields like R&D laboratories, industrial productions etc. due to its toxicity, flammability, environmentally hazardousness, and explosiveness. As per the literature scientists have already used carbon black, carbon nanotubes, graphene, black phosphorous, transition metal chalcogenides, MXene etc. But, there is some obstacle to use these type of materials as virgin such as, environmental stability (contamination, corrosion and moisture), selectivity of gas, high agglomeration rate etc. To overcome all these obstacles, these low-dimensional nanomaterials have been modified by doping, coating or decorated by some elements.A customized test setup is designed to perform the gas sensing studies, where a known and fixed volume of the test chamber is made including mass flow controllers, for controlling the dry/synthetic airflow including desired target gas inside the test chamber with or without humid condition. Also, a wireless hand-held device has been designed to measure the toxic gas concentration of the remote area. The detailed morphological as well as structural characterization by various techniques and gas sensing performance of this unmodified and modified low-dimensional materials have been extensively performed and their sensing mechanism is proposed. At the onset, we demonstrate for the first time a highly sensitive chemiresistive sensor for low-level humidity detection in ambient atmosphere by introducing graphene oxide (GO) and doped GO (Li-doped GO and B-doped GO) as a thin film in a facile manner. The incorporation of doping into GO induces a dramatic change in the sensing behavior of the base film (undoped GO). Later, a selective chemiresistive gas sensor for carbon dioxide gas detection at room temperature (~ 25 °C) was successfully fabricated, where ruthenium decorated tungsten disulfide (Ru@WS2) quantum dots (QDs) have been used as the sensing material. The Ru@WS2 QDs based sensor showed superior sensitivity and good selectivity to CO2 gas in comparison with VOC’s at room temperature than WS2 QD. Ru@WS2 QD shows less effect at different humid condition compares to WS2 QD as a CO2 gas sensor. After that, we explain the enhancement in gas-sensing performance of molybdenum disulfide (MoS2) nanoflakes (NF) by decorating with Lanthanum (La) at room temperature (25 ºC). It is noteworthy that La has introduced to MoS2, and its selectivity towards the volatile organic compounds (VOCs) and other toxic gases improved drastically. Afterward, a selective chemiresistive gas sensor of molybdenum diselenide/graphene (MoSe2/G) nanoflakes (NF) that detects ammonia (NH3) and nitrogen dioxide (NO2) gas at room temperature was successfully fabricated. The superior performance of this gas sensor makes MoSe2/G NF a potential candidate for NO2 and NH3 gas detection at room temperature. Due to the presence of graphene, MoSe2 NF becomes less hydrophilic which showed very less effect of humidity effect. Next, we present effective liquid exfoliation of few-layer phosphorene (FLP) from bulk black phosphorous (BP) in the presence of cationic surfactants like cetyltrimethylammonium bromide (CTAB), which is highly stable. It successfully stabilizes FLP in deionized water, which is consistent with obtained characterization and gas sensing studies. It is noteworthy that when polylactic acid (PLA) membrane introduced as a barrier layer in our fabricated Arduino based Bluetooth enabled handheld device, and it obstructs the environmental effect with a trace-level detection capability and negligible change over time (up to 30 days). At last, we demonstrate two-dimensional (2D) metal carbide MXenes (Nb2CTx) having the high metal conductivity for reduction in noise and their fully functionalized surface for detecting the signal towards higher sensitivity. The sensitivity of the Nb2CTx and Nb2CTx -CTAB found 0.543 and 1.686 ppm-1, respectively, towards NO2 gas. It is noteworthy that delamination by CTAB there is no environmental effect with a trace-level detection capability and negligible change over time (up to 30 days). Herein, the first time we discover the gas sensing characteristics of Nb2CTx -CTAB and witness an ultra-sensitive and selective response toward NO2 gas detection. Finally, a detailed conclusion has been drawn best on the fabricated various materials for humidity and toxic gas detection at room temperature.