# Highly Sensitive and Selective H\({}_{2}\)S Chemical Sensor Based on ZnO Nanomaterial
## 1 Introduction  
Hydrogen sulfide (H\({}_{2}\)S) is a colorless, highly flammable and toxic gas [1]. It is produced due to industrial activities including petroleum refineries, natural gas plants, paper milling, sewage treatment plants and taneries [2; 3; 4; 5]. H\({}_{2}\)S remains in the atmosphere from 1-42 days, depending on the season [6]. Its presence causes eye irritation, fatigue, headache, poor memory, dizziness, olfactory paralysis and respiratory distress. An increase in the concentration of H\({}_{2}\)S up to 700 ppm causes human death [7]. Consequently, the monitoring of ambient H\({}_{2}\)S levels, particularly in areas at hazardous waste sites, is needed. The information obtained can be used in combination with the known body burdens of H\({}_{2}\)S to evaluate the potential risk of adverse health effects in the population. H\({}_{2}\)S can also be released from meat, seafood, egg and milk products during their cooking and storage, and its production increases with the rise in temperature [8; 9]. Thus, the detection of H\({}_{2}\)S can be useful for the identification of food quality. The ingestion of food and water with high sulfur content increases urinary thiosulfate concentrations [10]. Moreover, blood sulfide levels were proposed as a biomarker [11]. Therefore, the development of small-size and portable H\({}_{2}\)S gas sensors is in high demand to provide environmental and human health safety. Metal oxides are very attractive materials for the fabrication of chemical sensors due to their ability to interact with different gaseous compounds [8; 12; 13; 14]. In this aspect, the preparation of nanoscale oxide materials seems to be more efficient to improve their functional performance and opens new perspectives for their application in chemical gas sensors [15; 16; 17; 18; 19]. Low-dimensional metal oxide nanostructures with different shapes have been used to develop high-performance gas sensing systems [20; 21; 22; 23; 24; 25; 26]. To date, the most-studied material for chemical sensing applications is SnO\({}_{2}\)[27; 28]. Other types of metal oxide nanostructures have been used as alternative materials to SnO\({}_{2}\)[25; 29; 30; 31]. In recent years, much attention has been devoted to ZnO owing to its high electron mobility and thermal stability [16; 32; 33]. However, achieving a high response and selectivity of ZnO towards H\({}_{2}\)Sis still a challenge. To enhance the gas sensing performance of ZnO it was functionalized with noble metal nanoparticles [34; 35; 36]. The noble metals improve the interaction of gaseous molecules with the sensing layer due to their catalytic activity [37; 38; 39]. Nevertheless, this is an expensive method and can increase the self-cost of the final device. In addition, the metal nanoparticles can lose their catalytic activity at relatively high operating temperatures due to the coagulation of particles on the support or can be poisoned by sulfur-containing chemical compounds [40; 41]. Recently, the sensing properties of membrane-coated ZnO materials have been investigated. The obtained results have shown that it is an effective strategy to improve the materials selectivity [42; 43]. To enhance the interaction between the gas molecules and sensing material, doped ZnO structures have been studied as well [15; 16]. Some dopant materials decreased the optimal operating temperature of ZnO towards H\({}_{2}\)S [44; 45]. However, the operation of metal oxide-based sensors below 200 \({}^{\circ}\)C can be affected by water molecules, which worsen the material's recovery abilities [18]. The surface morphology and the structure of semiconductor materials have crucial effects on their sensing properties since the gas sensing process relies on the adsorption/desorption reactions of gas molecules on the surface of a material [46; 47; 48; 49; 50; 51; 52]. The synthesis of metal oxide nanostructures, varying their morphologies, improves their response towards specific gases [15].