Papers by Keyword: Nitrogen Dioxide

Abstract: Transition metal dichalcogenides (TMDCs) nanomaterials, in particular Molybdenum disulfide (MoS₂), have been employed frequently as a basis for flexible gas sensors due to their extreme sensitivity to gas molecules, super mechanical and electrical properties, and large surface area. This work aims to study the behavior of the flexible gas sensor made of 2D-MoS₂ under exposure to nitrogen dioxide (NO₂) gas at the part per million (ppm) level. The mono-layered MoS₂ was successfully synthesized by Chemical Vapor Deposition (CVD). The formation of MoS₂ layers was confirmed by Raman spectroscopy and Photoluminescence (PL). Two different gas-sensing devices were fabricated by transferring two MoS₂ samples (obtained from two positions inside the CVD tube) onto paper substrates. Specifically, upstream sample S_up was obtained from an area near the MoO₃ source, and downstream sample S_down was obtained from an area far from the MoO₃ source. Both sensors showed a good response to a concentration as low as (1.5 ppm) of NO₂. Although a high response of 62.8% along with a fast response of 9 sec were recorded by S_down, the sensor showed a slow recovery time of 42 sec. On the other hand, S_up showed good stability with an appropriate response of 36.8% along with a reasonable response time and recovery times of 20 and 27 sec, respectively. Such behavior could be accredited to the difference in the reactivity in both MoS₂ samples. This work opens the way for further improvements in manufacturing MoS₂-based flexible gas sensors.

Abstract: Heterostructure thin films of indium and zinc oxides (IZO) were prepared by spray pyrolysis from an aqueous solution of the precursors at different substrate temperatures (T_S). The polycrystalline structure of bixbyite appeared at a low temperature. The crystallinity was enhanced with the emergence of the zinc oxide phase. By increasing the T_S to 623 K, the crystallite size was increased. SEM images reveal that the deposited sample at 523 K is composed of irregularly shaped nanoparticles with a lack of links. Increasing the T_S to 573 K increases the average particle diameters, and the particles appeared as polyhedrons well connected with cavities between them, which candidates for gas sensing applications. Increasing T_S to 623 K resulted in the particles merging. NO₂ gas sensor results confirmed the enhancement of IZO sensitivity performance at 573 K. Keywords: Gas sensor, thin film metal oxide, spray pyrolysis, In₂O₃– ZnO

Abstract: Herein, we report a facile synthesis of zinc oxide-reduced graphene oxide (ZnO-rGO) hybrid materials by two-step method. Firstly, rGO was synthesized by using graphite powder mixed with sodium nitrate, sulfuric acid and potassium permanganate via Hummers method. Synthesized rGO were dispersed in ethanol by ultra-sonication for a designated time period. Then, zinc oxide (ZnO) powder was added into rGO-ethanol solution and transferred into Teflon-lined stainless steel autoclave. The ZnO-rGO was produced by hydrothermal method at 180 °C for 120 and 180 min (here after referred to as ZnO(120)-rGO and ZnO(180)-rGO, respectively). The morphological and crystalline structures of synthesized rGO and ZnO-rGO were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). Synthesized ZnO-rGO was exposed to 100 parts per million (ppm) nitrogen dioxide (NO₂) gas at room temperature, 50 °C and 75 °C for testing its sensing performance. The results show that ZnO(180)-rGO hybrid materials exhibit high response to NO₂ gas at 50 °C and 75 °C. The electrical resistance of ZnO-rGO sensors decreased when the sensors responded to NO₂ gas, indicating a p-type behavior. Moreover, the ZnO-rGO hybrid materials can detect 100 ppm NO₂ gas with an operating temperature limit at 50 °C. The results imply that synthesized ZnO-rGO hybrid materials could be used as gas sensing device for ppm-level NO₂ detection at low temperature and consume low power.

Abstract: Graphene films were grown on SiC substrates by annealing in vacuum or in Ar flow. Gas sensors based on graphene films were made and tested on response to nitrogen dioxide. Graphene film is used in the sensor. The graphene film grown by annealing in Ar flow shows superior sensitivity compared to that annealed in vacuum. Both sensors exhibit good potential for environmental research and monitoring

Abstract: Ozone (O₃) and nitrogen oxides (NO_x) are closely related in the atmosphere. In ambient air, these pollutants always fluctuated depending on their emission sources and meteorological influences. The paper is aims to gain insight understanding of the monthly temporal variation of O₃ and NO_x concentrations to enable proper control strategies against these pollutants. One-year monitoring records from 1^st January to 31^st December 2009 of O₃ and NO_x at Pasir Gudang, were obtained from Department of Environmental Malaysia. Temporal analysis of O₃ and NO_x concentration fluctuation during annual and monthly were assessed using time series and scatter plots. The annual variations of O₃ concentrations were negatively correlated with annual variation of NO and NO₂ concentrations. Results suggest that NO concentration are higher than O₃ and NO₂ especially in May 2009. However, zero exceedences were recorded in the studied period for all pollutants against the Malaysia Ambient Air Quality Guidelines.

Abstract: The daily pollutant variations of PM_2.5, PM₁₀, SO₂ and NO₂ are useful for environment protection and advices for human health. Two years data from three different time period before Spring Festival (SF), after SF and after Lantern Festival (LF) in Shenyang city of year 2013 and 2014 was analyzed. Results showed that concentrations of PM_2.5, PM₁₀, SO₂ and NO₂ were higher in 2014 than those in 2013, especially for SO₂ and NO₂; Among the three periods, concentrations of four air pollutants followed the sequence of after LF > before SF > after SF and road traffic emissions are more contributive than firework combustion during the festival celebration. We conclude that air pollution in Shenyang City is worsening and more influenced by vehicular pollution.

Abstract: Complaints on poor air quality in an enclosed car park have been raised up among the public, which might cause serious health effects to the drivers, passengers, and labours who are working at the premises. Improper design of mechanical ventilation systems in a car park would result in a poor indoor environment. The exhaust emission of motor vehicle contains a variety of potentially harmful substances encompassing carbon monoxide, nitrogen oxides, sulphur dioxide, hydrocarbons, and fine particulates. In Kuala Lumpur, there is a great demand but a short supply of lands and building spaces. Thus, a large multi-storey underground car parks is a common solution for both, the government and developers. Although the health effects of the motor vehicle emissions and ambient air pollution are already known, but due to the nature of enclosed multi-storey car parks, these health risks are predicted to be intensified. Thus, it is crucial to investigate and evaluate the status of the air pollution in the enclosed car parks with emphasis on sulphur dioxide (SO₂) and nitrogen dioxides (NO₂). Samples were collected in one of the famous shopping malls in Kuala Lumpur using a GrayWolf Advanced Sense Direct Sense; Toxic Gas Test Meters from 8 am until 5 pm on weekdays and weekends. The results demonstrate that the concentrations of SO₂ and NO₂ on weekends is higher than weekdays. Besides, the concentrations for both weekdays and weekends have exceeded the standard limit set by the Malaysian Ambient Air Quality Guideline (MAAQG).

Abstract: A new hybrid material performed by the immobilization of indigo particles on nanocarbonaceous matrix has been developed, characterized and implemented with chemoresistors. If phthalocyanine-based chemoresistors provide a high sensitivity, a low threshold of detection and a partial selectivity towards oxidizing pollutants, indigo/nanocarbons hybrid material acts as a selective ozone filter from air sample and so strongly enhances the sensor selectivity towards nitrogen dioxide. The functionalization, highlighted by scanning electron microscopy, X-ray diffraction and thermogravimetric analysis, occurs in a non-covalent way and proceeds through pi-stacking interactions. With filtering yields higher than 99.5% for ozone and less than 5% for nitrogen dioxide, indigo adsorbed onto multi-wall carbon nanotubes has been identified as the most selective filtering material while exhibiting a much greater durability than indigo or nanotubes separately. Associated to phthalocyanine-based gas sensor, the sustainable, selective and real-time monitoring of NO₂ in ppb range has been successfully achieved.

Abstract: The conditions of oxidation of nitrite by manganese dioxide in the solution was studied, with sodium nitrite was used as the sample, respectively. Experiment results showed that the stronger oxidation to nitrite in strong acid, but nitrous acid is instable, escaping NO₂ out from the solution. When pH=3.5, the oxidation rate of nitrite with manganese dioxide was better. The concentration of nitrite tended forward to be stable after 30min. After reaction, the oxidizing power of manganese dioxide which was treated by dilute sulfuric acid wasn't enhanced. The manganese dioxide treated by acid reacted with fresh nitrite solution and the effect wasn't good. The process of oxidation could be go on and the concentration of nitrite would decrease obviously when fresh manganese dioxide used to react the same original nitrite. Maybe some active sites exist at the surface of manganese dioxide. When the reaction equilibrium was reached, the active point at the surface of manganese dioxide would probably have disappeared. The property of oxidation of manganese dioxide to nitrite could be used in treat NO₂ and nitrite. Combining the two methods which absorption of NO₂ by base and oxidation of NO₂ or nitrite by manganese dioxide, NO₂ would be treated completely and treatment cost would be lower.

Abstract: Taking nitrogen dioxide from atmosphere samples in Changsha as research object, GM (1, 1) dynamic prediction model of pollutants was built based on grey system theory and automatic monitoring data (nitrogen dioxide) of 2011. Pollution status of the next year was predicted by the model. Through model testing, the model can meet actual demands with its precision level is at the second and the third rank. Comparison between the predicted data and the measured ones shows that the higher the precision level is, the more accurate the model prediction will be. It is also obtained that the GM (1, 1) dynamic prediction model not only possesses potential value for further application, but also can provide the basis of the formulation of the local regional environmental planning and implementing measures related to atmospheric pollution control.