Nanocomposite Gas Sensors
Gas Sensors with Pre-Concentration Based on Chemically Modified Materials
Tech Area / Field
- INS-DET/Detection Devices/Instrumentation
- ENV-APC/Air Pollution and Control/Environment
- MAT-SYN/Materials Synthesis and Processing/Materials
8 Project completed
Senior Project Manager
Latynin K V
Moscow State University / Department of Chemistry, Russia, Moscow
- Institute of General and Inorganic Chemistry (IONKh), Russia, Moscow\nScientific and Technical Center of Measuring Gas Sensors, Russia, Moscow reg., Lyubertsy
- Micro-Sensor-Technologie Intertrade Handels GmbH, Germany, Munich\nMSA AUER GmbH, Germany, Berlin\nOLDHAM, France, Arras
Project summaryThe Project objective is the development of novel surface modified materials based on nanometer scaled metal oxides and micro/mesoporous molecular sieves of high selectivity to air pollutants and their applications in chemical sensors consisting of semiconductor detectors and pre-concentrators.
Air monitoring has become a major concern of worldwide environment policy, for the most part in large urban areas where the threat of industrial chemical agents discharge is higher and level of hazardous toxic and explosive products such as CO, NO2, SO2, CnH2n+2, H2S, and NH3 largely exceeds the World Health Organization recommendations. The problem of air monitoring is not resolved yet, in part due to cross effect of various pollutants and low level of their concentration (10–3 – 10–6 % in air under environmental conditions). Nowadays, very sophisticated and expensive techniques are used for gas analysis: Gas-Chromatography, Mass-Spectrometry, and Chromato-Mass-Spectrometry. Some of these methods permit the detection up to 10–12 g/m3 of different products in air. However, all the above techniques are only applicable in laboratory conditions. They are time consuming and require special trained personal. These methods do not allow continuous environmental monitoring required in process control and security systems such as early-warning detectors for protecting from ecological accidents. Evidently, new instrumentation for monitoring hazardous air pollutants is needed. The wide variety of existing types of gas detectors do not satisfy all the necessary requirements of high sensitivity, selectivity, long-term stability, fast response, and integration in information networks. Ideally, the detectors should continuously provide reliable quantitative information about atmosphere composition at ppm/ppb level of pollutants.
Solid state gas sensors based on semiconductor metal oxides present promising devices due to their high sensitivity up to ppb level, low cost, and low power consumption. The active part of commercial gas sensors such as those from Figaro, Drager, General Monitors, and Applied Sensor is based on SnO2 semiconductor. The SnO2 surface exhibits good adsorption properties and reactivity due to presence of surface and bulk oxygen vacancies, and active chemisorbed oxygen (O2-, O2-). Solid-gas interactions take place on SnO2 surface at moderated temperature through surface adsorption phenomena. These reactions give rise to the electrical conductance change and the value of sensor signal depends on the atmosphere composition. One of the main obstacles that hinder broad application of semiconductor gas sensors for air monitoring is their poor selectivity that increases the possibility of false alarm in the presence of non-toxic pollutants such as tobacco, perfumery, and solvents. The limitation of selectivity is caused by the non-selectivity of the adsorption process.
The novelty and technical future. The proposal focuses directly at attacking the problem of improving of selectivity of sensitive material and sensor on the whole. An important feature of the Project is an integrated strategy of air pollutants detection, which combines in one technological module the different materials: molecular sieve for separation and concentration of pollutants and complex nano scaled semiconductor materials (nanocomposites) for semiconductor detector. The selectivity of materials will be increased by means of geometrical factor via control of pore and crystallite size and by chemical factor with using of chemical modification of surface with different functional groups of fixed ratio of red/ox, acid/basic and hydrophilic/hydrophobic centers. The materials will be used for application in multi-channel system with each channel will be selective toward one chemical group of pollutants and will be composed of three main blocks: semiconductor detector, pre-separator and pre-concentrator.
The new type of gas sensor materials have been developed at Chemistry Department of Moscow State University based on composites of nanometer-scaled porous oxides (nanocomposites) with high specific surface area of about 150 –200 m2/g and controlled grain size within 3-10nm. The surface of semiconductor nanocrystallites SnO2 and In2O3 will be covered with mono-layer of metal oxides: Fe2O3, NiO, CuO, La2O3, MoO3, and V2O5, which exhibit catalytic activity toward different air pollutants. High long-term stability of nanocomposite structures has been demonstrated by authors of this Project. In such systems, the effect of rapid reversible change (switching) of chemical and physical properties induced by gas molecules is observed due to reorganization of electronic state of interfaces between the grains. In contrast to usual gas sensitive materials like SnO2 and ZnO, where gas adsorption process is of primary importance, in nanocomposites the reconstruction of electronic state of interfaces has the predominant effect that allows to improve the gas sensitivity and to lower the detection limit down to 10–100 ppb. The specificity of catalytic reactions as distinct from adsorption makes possible an increase in the selectivity of materials toward main air pollutants. The semiconductor materials will be deposited on the microelectronic chip with using of thick films technology. The microelectronic chip will be fabricated using original technology, developed by the authors of the Project (STC MGS). The sensitive semiconductor layer will be deposited on the thin dielectric membrane thickness of 1.2 μm and surface of 2x2 mm2. Such element allows the fabrication of sensors with low energy consumption of about 100 mW. The electrical contacts and heater of microelectronic chip will be fabricated of Pt.
The materials envisaged for pre-separator and pre-concentrator involve the novel micro/mesoporous molecular sieves with bimodal pore size distribution developed at Chem. Dept. MSU and IGIC RAS. These materials are based on microporous zeolites and mesoporous alumosilicates with unique pore structure composed of three dimensional pore system with uniformly sized pores of molecular dimensions. These materials preferentially adsorb molecules that fit snugly inside and exclude molecules that are too large. Due to this peculiarity they act as sieves on the molecular scale and therefore can show unique shape-selective properties. Novel micro/mesoporous molecular sieves combine the advantages of zeolites and mesoporous materials: high thermo stability, acid resistance, molecular sieve properties, high adsorption capacities and lack of diffusion limitations for adsorbate molecule transport. The applicants have developed synthetic approaches for pore size and geometry control in these systems, which will allow for systematic tuning and mastering of their porous characteristics towards development of the systems for selective pre-concentration and pre-separation of various gaseous mixtures. Along with molecular sieve properties, these materials exhibit ion-exchange, acid/base, red/ox and catalytic properties. These properties could in turn be altered by post-synthesis chemical modifying, which gives the additional advantage for selective pre-concentration and pre-separation. It is envisaged that pre-separation and pre-concentration will be based on the following procedure: during pre-concentration the gaseous flow will be passed through molecular pre-concentrator, which will selectively adsorb toxic compounds, these compounds will be further desorbed by pulse heating and directed to the molecular pre-separator, where the precise separation from concomitant gases will take place.
- The complex nano scaled semiconductor oxides of high surface area about 200 m2/g and high gas sensitivity to sulfur and nitrogen containing pollutants in air will be synthesized. The correlation between composition, microstructure, electrical and gas sensor properties toward H2S, NOx, NH3 will be established.
- The synthesis of chemical modified molecular sieves of controlled pores size and density of adsorption sites selective to H2S, NOx, NH3 will be realized. The feature of molecule transport in the conditions of pulsed heating will be determined.
- The system for concentration of sulphur and nitrogen containing gases will be designed.
- The microelectronic chip with low energy consumption <100mW and high stability will be developed using micromashining technique.
- An electronic system for sensor, gas pre-concentrator, and separator interconnection will be developed. The software will provide data acquisition as well as transfer of information by Internet.
- The prototypes of multi-channel gas detector will be fabricated and tested in laboratory and realistic conditions depending on humidity, temperature, and gas interference.
After finishing of the Project the exploitation of sensor systems will be implemented. The obtained results may be proposed to special design office or industrial groups.
Role of Foreign Collaborators/Partners. The world-famous European and USA specialists in gas sensor and sensing materials: Dr M.Niederhuber from Micro-Sensor –Technologie Intertrade Handels GmbH Germany, Dr. Lionel Colin from Gas monitoring Company OLDHAM S.A. France, Dr A.Davydov from NIST USA, DR. O.Kryliuk from University of Florida, Department of Chemical Engineering USA and Dr. J. Holmgren from UOP LLC USA are interested in the goals of the Project. The support of above noted persons is confirmed by corresponding letters. The role of Foreign Collaborators is very important in the Project. The close contacts with Foreign Collaborators will be effectuated during realization of all tasks of the Project to exchange of idea, discussion of obtained results, and looking-for the optimal solutions. The testing of sensor prototypes on the final stage of the Project may be realized with participation of Foreign Collaborators.
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