Ayo Afonja

Metal Oxide Semi-Conductor Gas Sensors in Environmental Monitoring

Metal oxide semiconductor gas sensors are utilised in a variety of different roles and industries. They are relatively inexpensive compared to other sensing technologies, robust, lightweight, long lasting and benefit from high material sensitivity and quick response times. They have been used extensively to measure and monitor trace amounts of environmentally important gases such as carbon monoxide and nitrogen dioxide. In this review the nature of the gas response and how it is fundamentally linked to surface structure is explored. Synthetic routes to metal oxide semiconductor gas sensors are also discussed and related to their affect on surface structure. An overview of important contributions and recent advances are discussed for the use of metal oxide semiconductor sensors for the detection of a variety of gases—CO, NOx, NH3 and the particularly challenging case of CO2. Finally a description of recent advances in work completed at University College London is presented including the use of selective zeolites layers, new perovskite type materials and an innovative chemical vapour deposition approach to film deposition.

Zeolite-Modified Discriminating Gas Sensors

The use of zeolites as transformation layers to enhance the response and discriminating power of solid-state metal-oxide-semiconductor gas sensors is demonstrated. Thick film sensors were prepared by screen printing layers of tungsten trioxide or chromium titanium oxide with various zeolites as overlayers. The sensors gas response was tested against carbon monoxide and ethanol in varying concentrations. Experimental result.,; show that it is possible to dramatically alter the response behavior of the devices: in the instance of ethanol gas with a zeolite Y-modified sensor. the response was increased by 2 orders of magnitude compared to the unmodified sensor. Computational modelling studies show that a combination of catalytic reaction and diffusion behavior are responsible for these changes. Such discriminatory behavior should prove useful in electronic noses and sensor arrays. (c) 2009 The Electrochemical Society. [DOI: 10.1149/1.3065436] All rights reserved.

Discrimination Effects in Zeolite Modified Metal Oxide Semiconductor Gas Sensors

The use of zeolites to enhance the response and discriminating power of solid state metal oxide semiconductor gas sensors is demonstrated. Thick film sensors were prepared by screen printing layers of chromium titanium oxide (CTO) or tungsten trioxide with various zeolites as over-layers. The sensors gas response was tested against two similar gases; ethanol and isopropyl alcohol. The modified sensors effectively discriminated between the two gases. An understanding of this discriminating behavior is elucidated through computer modeling of diffusion and reaction processes occurring in the zeolite transformation layer and sensor element. Our analysis suggests that the discrimination is the result of the size and shape selective behavior.

On the Development of a Sensor Module for Real-Time Pollution Monitoring

In a Wireless Sensor Network (WSN) application for ambient pollution monitoring, which involves the deployment of a large number (i.e. ~100) of sensors in India and UK to obtain fine-grain pollution data, the selection of gas sensors and the validity of sensor measurements against ground truth are vitally important to the success of the system. Electrochemical and solid state gas sensors are low- cost, small sensors that are traditionally used for examining exhaust emissions from vehicles, and hence, they are specifically designed to measure a much higher gas concentration than in our measured environment. In this paper, we evaluate the use of gas sensors in a pollution monitoring project using pervasive sensor technology by literature comparison of the different types of gas sensors available commercially; and a discussion on the calibration process of a set of selected Oxygen (O2), Carbon Dioxide (CO2) and Carbon Monoxide (CO) sensors. The results show that the electrochemical CO sensors exhibit linearity against the range of concentrations that we are interested in (0-20ppm). This paper presents in-depth knowledge on sensor selection and calibration against the ground truth to the research community interested in enabling practical development and deployment of WSN gas pollution monitoring systems.

Discrimination effects in zeolite modified metal oxide semiconductor gas sensors

The use of zeolites to enhance the response and discriminating power of solid state metal oxide semiconductor gas sensors is demonstrated. Thick film sensors were prepared by screen printing layers of chromium titanium oxide (CTO) with various zeolites as over-layers. The sensors gas response was tested against two similar gases; ethanol and isopropyl alcohol. The modified sensors effectively discriminated between the two gases. An understanding of this discriminating behaviour is elucidated through computer modelling of diffusion and reaction processes occurring in the zeolite transformation layer and sensor element. Our analysis suggests that the discriminating behaviour is the result of a size and shape selective catalysis.

Gas sensing properties of composite tungsten trioxide-zeolite thick films

The effectiveness of using zeolites as a method of improving selectivity in semiconducting metal oxide gas sensor responses to different test gases is demonstrated. Screen printed tungsten irioxide (WO3) thick films overlaid with the hydrogen form of four types of zeolites were tested to different concentrations of NO2. ethanol and CO. The sensing results were shown to be reproducible with gas specific enhanced and diminished responses, which indicate that an array of highly discriminating composite metal oxide-zeolite sensors could be produced for electronic nose applications. ©The Electrochemical Society.

Evaluating Zeolite‐Modified Sensors: towards a faster set of chemical sensors

The responses of zeolite‐modified sensors, prepared by screen printing layers of chromium titanium oxide (CTO), were compared to unmodified tin oxide sensors using amplitude and transient responses. For transient responses we used a family of features, derived from the exponential moving average (EMA), to characterize chemo‐resistive responses. All sensors were tested simultaneously against 20 individual volatile compounds from four chemical groups. The responses of the two types of sensors showed some independence. The zeolite‐modified CTO sensors discriminated compounds better using either amplitude response or EMA features and CTO‐modified sensors also responded three times faster.

Zeolite-based discriminating gas sensors

The use of zeolites to enhance the discriminating power of solid state gas sensors is demonstrated. Screen printed tungsten trioxide (WO3) and chromium titanium oxide (CTO) thick films overlaid and admixed (CTO only) with different forms of zeolites are shown to give different and in some cases, enhanced responses to a variety of gases. We attempt to rationalise the noted enhancements through computational studies of the adsorption, diffusion and reactivity of the various species in the zeolites, with some success, in order to allow future design of electronic nose devices.