IMPLEMENTING MOX NANOSENSORS FOR AMBIENT AIR APPLICATION

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1 IMPLEMENTING MOX NANOSENSORS FOR AMBIENT AIR APPLICATION F. Adrover, C. Jaeschke, A. Parellada Nanotechnology and Nanosensors Online Course. Coursera Technion Israel Institute of Technology June 10, Felanitx (Spain), Tübingen (Germany), Barcelona (Spain)

2 Table of content 1. Abstract Introduction Literature review Project description MOX gas sensors Method for Fabrication Characterization Power Consumption Gas specific Sensor Characterization Improvements Real-life application of smelling VOCs in the ambient air Sensing mechanism in the case of an exposition to formaldehyde Conclusions and recommendartions References... 9 Table of figures Figure 1. Principle of the transducer Figure 2. Commercial gas sensors with a TO-39 socket Figure 3. Reaction scheme of the formaldehyde detection.... 8

3 1. Abstract In this report we want to show, the purpose of using nanotechnology and nanosensors for imitating the human sense of smell. Regarding this topic we are going to implement metal oxide (MOX) gas sensors in a so-called Smart intelligent house or flat. These sensors are able to detect displeasing odours each room and the evaluation unit could decide on its own to open a window or to turn on the ventilation system to get fresh air and to remove the unpleasant smell or could give the owners a message on their mobile phone or tablet. The evaluation unit consists out of a data acquisition and analysis system using pattern recognition methods and algorithms. The resistance of the active sensing layer changes due to contact with the gas to be detected. In the ideal case, the gas reacts with the sensor surface in a completely reversible reaction. Keywords: MOX, nanotechnology, nanosensors, smell human sense, ambient air detection, VOCs, displeasing odour, Smart intelligent house 2. Introduction A gas sensor is a device that can be used to detect different types and composition of gases such as ethanol, CO 2, CO, etc. This report is dedicated to explain how the metal oxide (MOX) gas sensors are working, how can we fabricate them and which features can characterize them for implementing a day life application using the MOX principles. The gas sensors based on MOX have an important role in environmental monitoring, air quality, chemical process controlling, personal safety, process controls, detection of toxic environmental pollutants in human health, preventing of hazardous gas leaks, and we could expand this until having a huge list. [1] It is well-known that the sensitivity characteristics of these sensors strongly depend on the morphology of metal oxide semiconductor (MOS). 3. Literature review The following paragraph will be focused on which is the State of the art related with MOX sensors. In past decades, gas sensors based on the metal oxide semiconductors (MOSs) have been studied in diverse field for wide applications. One of the main reasons because of some people is putting such an effort to build new devices is because of the cheap price of this technology. 2

4 There is the need to say that is not clear some functionality of the sensors, and that is why some people prefer using silicon nanowires or gold nanoparticles instead of MOX solutions. Probably in the future the applications will be adopt different technologies at the same time instead of usi g just the po er of just one of them. Nowadays there are companies which have a large product range of ambient air gas detection solutions, but they are not able to detect displeasing VOCs. There is no existing working smart home solution which is able to help the residents with artificial intelligence. 4. Project description In our project, we would like to show a possibility of imitating the smell as one of the important human senses with nanotechnologies and nanosensors. Especially to improve the sense of smell could be very useful because humans are not able to smell CO and it is a great advantage to detect VOCs in the ambient air before a human nose could smell it. The following part will describe general information about metal oxide (MOX) gas sensors. 4.1 MOX gas sensors Gas sensors are used for the detection of gaseous substances and compounds. In the presents of a target gas, the sensor transduces the chemical information into an electrical signal (see figure 1). Figure 1. Principle of the transducer. Such a chemical sensor is assembled by different parts as described in the following. The receptor is formed by the sensitive layer and the transducer. The sensitive layer is in direct contact to the gaseous sample and due to the contact with a target gas the sensitive layer is altered and its physical 3

properties changes. These changes are translated into an electronic signal by the measurement electronics (transducer). Generally sensors are constructed in the following way.

5 properties changes. These changes are translated into an electronic signal by the measurement electronics (transducer). Generally sensors are constructed in the following way. On an alumina (Al 2O 3) substrate, there is a platinum heater at the bottom. This heater is needed to set the operating temperature of the sensor via applying a voltage. At the top, there are interdigitated platinum electrodes, where the changes in the resistance of the sensor are measured. Above this interdigital-structure there is the sensitive layer (e.g. SnO 2). The substrate separates the heater from the electrodes at the top. Because of the fact that there is a metal oxide as a sensitive layer at the top of the sensor, these sensors are called metal oxide gas sensors, short MOX sensors or MOS - sensors for metal oxide semiconductor gas sensors. SnO 2 is an n-type semiconductor because of oxygen vacancies in the crystal lattice. These certain metal oxides, such as SnO 2, change their conductivity under a gas exposure. In the case of an oxidizing gas the conductivity decreases and in the case of an exposure to a reducing gas the conductivity will increase. The sensor layout of commercial sensors is mostly the same one as mentioned above. But because of the miniaturization it is a little bit different. These sensors are mounted in a metal can with a socket, for example the so-called TO-39 socket, as can be seen in figure 2. Figure 2. Commercial gas sensors with a TO-39 socket. MOX sensors consist of the same components as the planar gas sensor. The substrate separates the heating electrodes at the bottom from the measuring electrodes at the top of the sensor. The commonly used sensitive layers in commercial sensors are made out of tin oxide (SnO 2) [2], tungsten oxide (WO 3) [3] or zinc oxide (ZnO) [4]. The properties of the sensitive layer can be adjusted through doping materials, for example to reach a higher sensitivity to a target gas. The sensitive layer is located directly over the interdigital-electrodes. Heater and sensor electrodes are connected to the pins of the TO-39 socket. This layout sometimes differs for different suppliers. 4

6 4.2 Method for Fabrication There are some different techniques of manufacturing sensitive layers for MOX sensors. Depending on which is our final goal and which are the conditions we are working on, is better using one or another technique. Nowadays, the companies and researchers who are developing these sensors are mainly using three different sensors. These sensors can be distinguished according the morphology they have: - Thick film technology (Taguchi-type): most of the times they are produced via screen printing. They are recognized as 3D sensors. The working principle is using a porous sintered block [1]. - Thin film technology: This type of sensors are recognised as 2D sensors. Thin film sensors are produced by chemical vapor deposition or physical vapor deposition [1]. - Nanowires: The MOS nanowires can be synthesized by thermal oxidation technique. This technique has been successfully used for synthesizing ZnO or CuO by simply heating pure Zn and Cu material source, respectively. This are the most innovative and they can be known as third generation of MOX sensors. The sensing mechanism is based in chemisorption and change transfer interaction with the gas. They are currently helping the understanding of adsorption between gas and metal oxides. The nanowire fabrication routes can usually be categorized by bottom-up or top-down. Recently, advances in bottom-up fabrication processes have opened up the possibility of a synergy between bottom-up and top-down processes to achieve the benefits of both. Nanowire sensors can be described as having a well-defined geometry, high surface to volume ratio and good crystallinity. [5] 4.3 Characterization The MOX sensors can achieve very different features depending on the method of fabrication, sizes and morphologies of the nanostructures explained before. Depending on the application, we are going to choose the best type of MOX sensor which is fitting for that purpose. 5

7 The MOS gas sensors have some advantages in front other types of sensors because of their miniaturization, low power consumption, ultra-fast response times, simple construction, good sensing properties, and high compatibility with microelectronic processing. So, they have rapidly gained attention over the years Power Consumption The difference in power consumption of metal oxide gas sensors is because of the different designs we can have on the applications. Standard sensors, require approximately 1.3 W of power (at 350 C). The power consumption can be reduced by the thermal decoupling of the sensor from the housing, for example through the use of micro-hotplates. In this case, the power consumption will change to less than 150 mw. Thin film sensors need a pretty high power consumption, being a limitation for some applications Gas specific Sensor Characterization Due to their chemical composition and properties, metal oxide gas sensors are well-suited for a wide range of applications and for the detection of all reactive gases. The detection limit will depend on the material that is used for sensing and the gases which are going to be detected. The typical operating temperatures range can be between 300 C and 900 C. If we have a good sensor in the proper conditions, we can detect from a few ppb Improvements For improving the results, there is the chance to implement a software processing. Algorithms are implemented to the MOXs gas sensors results for getting a maximum and optimal selectivity, drift compensation and for self-calibration. 4.4 Real-life application of smelling VOCs in the ambient air Semiconductor gas sensors can be used for a wide array of applications, ranging from safety equipment (explosion, leakage, fire, contamination and poisoning protection) up to emissions and air quality monitoring, quality assurance, process instrumentation and measurement technology. For 6

8 example, gases such as carbon monoxide (CO), nitrogen oxide (NOx), ammonia (NH 3), sulfurous gases (H 2S, SO 2) and hydrocarbons (C xh y) as well as volatile organic compounds (VOCs) can be detected. Now, we are going to implement an application directed to an intelligent house: Sensor systems with the MOX-nanosensors could be placed in different places (like in the kitchen, in the living room, in nursery, in the toilet and in the sleeping room) in the house or flat. These nanosensors work in a network with the other sensors, which are located all around the house, and the data acquirement and data analysis will be done by a computer system, which is located in the basement. These system is responsible for the results of the different sensors. This multi sensor system for ambient air analysis is utilized with an advanced software for pattern recognition methods (like principal component analysis (PCA), principal component regression (PCR) or k-nearest neighbours algorithms (KNN)) for evaluating the outputs of the sensors. The data analysis software reduces the noise of the sensors via using and applying filters and feature extraction tools. In the case of contaminated ambient air the system is able to start the ventilation system of the house or to open the window electrically to clean the air in this room. This is a great advantage because these systems can work without humans, so these systems are able to help old people and also babies, because you are able to set your parameters for fresh air in your house or flat as you want and the system takes care of the actual value and the given value. These application could make the human life easy, the baby could sleep in its bedroom with the best value of fresh air during the parents are not at home. Another example for our real-life application are nanosensors in the kitchen. Sensors could control the hood. The nanosensors are placed around the cooking plate and the oven so that these kitchen sensors can detect volatile organic compounds (VOCs) which are coming from the displeasing smell of cooking. The pattern recognition system will evaluate the smell and would start the kitchen hood at the needed power. Thereby it is possible to reduce the power consumption, because the kitchen hood is working on the high level only if it is needed. This sensor in the kitchen hood is also able to detect fire. Another sensor is placed in the refrigerator to detect spoilt meat or tainted food. These application enhances the security of the feeding and it helps people to decide is the food okay or not, especially children and older people. Our real-life application could also be placed in the bathroom or toilets, because here is the most common place of undesired and displeasing odour of humans. To reduce this, nanosensors could be placed around important detecting points and the data acquisition and analysis software is able to detect the bad smell. The software starts the ventilation process and could spray good smelling fragrances directly into the bathroom or toilet. The pattern recognition system is trained for the used perfume so that the software is able to differentiate between the used good smelling odour and displeasing ones (e.g. other fragrances or human odours). 7

9 The sensing mechanism for formaldehyde of these MOX-nanosensors is described in the following section Sensing mechanism in the case of an exposition to formaldehyde Formaldehyde reacts with the ionosorbed oxygen ions at the surface of the semiconductor. CO 2 and water will be formed and the previously trapped electrons are released into the conduction band of the semiconductor. The sensor resistance is decreasing during the form equation (1) [6]: (1) Through a following in situ DRIFTS study Zhang et al. Also confirmed each step starting from formaldehyde with all important intermediates (dioxymethylene, formate species and polyoxymethylene) to the formation of CO 2 and water, as can be seen in the equations (2) (6) [6]: (2) (3) (4) (5) (6) Other molecules of formaldehyde can be polymerized to linear polyoxymethylene during the creation of CO 2 and water. In figure 3 the possible gas-sensing reaction mechanism of formaldehyde and the decomposition is shown. Figure 3. Reaction scheme of the formaldehyde detection [6]. 8

10 Moreover Zhang et al. (2013) reached an excellent formaldehyde gas-sensing property with the use of nano-sno 2 powder for the sensitive layer. 5. Conclusions and recommendations This report is showing how to use the MOX technology for implementing a Smart Sensing for Houses. As we have seen, the Metal Oxide Gas Sensors are a good candidate as a gas sensing device, because of their miniaturization, great stability, fast response times and simple construction. Moreover, the size and morphology are playing a big role on the sensitivity characterization, surface to volume ratio and depletion layer width. Our gas sensor applications are based on the important role of MOX in environmental-house monitoring, air quality, chemical process controlling, personal safety. Preventing any type of possible damage for the people is living there and giving them a better and more comfortable live. Other technologies like Silicon Nanowires and Gold Nanoparticles could be used with MOXs, because in some cases they have better implementation. For instance, they can have flexible substrates, and better power consumption because they dont need to heat up to very high temperature for sensing. 6. References [1] H. Haick, Coursera le ture otes anotechnology and nanosensors, [2] N. Tagu hi, Jp. Pate t No. -,. [3] J. Polleu, A. Gurlo, N. Bârsa, U. Wei ar, M. A to ietti, a d M. Nieder erger, Te plate-free Synthesis and Assembly of Single-Crystalline Tungsten Oxide Nanowires and their Gas-Sensing Properties, Angew. Chemie, vol. 118, no. 2, pp , Jan [4] X. Chu, T. Che, W. Zha g, a d H. Z. B. hui, I estigatio o for aldeh de gas se sor ith Z O thi k fil prepared through i ro a e heati g ethod, Sensors Actuators B Chem., vol. 142, no. 1, pp , Oct [5] R. G. Hobbs, N. Petkov, and J. D. Hol es, e i o du tor a o ire fa ri atio otto -up and topdo paradig s, Chem. Mater., vol. 24, no. 11, pp , [6] Z. Zha g, K. Hua g, F. Yua, a d C. Xie, Gas-sensing properties and in situ diffuse reflectance infrared Fourier tra sfor spe tros op stud of for aldeh de adsorptio a d rea tio s o O fil s, J. Mater. Res., vol. 29, no. 01, pp , Nov

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