Repositorio Institucional de Documentos

Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Universidad De Zaragoza Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites By Thomas Hoole Directors: Miguel Urbiztondo and Pilar Pina Iritia Zaragoza, February 2013 Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Contents 1. Introduction 1.1. What is a gas sensor? 1.1.1. Transducers 1.1.2. Signal processors 1.1.3. Sensing materials 1.2. Zeolites 1.2.1. Properties of zeolites 1.2.2. Types of zeolites 1.3. Zeolites as gas sensors 2. Project Aim 3. Experimental 3.1. Interdigital capacitors as transducers 3.1.1. Interdigital capacitors with zeolite as sensor devices 3.1.2. Zeolite nanocapacitors 3.2. Synthesis of sensing material: zeolite 3.2.1. Crystal growth 3.2.2. Synthesis of Small crystals of zeolite 3.2.3. Synthesis of Large crystals of zeolite 3.3. Incorporation of sensing material and final sensor preparation 3.3.1. Standard interdigital capacitor 3.3.2. Zeolite nano capacitors 3.4. Experiment set up Index 1 1 2 3 3 5 6 8 9 10 11 11 12 13 14 14 15 16 18 18 18 20 Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Contents 4. Result 4.1. Electronic behaviour of sensors 4.1.1. Description and behaviour of IDC 4.1.2. “Nano” interdigital capacitors (NanoIDC) 4.2. Synthesis of sensing material: zeolite 4.2.1. Synthesis of Small crystals of zeolite 4.2.2. Synthesis of Large crystals of zeolite 4.3. Zeolite deposition 4.3.1. IDC deposition 4.3.2. “nano interdigital capacitors 4.4. Zeolite sensor characterisation 4.5. Dynamic gas characterisation 4.5.1. IDC gas characterisation 4.5.2. NanoIDC gas characterisation 5. Conclusions and Further work References Index 22 22 22 25 28 28 30 31 31 32 34 36 36 40 43 44 Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Page 1 1. Introduction The ability to sense different environmental parameters is extremely desirable in industry. Temperature, motion, humidity, pressure, sound and the presence of environmental molecules are just a few examples of important conditions many different commercial areas wish to detect and control. In particular, gas sensors are needed in the areas of industrial processing and the automotive industry [1]. Gas sensing in industry is important as increasing the control of gas flows and combustions can help to produce inexpensive, enhanced products. The automotive industry needed gas sensors due to the increase in restrictions in emissions [2]. Other industries that require gas sensors include but are not limited to: chemical processing, food, electronics, pharmaceuticals and textiles [1]. Gas sensors are also important for everyday human comfort technologies and in toxic environments for health and safety purposes. There is a huge drive for research into different techniques for improving gas sensing equipment because of these needs. Traditionally the aim of gas sensor research has been to improve selectivity, sensibility, response and recovery times. Currently, the research drive is focused specifically on the improvement of selectivity as other parameters are deeply studied. As well as these parameters there is a particular interest in sensor miniaturization for cost and performance purposes [3]. By reducing the size of the sensors, fewer materials are used and therefore the output costs can be reduced. When working with gas sensing the presence of natural moisture is very important. This is because moisture can appear near enough everywhere. The amounts and the effects of moisture need to be taken into account when detecting certain chemical species. This is to limit the interference effects that can take place while attempting to sense specific analytes [1]. 1.1 What is a gas sensor? Sensing devices are generally made up of three main parts. These are: a sensing material, a transducer and a signal processor. The sensing material is sensitive to a certain environmental stimulus. When this stimulus is introduced to the sensing material its properties are altered in a specific way. The job of the transducer is to detect the change in the sensing material properties and convert this change into an electrical signal. The signal processor receives this electrical signal and quantifies it into results which are comprehendible. Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Page 2 Fig. 1: Parts of a gas sensing device Transducers The transducer is probably the most important part of the sensor. It is used to convert one type of energy to another. There are two types: thr input and the output transducer. The input transducer converts a physical signal to an electronic current and the output transducer does the opposite. The types of transducer used in sensors are input transducers. Their function is to convert some sort of physical input, detected by a sensing material, into an electric signal. This can then be processed into something understandable to a user by a signal processor. The electrical signal, which the transducer outputs, is proportional to the strength or intensity of the physical quantity detected. A common example of an input transducer can be seen as the microphone. This converts vibrational energy into an electrical signal. Other examples of devices which use input transducers include motion detectors, touch screen devices and electronic weighing devices. Fig. 2: The transducer of a condenser microphone [4]  An electronically charged diaphragm is the sensing material part of the microphone  A current is passed through the back plate and a capacitance is created across the gap  The sound waves travel towards the diaphragm and this makes it vibrate  The change in distance between the back plate and the diaphragm changes the capacitance  This can then be quantified by a signal processor Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Page 3 The microphone is an example of a capacitive transducer. The concept of using a change in capacitance as a sensing device is commonly used. Around 75% of the humidity sensors are based on capacitive techniques [1]. This is due to the parameters that can cause a change in the capacitance of a device. Eq.1 Eq.1 shows the capacitance of a device, where C is the capacitance, A is the surface area, D is the distance between the electrodes, is the permittivity of the air and is the permittivity of the material in-between. The equation shows that the capacitance of a device can be affected by the surface area of the electrodes, the permittivity of the air and the material and the distance in between the two plates. Any external source that can alter these parameters can be detected using a capacitive transducer. This results in capacitive transducers having a wide variety of applications as sensing devices. To complete the sensing device, the transducer requires a signal processor and a sensing material. Signal processors The signal processor is important in a sensing device to show and record sensed changes in a comprehensible way. Their job is to receive the electronic signals given by the transducer and to quantify them into something that can be measured and understood. It is important that the signal processing method is produced and calibrated in the most accurate way possible to help avoid noise and other interference factors. For example if the signal processing’s quantified values are too big then large amounts of information can be lost. The smaller these values are the more information can be saved. Sensing materials The sensing material is the part of the sensor that initially detects the environmental conditions. These conditions affect the properties of the sensing material in a way which can be measured. A sensor’s ability has a high dependence on the type of sensing material used. For the most effective sensor, a sensing material is needed that is strongly affected by the desired conditions; also it is important that these conditions are easy to measure. There are many examples of materials that have been used for sensing such as: polymers, carbon and tin oxide nanotubes, anodic alumina and porous materials [1]. Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Page 4 The nature of polymers makes them good sensing materials. They are used in sensing devices as they have a great variation of flexibility in use; they are cheap and because of their functional groups. Carbon and tin oxide nanotubes have been used due to their improved electrical and chemical characteristics at the nano scale. Porous materials have been used as sensing materials especially in gas sensing as their internal properties can be affected when molecules enter inside them. They also have selectivity properties and because of this they are possibly the material with the most potential. A good example of this type of material is zeolite. Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Page 5 1.2. Zeolites As previously mentioned the majority of research into sensing devices is based upon large improvements in sensibility and selectivity for specific analytes. The problem is that for a lot of these cases the sensing devices are limited to ideal conditions. There is also research into materials that can be used as sensors and are also easier to manipulate for several different purposes; an example of these materials are zeolites. Zeolites are materials that have become valuable in sensing equipment due to their internal frameworks, their ion change properties and their control in synthesis. They are natural occurring materials but can also be synthesised with high control over their internal structure, framework and ionic properties. This makes them an excellent material for mass manufacturing and industry. To date, there are approximately 34 different types of natural occurring zeolites known [5]. Zeolites that can be artificially synthesised in a lab have greater control over framework and properties. Nowadays there is estimated to be over 150 different artificially synthesised zeolite frameworks with exact channel and pore sizes inside of the crystals [1]. Zeolites are aluminiumsilicates crystalline porous materials. The chemical composition for zeolites can be written as [3,5]; They are made up of and tetrahedral blocks formed in a three-dimensional formation via the oxygen atoms [6]. The tetrahedral blocks are linked together to form ring like structures [7]. This leads to crystal lattices with very small channels, pores and cages. Fig. 3a and 3b: The basic building blocks of zeolite [8] (left). image of a zeolite with FAU framework [9] (right). Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Page 6 In a complete siliceous structure with Si tetrahedral, silica with a neutral charge is created. In zeolites the aluminium atoms replace some of the silicon ones and are alternatively bound between shared oxygen atoms. Si has a +4 charge. When the +3 charge of aluminium is introduced to replace the silicon it creates a negative charge inside the framework [8]. Extra framework cations electrostatically bound to the host are allowed to move along the channel to compensate and create an overall natural framework [7]. Properties of zeolites The zeolites pores give a large surface to volume ratio which means that they are excellent at adsorbing. The channels that are created inside of the material can be used to trap, adsorb and filter different molecules. The pores have a uniform sub-nano size throughout the entire crystal [1, 6] and because of this they have been given the name “molecular sieves” [5]. Due to the uniformity of the small channel sizes in zeolites, only molecules with certain diameters that are smaller than the pore entrance may enter. This is not possible for larger molecules. F.S Stone et al. [12] show an example of this where zeolites were used to hydrate isopropanol. This can be used for processes where specific selectivity is needed. For example, the pores can be used to dry liquids or to separate gases by trapping molecules inside of the pores [6]. Specific zeolites can be carefully chosen to improve selectivity processes. This can be seen when comparing zeolite A and ZSM-5. Molecules between 4-6Â diameters in size can enter the pores of ZSM-5 (4Â) but not zeolite A (6Â). For adsorption, zeolite A has a bigger cubic capacity when compared to ZSM-5 although the amount of adsorption can be affected by other parameters such as the molecules polarity. The extra framework cations inside of the crystals allow for ion exchange to take place inside the material. This is due to the negative charge created by the aluminium ions. In catalysis, the exchangeable cations allow for numerous applications [6]. This property has been used in the oil and gas industry to crack hydrocarbons. Currently around 95% of all cracking processes in the oil industry are done using zeolite material [5]. The amount of ion exchange to take place can be dependent on the amount of cations. For example zeolite A is greater for ion exchange then ZSM-5. The electrostatically bound extraframework cations motion in zeolites can be interfered with the addition of guest molecules. This property allows them to be used in sensors based on transducers with electrical behaviour, detecting changes in impedance (resistance) and Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Page 13 By depositing zeolites over the top of the IDC; when the analyte is introduced the pores of the zeolite help to trap more of the analyte closer to the electrodes. This results in an increase in change of permittivity and therefore a more sensitive sensing device. Zeolites also have the benefit over other materials, as previously mentioned, that they can be highly selective. A problem with this configuration is that the change in capacitance is limited only to the area above the capacitor. This is because the analyte is not able to change the capacitive of the substrate effectively. To further increase the sensitivity of the sensor it was proposed to change the substrate for a sensing material, zeolite in this case. By doing this, a change of capacitance can be achieved by, not only the top layer of zeolite but also, the changes in the substrate. Moreover, to get further sensitivity a nanocapacitor could be created on top of a zeolite crystal. This idea gives nano scale benefits as well as the increase in sensitivity. Fig. 8: Capacitive effect in IDC chips and nano capacitor Zeolite nanocapacitors With the use of nano sized technology, sensing equipment can be developed with improved characteristics. This is due to such benefits as increased surface area/ volume ratios, electrical and mechanical properties and also molecule selectivity [16-18]. Current research based around nanosensors is aimed at making use of singly synthesised nano structures that can be manipulated to work as a sensing device; the sensing material and the transducer. The sensors take advantage of the nano properties of increased sensitivity to parameters such as capacitance and resistance which can be transferred to the sensor’s sensitivity [16]. The nano aspect of the project also can aid towards miniaturisation and as less material is used the product can be lower-cost [17]. By taking advantage of these nano characteristics and using them with the zeolite characteristics, the sensor should have an improved sensibility and selectivity. Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Page 14 3.2 Synthesis of sensing material: zeolite For this project different types of zeolite crystals are required to test their different sensing capabilities. The parameters that will be compared will be: a difference in structure, changes in Si/Al ratios and variation in crystal size. The difference in structure will change the pore size of the zeolites. This should affect the types of analytes that can be adsorbed, the rate of adsorption and the capacity of adsorption. The types of structures that will be produced will be MFI and LTA. By producing zeolite sensors with different Si/Al ratios, an understanding of the effect of polarity can be obtained. This should affect the sensing abilities of different analytes in different ways. The crystals that will be synthesised will be: silicalite (Si/Al = ∞), ZSM-5 (Si/Al= 100) and zeolite A (Si/Al = 2) A variation in crystal size will allow different sizes of sensors to be created. With small crystals sensors will be made with IDCs and with large crystals nano sensors. By creating different sizes in sensors it will be able to compare the sensing ability from the micro to the nano scale. Zeolite crystal growth To synthesise zeolite in a hydrothermal process there are several different reagents needed; a source of silica, a source of alumina and a mineralizing agent such as OH-. Also for a high Si/Al ratio zeolite such as silicalite, organic molecules may be required to help create the structure. The molecules can be used to fill voids, balance charge and/or as structural templates to create high-silica zeolites [8]. To create crystals the process moves from a metastable phase where the reactants are mixed, to ion transportation where monomers are produced, to a final stable phase where crystallization takes place. Fig. 9: The growth of zeolite crystals Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Page 15 Synthesis of small crystals of zeolites Material type Molar ratio Method Temperature Synthesis time Ref. Silicalite 9TPAOH: 25S : 480 Oven 100ºC 15 hours [21] ZSM-5 :200Si :14 :9.5TPAOH:9400 Microwave 180ºC 15min [21] Zeolite A 13.4 O: 0.3 : 1.8 : 11.25 : 700 Rotatory oil bath 100ºC 8-12hours [20] Table 1: Small zeolite crystals Silicalite The small silicalite crystals were created using an oven method. To create the silicalite 48.6g of and 0.27g TPAOH were mixed together until homogenous. Then 1.12g of TEOS was added to the mixture drop by drop. The mixture was stirred for 8 hours until completely homogenous. The mixture was then placed in to a vessel and into the oven for 15 hours at 100ºC. Once completed the mixture was centrifuged at 8500rpm for 45 minutes. The seeds were then cleaned. This process was repeated 3 times. ZSM-5 The ZSM-5 crystals were created using a microwave method. The process was done starting with 0.2737g of NaOH mixed with 38.9607g . 2.5770g of TRAOH were added to this mixture and stirred for an hour until homogenous. Then 0.0513g of aluminium were added and again stirred for an hour until homogenous. Finally 8.1373g of LUDOX solution were added and this was then stirred for 24 hours. The crystals were then produced in the microwave. This method was done at 180ºC at 300W. For the small crystals the synthesis time was 15 minutes. Zeolite A The small zeolite A crystals were grown in a rotating oil bath. The process was done by creating two solutions; the silica part (solution 1) and the aluminium part (solution 2). Solution 1 was made using 6.39g of silicon Ludox diluted in 5.68g of . Solution 2 was made using 2.13g isoprapoxide aluminium mixed with 14.20g TMAOH and 19.88g Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Page 16 which were stirred till homogenous. Solution 2 was mixed with 2.8g of 1 mole NaOH and 70ml of . Solution 2 was then added over solution 1 and was left to mix for 48 hours. The solution was placed in a rotating oil bath at 100ºC until it turned white. A centrifuge was then used to purify the samples. This was done at 400rpm for 10 minutes. The supernatant was then removed, replaced with water and then mixed in a sonic bath. After completing 3 times the crystals were then dried in an oven at 80ºC for 24 hours. Synthesis of large crystals of zeolites Table 2: Large zeolite crystals Silicalite Large silicalite crystals were synthesised using an oven method. 11.80g TEOS and 9.02g of water were added drop by drop into a vessel and stirred at room temperature. They were left to stir for 1 hour and then 11.38g TPAOH 1M were added and stirring was continued for another 3hours. (aging time of the gel) When the aging was finalized a highly diluted gel was obtained. The gel was introduced into a Teflon autoclave with a steel casing as the process produces high temperatures. The autoclave is introduced into the oven at 130ºC for 12 hours. After this time the autoclave was cooled to complete the growing process. The mixer was then centrifuged at 2750rpm and 20 minutes to separate the crystals from the gel. ZSM-5 The synthesis process for the large ZSM-5 crystals was the same as for the small ZSM-5 crystals only changing the synthesis time. For the large crystals the synthesis time as 90 min. Material type Molar ratio Method Temperature Synthesis time Ref. Silicalite 40 : 9TPAOH: 9500 Oven 130 ºC 12 hours [22] ZSM-5 :200Si :14 :9.5TPAOH:9400 O Microwave 180ºC 90min [21] Zeolite A 1.7 : : 0.7 : 165 : 6.1TEA Oven 85ºC 4 days [19] Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Page 17 Zeolite A Large zeolite A crystals were synthesised in an oven. To prepare the synthesis gel, 0.99 g of NaOH were diluted with 4.95g of in a vessel. During continuous stirring, 0.66g of aluminium flakes were added slowly and heated up to 90ºC for 2 hours. Once the aluminium was completely dissolved, 1.16g of sodium metasilicate, 5.52g of triethanolamine (TEA) and 12.82g of water were added. While this solution was under stirring, solution 2 could be prepared. For that purpose, 5.58g of TEA with 17.78 g of were mixing in a vessel and 0.58 g. of TEOS were added under continuous stirring until a homogeneous solution was obtained. After 2 hours mixing, solution 2 was poured over solution 1 and stirred for 6 hours. The final solution was introduced into an autoclave at 85ºC for 4 days. After several centrifugations at 4500 rpm for 15 min followed by a sonication step, the zeolite A crystals with the appropriate size had been achieved. Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Page 18 3.3. Incorporation of sensing material and final sensor preparation The final part of creating the zeolite sensors was to introduce the zeolite crystals on to the chips. Due to the different types of sensors, different methods were proposal for introducing the zeolites to the different types of chips. Standard Interdigital capacitor (IDC) The IDC and zeolite sensors were prepared using micro-drop deposition. The process involved the suspension of the crystals in ethanol. This was done by placing 0.015g of the crystals with 3ml of absolute ethanol into different vessels. These vessels were then placed into a sonic bath for 10 minutes. This was done to make the suspension homogenous, allowing agglomerations to sink to the bottom of the vessel leaving single crystals dispersed. Using a micro-syringe, a single droplet (10ul) of the solution was carefully deposited on to the chip. This was left for 10mins. Over this time the ethanol evaporated and the crystals adhered to IDC surface by electrostatic forces. The process was repeated until around 50ul was achieved (usually repeated 5 times). Fig. 10: Micro deposition of zeolite crystals Zeolite nano capacitors Sensors were created with nano capacitors on top of crystals to show the sensing properties on the nano scale. The nano capacitor was created using a focused ion beam (FIB) in a clean room of 10 000. The process for creating the nano capacitors started using the micro-drop deposition as previously discussed with the IDC sensors but with further dilution. This was done to deposit fewer crystals and to give a better chance of having single crystals deposited in the horizontal plane. The chip used in this case was a simple 4 line micro adapter. These chips where made of Si with a thin layer of Si . Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Page 19 Fig. 11: SEM image of simple 4 line micro adapter The FIB has the capability of drawing lines of palladium over this type of material with a minimum width of 200nm. Due to the limitations of the ion beam width crystals of micro size were needed to create a capacitor which was several electrodes thick. It was also important that the surfaces were planer to improve the accuracy of the capacitor. In the clean room, the chip was introduced to the FIB vacuum chamber and pressurized. In the process the first step was to examine the chip to find a suitable crystal. A capacitor pattern was placed on top of the crystal and the ion beam was turned on to create it. An ion beam is used to ionise a palladium precursor on to the samples surface. Three types of materials were used for the nano sensors. These were: silicalite, Zeolite A and ZSM5 crystals. Wires were drawn from one of the four way connections to the single crystal. The wires and the capacitor were then connected by creating lines up the vertical axis of the crystal. A capacitor was also drawn on the opposite side over the substrate. This was done to create and test a blank reference. Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Page 20 3.4. Experimental set up To test the sensors features an experimental system was designed. The idea was to test the changes in capacitance and resistance when introduced into certain environments. To do this the sensors were placed inside a gas chamber. The gas chamber was connected to several mass flow-meters which were in turn connected to several different types of analytes. The mass flow-meters were controlled by a computer system. The sensor itself was connected to an impedance analyser that could measure the output. Fig. 12: Experimental set-up Using this set up an accurately controlled environment could be created inside the gas chamber under automated control. There were three main analytes used to test the sensors parameters. These were; ethanol, toluene and water. These were chosen because of their known variations in behaviour with zeolite material. Water and ethanol are known polar molecules and toluene not. Therefore the analytes should produce different results when coming into contact with hydrophobic or hydrophilic zeolite crystals. As well as this, nitrogen was connected to build an inert atmosphere and as a reference point. To produce the analytes in gas form a controlled nitrogen flow was passed through a saturator containing the desired analyte. This flow then passed out to the gas chamber. The gas chamber was made of steel and had a cubic capacity of 200 Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Page 21 A 4294A precision impedance analyser was used to measure the sensors response to different analytes. The impedance analyser works with a frequency range of 40Hz - 110MHz with a resolution of 1mHz. It works by taking sweeped measurements of voltages and frequencies to see the behaviour of a load. It was connected to the sensors using 4 different wires. This was used to cancel out large parasitic effects. The voltage and frequencies were introduced into the sensor and the response is recorded. Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Page 22 4. Results 4.1. Electrical behaviour of sensors Description and behaviour of the IDC The IDC was made over Pyrex (SiO2) with an aluminium thickness of 100 nm. The design has two capacitive and two resistant zones. Different separations between electrodes from 3, 5, 10 and 20 microns were produced. Fig. 13: Scheme of 20 microns IDC with double capacitor and resistance For the sensor tests, it was very important to have control over the temperature. This is because as temperature increases, the ionic current between the OH groups improves. This produces an improved response in capacitance and resistance. As temperatures continue to rise, up to 500ºC, the cations inside of the zeolites have increased mobility and therefore there is an increased current flow [1]. There are two problems with this temperature though. The first problem is that, as previously mentioned, as the temperature increases the adsorption affinity decreases. The second problem that occurs is that it is practically difficult to experiment using these temperatures. Due to these factors, 150ºC was chosen. This was done using the IDC resistance; one resistance was used for monitoring the temperature while the other was used for heating. This was found by heating the IDC chip inside an oven and measuring the resistances at different temperatures. Afterwards, a voltage was introduced to IDC and the resistance was measured. From this a voltage to temperature ratio could be calibrated. (For further results a trend line was used). This type of heating was done to show how a heating process could be achieved in a commercial point of view. Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Page 29 ZSM-5 Fig. 20a and 20b: XRD (left) and SEM (right) images of small ZSM-5 crystals The XRD shows that the crystalline structure fits with a MFI type material. With SEM images round shaped crystals could be observed. Their size was smaller than 200nm. Zeolite A Fig. 21a and 21b: XRD (left) and SEM (right) image of zeolite A small crystals. The XRD image shows that the material created is zeolite A. This is known from the fact the measured results have a strong match with previous known characteristics. It can be seen that there is a strong match in the peaks between the known sample and the sythesised sample. The SEM image shows that there are some homogenous single crystals created. These have a slightly square shape with flat surfaces. The size of these crystals is just under 200nm. Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Page 30 Synthesis of Large crystals of zeolite Fig. 22a, 22b and 22c: SEM of big silicalite (left), ZSM-5 (centre) and zeolite A (right) crystals The large silicalite crystals can be seen in Fig. 22a. They have a coffin like shape and have a width of around 2μm and a length of around 5μm. A SEM image of the large ZSM-5 crystals synthesised can be seen in Fig.22b. The image shows that the crystals are fairly homogenous. The average size is around 4μm. Their shape is rectangular with flat surfaces. Fig. 22c is of the sythesised large zeolite A crystals. There are single square crystals with the size of around 2μm. For using these crystals for the NanoIDC some agglomerations were needed to be removed. The crystals were mixed with ethanol and then placed in a sonic bath for 10mins. The sonication made the agglomerations sink to the bottom and therefore the single crystals were left floating. These were then removed and used in the deposition process. Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Page 31 4.3. Zeolite deposition IDC deposition The process of depositing the zeolite on to the IDC chip, as previously mentioned, was done by micro drop deposition. Figs. 23: Micro-drop deposition on to the IDC Fig 23 shows the process of depositing the zeolites on to the IDC. It was done with a micro syringe and deposited slowly to produce the smallest drops possible. The drops were left for the ethanol to evaporate and this left the crystals distributed across the IDC. The step was repeated to increase the amount of zeolite crystals. Fig. 24a and 24b: IDC with zeolites deposited on top. Figure 24a and 24b show images of the IDC chips with the zeolites deposited on top of them. From the images it can be seen that crystals were spread across the IDC fairly homogeneously covering the IDC’s electrodes equally. At the top of Fig. 24a the three double micro contacts can be seen. They were connected to wires to measure the responses. Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Page 32 “Nano” Interdigital capacitors (NanoIDC) The NanoIDCs were produced by depositing a lesser amount of crystals onto the Si chip than the IDCs. This was then taken to the clean room and a nanocapacitor was drawn on top of the crystal and connected to micro contacts. As well as this the blank nanocapacitor was created as a reference (as mentioned above). Fig 25a-h: Ion beam creation process of ZSM-5 nanoIDC and blank nanocapacitor Fig. 25 shows the process of producing the nanocapacitor on top of the zeolite crystals. Firstly the blank nanocapacitor was created on top of the substrate. This was done by finding an area without crystals and then using the ion beam to create the capacitor. This can be seen in fig 25a. The capacitor was then connected to the micro contacts which can be seen in fig. 25b. To create the nano capacitor first a suitable crystal was found. This consisted of finding a single crystal that was square in shape and that had a flat surface. It was also important that the crystal was in the horizontal plane. These parameters made the process of producing the nanocapacitor easier. A crystal was chosen that was close to the micro contacts to help reduce the time it took and the chance of error. The capacitor design was created on top of the crystals. To connect the capacitor to the substrate, wires were created down the sides of the crystal. These were then connected in turn to the micro contacts. Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Page 33 This technique was done for the three types of big crystals, Silicalite, ZSM-5, and zeolite A. Images from the creation of the zeolite A nanocapacitor can be seen in Fig. 26. Fig 26: creation of Zeolite A nanocapacitor Table 3 shows a summation of all the sensors prepared in this work stating their main characteristics; Name Zeolite Si/Al Fabrication method Sil. IDC small Silicalite ∞ Micro-drop ZSM-5 IDC small ZSM-5 100 Micro-drop ZA IDC small Zeolite A 2 Micro-drop Sil. NanoIDC Silicalite ∞ FIB ZSM-5 NanoIDC ZSM-5 100 FIB ZA NanoIDC Zeolite A 2 FIB Table 3: classification of sensors Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Page 34 4.4. Zeolite sensor characterisation When the zeolites were deposited on to the IDC chips their electrical behaviour had changed. Similar to the substrate in the nanocapacitors the zeolites had created an alternative path for the electric to flow through at low frequencies. Just like the nanocapacitors the IDCs behaviour could now be determined as a RC circuit. This can be seen in previous works such as [1]. Fig. 27: RC circuit produced in IDC sensor To increase the effectiveness sensors the ZSM5 IDC sensor was tested at different temperatures. By testing at different temperatures the optimum sensor characteristics could be found. The test was done under pure nitrogen at temperatures ranging from 25ºC- 150ºC. Fig. 28: Heat test of ZSM-5 of a IDC Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Page 35 The ZSM5 IDC tests at different temperatures showed that the device had a RC circuit response although this was more difficult to see at lower temperatures. This can be seen from the graph as when the voltage, and therefore heat is increased so did the resistant elements of the sensor. This can be seen in the impedances results were impedance is reduced when the temperature increases. The ideal temperature was chosen to be around 150ºC. The preliminary tests showed the effect that temperature had on the sensing response but the response can also be affected by the presence of different analytes. Similar experiments, but under different atmospheres, have been carried out, where mainly changes in the resistance have been registered. From a more functional point of view, the optimal frequency (better signal/noise ratio) was found. The concept was to measure the changes in the capacitance and resistance of the system using this frequency over a longer period. The optimal frequency was found by comparing the sensors response when introducing nitrogen to introducing water. The frequency was chosen which produced the biggest difference in impedance between the water and nitrogen results. The results for the preliminary tests showed that the best working frequency was 7.5KHz. Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Page 36 4.5. Dynamic gas characterisation IDC gas characterisation The different chips were then tested using an automatic program. This was done with the gases introduced in steps. The optimal temperature of 150ºC and a frequency of 7.5KHz were used. Firstly a flow of nitrogen was introduced to the sensor for 6 hours to create a referenced atmosphere. Then steps of the analyte were introduced changing the concentration of ppm depending on the composition of the analyte. With water it ranged from 4000- 35000ppm, with ethanol 7500-62000ppm and with toluene 3200-25000ppm. Each step would last an hour with an hour gap in between of pure nitrogen to test the sensors ability to recovery. As an example of this type of experiment the results for the ZA IDC are present in Fig. 29. Changes in the resistance and capacitance for the three analytes could then be deeply analysed. 0200 400 600 800 1000 1200 1400 5.0x107 1.0x108 1.5x108 resistance (ohm) time (min) h2o 0 6000 12000 18000 24000 PPM 0200 400 600 800 1000 1200 1400 1.6x107 1.8x107 2.0x107 resistance(ohm) time (min) ethanol 0200 400 600 800 1000 1200 1400 -3x1011 -2x1011 -1x1011 0 1x1011 2x1011 resistance(ohm) time (min) toluene 0 15000 30000 45000 60000 PPM 0 10000 20000 30000 40000 PPM Fig. 29: Zeolite A small IDC resistance (Where the red signal is the amount of analyte and black is the responses) Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Page 37 0200 400 600 800 1000 1200 1400 9.8p 10.0p 10.2p 10.4p h2o capacitance(F) time(min) 0 6000 12000 18000 24000 PPM 0200 400 600 800 1000 1200 1400 10.0p 10.0p 10.1p 10.1p ethanol capacitance (F) time(min) 0200 400 600 800 1000 1200 1400 8.0p 16.0p 24.0p 32.0p 40.0p toluene capacitance (F) time (min) 0 15000 30000 45000 60000 PPM 0 10000 20000 30000 40000 PPM Fig. 30: Zeolite A small IDC capacitance The results from the gas chamber experiment show that the increase of concentration of the different analytes cause a reduction in the resistance and an increase in capacitance in the small ZA IDC chip. This was expected as the analyte will be adsorbed into the zeolite increasing the permeability, which is directly related with the capacitance. On the other hand, an increment of water, for instance, could increase the ionic conductivity, making electron transport easier and therefore decreasing the resistance. The resistance results of the ZA IDC showed that ethanol and water were the best suited for this chip. Toluene was insensible. This can be seen by the responses in steps in-line with the analyte steps. The capacitance results of the ZA IDC show that the most sensitive gas was ethanol then water. This can be seen by the large steps produced in the results due to the steps in analyte. Toluene showed little signs of sensibility and had a high signal to noise ratio. These results are due to the fact the Zeolite A is a hydrophilic zeolite. This means that it has better attraction to the polar molecules water and ethanol. This also explains why toluene was insensible. Using these types of experiments, (dynamic gas testing), the calculation of the sensitivity, selectivity, response time and recovery time could be calculated. The sensitivities of the sensors were calculated by dividing the change in capacitance or resistance by the amount of ppmV of each analyte. The limit of detection (LOD) was calculated by dividing three times the noise in the response by the sensitivity. The noise was calculated by taking enough random points during the first 6 hours of testing and finding the standard deviation. Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Page 38 These experiments were done for all the different chips and the important results were calculated. The results can be seen in tables 4-6. H2O Capacitance ( r = 1.85D) Resistance Sensitivity (pF/ppm) LOD (ppm) Response time (min) Recovery time (min) Sensitivity (Ohm/ppm) LOD (ppm) Response time (min) Recovery time (min) Sil. IDC small 1.16E-06 1900 8.3 10.2 n/a n/a n/a n/a ZSM-5 IDC small 1.16E-05 530 1.9 2.5 19952 580 2 2 ZA IDC small 1.94E-5 340 1.6 21.8 381141 8.95 2.6 2.6 Table 4: IDC water results Ethanol Capacitance ( r = 24D) Resistance Sensitivity (pF/ppm) LOD (ppm) Response time (min) Recovery time (min) Sensitivity (Ohm/ppm) LOD (ppm) Response time (min) Recovery time (min) Sil. IDC small 1.48E-06 1490 8.4 8.3 n/a n/a n/a n/a ZSM-5 IDC small 2.8E-03 2.2 6.5 24.8 12323 820 4,4 8.8 ZA IDC small 2.3E-6 520 1 1.9 118 4900 0.4 9.3 Table 5: IDC ethanol results Toluene Capacitance ( r = 2.3D) Resistance Sensitivity (pF/ppm) LOD (ppm) Response time (min) Recovery time (min) Sensitivity (Ohm/ppm) LOD (ppm) Response time (min) Recovery time (min) Sil. IDC small 4.17E-06 520 4.3 5.5 n/a n/a n/a n/a ZSM-5 IDC small 8.4E-03 0.62 6.4 23 362629 50 1.45 16.2 ZA IDC small 1.24E-4 25 1.6 3.2 n/a n/a n/a n/a Table 6: IDC toluene results Using these results, observations could be made about the different sensors abilities. Firstly, this was done, focusing on the capacitance results: At a glance, it would be easy to think that zeolite A would adsorb more water and ethanol (polar molecules) than other zeolites due to its hydrophilic behaviour. However, the response was not as expected. This is because these zeolites need a more intense drying process, 150º C is not enough. Due to this, the zeolite had all its pores blocked by previously adsorbed ambient water. This resulted in less increases in sensitivity when the concentrated water was introduced. Gas Detection of Different Polarity Molecules by Capacitive Sensors Based on Zeolites Thomas Hoole Page 45 [15]-Isabella Marr, Anna Nützel, Daniela Schönauer-Kamin, Ralf Moos, Sensing of NO, NO2, and NH3 with Zeolite-Based Impedimetric Gas Sensors,Dept. of Functional Materials, University of Bayreuth, Universitätsstr. 30, 95447 Bayreuth, Germany [16]- S. Darbari, Y. Abdi, S. Mohajerzadeh, Branched carbon nanotubes to realize a novel capacitive sensor and actuator device, Sensors and Actuators A: Physical, Volume 167, Issue 2, June 2011, Pages 389-397 [17] - Kansong Chen, Kun Xie, Xinran Feng, Shengfu Wang, Rui Hu, Haoshuang Gu, Yang Li, An excellent room-temperature hydrogen sensor based on titania nanotube-arrays, International Journal of Hydrogen Energy, Available online 15 July 2012, [18] - Yi Zeng, Tong Zhang, Lijie Wang, Minghui Kang, Huitao Fan, Rui Wang, Yuan He, Enhanced toluene sensing characteristics of TiO2-doped flowerlike ZnO nanostructures, Sensors and Actuators B: Chemical, Volume 140, Issue 1, 18 June 2009, Pages 73-78, [19]- Xiaobo Yang, Daniel Albrecht, Jürgen Caro, Revision of Charnell’s procedure towards the synthesis of large and uniform crystals of zeolites A and X, Microporous and Mesoporous Materials, Volume 90, Issues 1–3, 20 March 2006, Pages 53-61 [20] -Castro, Miguel Ángel Urbiztondo. Microdispositivos basados en zeolitas: sensores y otras aplicaciones. s.l. : Universidad de Zaragoza, 2008. [21] – I. Marin Andrews, intergrated micro reactors with an ultra-high surface to volume ratio covered with nano structured materials, Final masters project, Universidad de Zaragoza, (2011) [22] - Lai, Z., Bonilla, G., Diaz, I., Nery, J. G., Sujaoti, K., Amat, M. A., et al. Microstructural optimization of a zeolite membrane for organic vapor separation; Science, volume 300, (2003), Pages 456-460.