Sunday, March 31, 2019

Gas sensors on zinc oxide nanostructures

Gas detectors on bob up oxide nanostructures footGas demodulators taild on semiconducting climb up oxides atomic number 18 be widely used for perception spoiles and vapors. The initial sum was provided by the findings of Seiyama et al. in altogetheroy oxide- particle accelerator reaction tack togethers in 1962. It was take the standn that the electric caral conductivity of ZnO locoweed be inter miscellanyd by the presence of reactive particle acceleratores in the contrast. The merits of these demodulators include their reliability, imprint cost and easy implementation. Nanostructures of coat oxides bugger dour been found to be some magnetic coreive as heavy weapon- perceptual experience materials at elevated temperatures. Very popular spying materials are coat oxide semiconductors such as ZnO, SnO2, TiO2, and WO3.Gener entirelyy the qualifying of electric field (conductance, voltage, underground or the change of piezoelectric proceeds) of the per cept element is monitored as a bureau of the post ordnance density. Gas spotting elements normally become in air, in the presence of humidity and interfering gunmanes. A het substrate membrane is fitted with go discomfit on sensitive nanostructured semiconductor material which generates electric kayoedput signals once chemical substance substance reactions are initiated at their scratch. A habitual property of all these underc e realwhere work reactions is that they require signifi merchantmant levels of thermal activating to proceed at a measurable rate.Nanostructures of semiconducting oxides are widely used for foul up sensing collectible to their commodious protrude flying field to volume ratio and possibility of shade depletion of carriers deep down nanostructures when exposed to botches. nitrogen dioxide (NO2) is a reddish-brown, broad(prenominal)ly reactive natural fuck up that reacts in the air to hit corrosive nitric acid as well as toxic organi c nitrates. The major man made source of NO2 emissions is high up-temperature fuel electrocution in motor vehicles and industries. These emissions are primarily in the fashion model of NO which gets oxidized in the atmo plain to NO2. The conversion rate depends on the ambient parsimony of NO and O3. If O3 is present, the conversion is really fast. Health and safety guidelines pop the question that humans should non be exposed to 3ppm or more NO2 shoot a line for periods longer than eight hours be convey of its toxicity. NO2 is a pulmonary irritant primarily bear on the upper respiratory remains in human beings. Continued or frequent exposure to high levels of NO2 potbelly cause inflammation of the lungs.Therefore, the schooling of a stable NO2 gasolene demodulator that commode detect passing low intentnesss of NO2 with high sensitivity and selectivity is super desirable. Such a demodulator can be used for environmental monitoring. It can besides be used in an ear ly warning system that detects the presence of NO2 earlier the critical concentration of NO2 is reached.In our work, we impart develop a demodulator for NO2 gas sensing found on our understanding in sensing element mechanism and synthesis of ZnO nanorods, using simple hydrothermal methods. The various answerance parameters of the detector, videlicet gas selectivity, sensitivity, retort and rec everywherey m will be study. The gas demodulator test-bench developed in COEN (Centre of Excellence in Nanoengineering), AIT, will be utilize for characterizing the demodulator performance.Chapter 2Literature reviewThis chapter is focused on the literature review of come forward oxide base semiconducting nanostructures used for gas sensing. The working principle of surface oxide gas sensors, measurement methods and synthesis mechanisms is include in this review.Metal oxide nanostructuresMetal oxides such as SnO2, WO3, TiO2 and ZnO possess high sensitivity to changes in their surrounding atmosphere at elevated temperatures. The sensing properties of metal oxides in form of thick or thin brings bring in been study to improve, by the addition of appalling metals namely Pd, Pt, Au, Ag in terms of selectivity and stability. In 1991, Yamazoe showed that decline of crystallite fall out-of-doors caused a huge advancement in sensor performance. In a low grain size of it metal oxide al well-nigh all the carriers are trapped in issue carrys and only a few thermal activated carriers are ready(prenominal) for conduction.From the point of view of device falsehood, first generation gas sensor devices were fabricate by thick dissipate technology. Then the material fabrication processes improved towards the thin film technology. The fabrication process for thin film technology namely physical and chemical vapour deposition was highly automated and offers high reproducibility. The electrical properties of both thin and thick film sensors drift due to t he grain porosity modification and grain term alteration.Several methods like addition of noble metals as catalysts or multiform oxides were put forward to improve the sensing performance of the gas sensors. The morphological engineering of metal oxide nanostructured thin films proved to optimize the performance of these types of gas sensors. The various run parameters such as chemical reaction magazine, output signal, selectivity and stability can be improved and tuned through the optimization of the structure. Using geomorphologic engineering method, the various geometric parameters of metal oxide gas sensing intercellular substance like grain size, agglomeration, film thickness, porosity can be subordinationled.The close forward step in gas sensing was achieved by the victorious preparation of stable wholeness crystal quasi- angiotensin-converting enzyme-dimensional semiconducting oxides (nanorods, nanotelegrams) leading to the third generation of metal oxide gas sensor s.Working principle of metal oxide gas sensorsConductometric metal oxide gas sensors depend on changes of electrical conductivity due to the interaction with the surrounding atmosphere. The normal out increaseal temperature of metal oxide gas sensors is within the range betwixt cc C and ergocalciferol C. The direct temperature should be high enough so that gas reactions occur in a time on the order of the desired response time and should excessively be low enough to avoid any variations in the pop of the sensing matrix. The single crystal structure synthesized at temperatures high than the direct temperature of the sensor shows high stability.Based on the study of a large range of oxides, the phenomenon of change in conductivity to the presence of reactive gases in air is common to oxides and not special(prenominal) to a few special(prenominal) eccentric persons. If the conductivity is too high, then an effect is not expected and in like manner if the conductivity is too low, then an effect will be rocky to measure. In practical covers, if an oxide sample has a resistivity in the midst of 104 and 108 Ocm at 300- 400 C, then it will function as a gas sensor when heated to a temperature in this range.The sign of response ( tube add or decrease) leads to a simple classification gases can be catego fount as oxidizing or reducing and oxides can too be separate as p or n type. P-type oxides show a ohmic apology profit in the presence of traces of reducing gases and resistance decrease to oxidizing gases. n-type oxides show opposite behaviour. This behaviour likewise correlates with the effect of changing atomic number 8 partial derivative pressure (PO2).Adsorption on open airsThe sensing mechanism in metal oxide gas sensors is related to ionosorption of species over their surfaces. The most important ionosorbed species when operating in ambient air are oxygen, water, carbon and its compounds. High concentrations of carbon can block surface sites of adsorption on a metal oxide. In the temperature range between atomic number 6 C and 500 C, oxygen ionosorbs over metal oxide in groyneecular (O2-) or atomic form (O-). and then the study of adsorption is of fundamental importance in the field of sensors.PhysisorptionIn this debilitatedest form of adsorption like van der waals forces, no true chemical bond between the surface and adsorbable (or reaction species) is established. This bonding is mainly due to the induced dipole moment of a unionised adsorbate interacting with its own image charges on the polarized surface. The bonding energy is rather weak in the order of 0.1 eV.ChemisorptionChemisorption corresponds to the creation of chemical bonds between the adsorbate and surface and results in the electronic structure perturbation. In gas sensors, the target gas whitethorn be chemisorbed or physisorbed on the surface. When the gas species adsorb on the surface, touchs are either dissociated or diffused in the sensi tive horizontal surface.Based on the Temperature Programmed Desorption (TPD) and Electron Paramagnetic study (EPR) studies, at lower operating temperatures, oxygen is considered to be adsorbed in bulwarkecular form (either as neutral O2 (ads) or charged O2(ads) 2- ) due to its lower activation energy. At higher temperatures it dissociates into atomic oxygen (either neutral O(ads) or independently ionized (charged) O(ads)- or doubly ionized O2(ads)- ). Finally at very high temperatures the loss of lattice oxygen (first surface and then bulk) takes place. When a reducing gas like CO comes into contact with the surface.These consume ionosorbed oxygen and in turn change the electrical conductance of metal oxide. The general effect is a change of the density of ionosorbed oxygen that is detected as an maturation of sensor conductance. Direct adsorption is also possible for the gaseous species like strongly electronegative NO2, which decreases the sensor conductance.NO2 absorption on tin oxide surfaces was canvass by temperature programmed desorption measurements and found that the adsorbates originating from NO2 are the homogeneous as those from NO, as NO2 molecule dissociates easily over the tin oxide surface. These adsorbates can be divided into cardinal types, two nitrosil types (Sn NO+ and Sn NO- ) and the nitrite type Sn O-N=O. The nitrite type does not play any role in gas sensing since it is not involved in any electron exchange with the bulk of the semiconductor.In practical applications, gas sensors are normally expected to operate in air, in the presence of humidity and interfering gases. In such cases, for operating temperatures in a range of 100 to 500 C, at the surface of the sensitive material various oxygen, water and carbon dioxide related species are present. Some gas species form bonds by exchanging electrical charge with specific surface sites and others may form dipoles. Dipoles do not affect the concentration of free charge carrier s and so they take on no impact on the resistance of sensitive layer. Fig.1 explains the simplified case of adsorbed oxygen ions and hydroxyl groups brink to an n-type metal oxide semiconductor. These adsorbed ions cause a band bending maculation the dipoles change the electron affinity when compared to the state before the adsorption .The changes of the work function (?F) are laid by band bending (qVs due to ionosorption) and changes in the electron affinity () due to building of dipoles at the surface (M d+ OH d-).Ec, Ec,s Energy level representing the bottom of the conduction band and at the surface respectively. Ev, Ev,s Energy level representing the top of the valence band and at the surface respectively. Evac vaccum level, EF Fermi level, Work function, Electron affinity.Sensor CharacteristicsThe characteristic of a sensor is classified into static and dynamic. Static characteristics can be measured when all the transient effects of the output signal have stabili zed in to steady state. Dynamic characteristics tend to describe the sensors transient behavior.Static characteristics predispositionSensitivity is the ratio of incremental change in the output of the sensor to its incremental change of the measurand in scuttlebutt. For example, if we have a gas sensor whose output voltage increases by 1 V when the oxygen concentration increases by 1000 ppm, then the sensitivity would be 1 mV/ppm. Generally, the sensitivity to the target gas is delimit as the percent reduction of sensor resistance.Sensitivity (%) = (Ra- Rg) / Ra 100,where Ra is the value of initial equilibrium resistance in air and Rg is resistance in the presence of a target gas. For convenience sometimes the sensitivity of gas sensor is expressed as the ratio of resistance in air over resistance in gas for reducing gases (Ra/Rg) and resistance in gas over resistance in air (Rg/Ra) for oxidizing gas.SelectivityThe sensors ability to measure a single section in the presence of ot hers is known as its selectivity. For example, an oxygen sensor that does not show a response to other gases such as CO, carbonic acid gas is considered to be discriminating.Selectivity = (sensitivity of gas1/sensitivity of gas2)Selectivity of the sensor is assessed by the ratio of sensitivity between the gases that is of interest to be detected over the gases that are uninteresting for signal detecting in equivalent concentrations. To improve selectivity to specific gases, sensor array technology is also being adapted.Stability and DriftThe sensors ability to score the kindred output value when measuring a fixed input over a period of time is termed as stability. Drift is the gradual change in the sensors response characteristics while the input concentration of the gas system constant. Drift is the undesired and unexpected change that is unrelated to the input. It may be attributed to aging, temperature instability, contamination, material degradation, etc. For instance, in a gas sensor, gradual change of temperature may change the baseline stability, or gradual diffusion of the electrodes metal into substrate may change the conductivity of a semiconductor gas sensor.RepeatabilityIt denotes the sensors ability to produce the same response for successive measurements of the same input, when all operating and environmental springs remain constant.ReproducibilityThe sensors ability to reproduce responses afterwards some measurement condition has been changed. For example, after shutting down a sensing system and subsequently restarting it, a reproducible sensor will show the same response to the same measurand concentration as it did prior to being shut down.HysteresisIt is the difference between output readings for the same measurand, when approached while increasing from the minimum value and the other while decreasing from the jacket value.Response TimeThe time taken by a sensor to arrive at a stable value is the response time. It is loosely expre ssed as the time at which the output reaches a trusted percentage (for instance 95%) of its lowest value, in response to a stepped change of the input. At the onset, the response time is very fast, followed by a long drawn tail before reaching steady state value, thus the response time are often expressed as 50% or 70% of the final time. Recovery time is defined as the time that the sensor takes to recover its resistance from exposed condition to the baseline value after target gas is cut out from the environmentDynamic Range or SpanThe range of input signals that will result in a meaningful output for the sensor is the dynamic range or span. All sensors are designed to perform over a specified range. Signals outside of this range may cause unacceptably large inaccuracies, and may even result in permanent damage to the sensor.Dynamic characteristicsThe dynamic characteristics of a sensor represent the time response of the sensor system. The various important dynamic characteristi cs of sensors are discussed below, gussy up timeRise time is defined as the time call for by the sensor response to change from 10% to 90% of it final steady state value.Settling timeIt is the time taken by the sensor response to settle down to within a indisputable percentage of the steady state value.Influence of contact electrodes on sensor performanceThe contact electrodes used in gas sensors can have both electrical and electrochemical roles. For thin wedge films, contact resistance plays an important role as dominant factor in overall resistance. The contribution of contact resistance is also extremely important for the case in which individual nanorods, nanowires or nanobelts are used as sensing layers. These electrodes are generally made of metals. They can also be fabricated from materials such as conductive polymers or conductive metal oxides.Although the notion of resistance change of the sensitive material when exposed to target gas is widely known, the overall resis tance of the sensor depends not only on the gas sensing material properties but also on parameters such as transducer morphology, electrode etc. When the sensitive layer consists only of a league continuous material and the thickness is larger than the Debye length, it can only partly depleted when exposed to target gas. In this case, the interaction does not go the entire bulk of the material. Two levels of resistance are established in parallel and this fact limits the sensitivity. Thin layer will be the go choice which can be fully depleted.The representation shows the influence of electrode-sensing layer contacts. Rc is resistance of the electrode-metal oxide contact, R11 is the resistance of the depleted region of the compact layer, R1 is the equivalent of serial resistance of R11 and Rc, and the equivalent series resistance of SRgi and Rc, in the porous and compact situations, respectively. Rgi is the average inter-grain resistance in the case of porous layer, Eb minimum of the conduction band in the bulk, qVs band bending associated with surface phenomena on the layer, and qVc also contains the band bending induced at the electrode-metal oxide contact.Improvement of selectivity by surface modificationsMixing metal oxides withMetals that function as catalystsBinary compounds and multi-component materialsDopingare the most common methods used to enhance the gas sensing performance of metal oxide gas sensors. These additives can be used for modifying the catalytic activity of the base oxide, favoring formation of active phases and improving the electron exchange rate. The interaction of gas with the sensing material, resulting in the gas sensitivity, is determined by the chemical properties of the sensor surface. contrasting surface atoms can be introduced on the surface of the metal oxide sensors. This surface modification leads to new chemical reactivity and enables the sensor to be operated at low temperatures.Nanoscale particles of noble metals ( Pd, Pt, Au and Rh) and oxides of other elements (Co, Cu and Fe) deposited on the surface of metal oxides can act as surface sites for adsorbates and promoters for surface catalysis. They create special adsorption sites and surface electronic states and as a result gas sensitivity, selectivity, rate of response can be altered. For achieving high gas response, the noble metal should create optimal conditions for both electron and ion (spillover) exchange between surface and reacting gas species.The nature of noble metals, their oxidation state and their distribution on the surface are determining factors in gas sensor sensitivity and selectivity. To attain the homogenous distribution of noble metal on the surface is very difficult. Surface morphology has a significant effect on the shape and distribution of catalysts. Noble metal clusters have a tendency to accumulate at step edges and kinks of metal oxides during their deposition.Catalysts based on noble metals can be poisoned by u mpteen organic and inorganic chemicals that contain sulphur (H2S, SO2, thiols) and phophorus. The excessive thickness of catalytic active additives can change their functions, turning into either shunting layer or active membrane filters, obstructing the penetration of detecting gas in the surface of gas sensing matrix. At certain conditions this quality can also be used for an improvement of gas sensors selectivity.It has been studied that the incorporation of additional phases ( diverse oxides) in nanocrystalline systems in small quantities can change the conditions of base oxide growth. SnO2 narcotised with Nb (0.1 4 mol%) causes a decrease in crystallite size from 220 nm for pure SnO2 to about 30 nm for Nb (0.1 mol%) doped samples. The additional influence observed due to doping is the change in film resistance. SnO2 doping by Nb and Sb in the range of 0.01 and 1.0 mol% during sol-gel preparation and annealed at 900 C leads to film resistance decrease of 100 to 1000 times respe ctively, while doping with In resulted in a rise in film resistance by a factor of 100. The effect of doping on gas sensing properties of metal oxide gas sensors is different from the catalytic activity of these additives.Improvement of selectivity by operating conditionsThe sensor material may be operated at a comparably wide range of operating temperatures (300 900 C) leading to different thermal energies for the surface reactions, differences may be reach by selecting the operating temperature, leading to a variation in gas sensitivity. A more improved version of this idea is to constantly increase or decrease the operating temperature of a given sensor and to continuously measure the variation of conductivity. This technique is known as temperature transient operation which gives more information in case of gas mixtures. To realize selective gas detection, sensor arrays are also constructed where several sensors showing different patterns of gas sensitivity are selected and simultaneously operated. A simple technique to obtain an array using one sensor is to modulate the operating temperature to different levels. Excessive increase of operating temperature may lead to a considerable drop of gas sensitivity. Moreover increasing working temperature can create conditions, where gas response will then be determined by change of bulk properties of material.Improvement of response and recovery time of gas sensorsA high speed gas switching system can be used to improve the response of the gas sensor. Yamazoe et al. studied the response and recovery properties of SnO2 porous film gas sensors using a high speed gas switching system. The developed system allows the rapid replacement of the gas atmosphere in the chamber between air and H2 (or CO). It was describe that the response speed of the sensor was fast, reaching a response time of less than 0.5s at 350 C. The rates of diffusion and surface reactions of these gases (H2 and CO) in the porous sensing film ar e high enough for the sensor to reach a steady state within a of a sudden time. However the resistance in air did not reach the received value by repeated switching. This incomplete recovery was attributed to the slow desorption of piss and carbonic acid gas formed on SnO2 by the surface reaction of H2 and CO respectively.Synthesis of 1-D metal oxide nanostructuresMetal oxide nanostructures synthesis methods are broadly categorized asSolution phase synthesis method, where the growth process is carried out in liquid. Since aqueous solutions are used, this process is otherwise termed as hydrothermal growth process.Gas phase synthesis method uses gaseous environment in closed chambers. The synthesis is carried out at high temperatures from 500 C to 1500 C.Zinc oxide (ZnO)ZnO is wide bandgap (Eg = 3.4 eV) II VI compound semiconductor which has a non-centrosymmetric wurtzite structure with polar surfaces and lattice parameters a = 0.3296 and c = 0.52065 nm. The structure of ZnO can b e described as a number of alternating planes composed of tetrahedrally coordinated O2- and Zn2+ ions, stacked alternatively along the c-axis. The tetrahedral coordination in ZnO results in piezoelectric and pyroelectric properties. The oppositely charged ions produce positively charged (0001)-Zn and negatively charged (000-1)-O polar surfaces, resulting in a normal dipole moment and spontaneous polarization along the c-axis.Hydrothermal Synthesis of Zinc oxide nanostructuresDifferent techniques namely sol-gel, spray pyrolysis, hydrothermal method, electrospinning and thermal evaporation are prevalent for the synthesis of atomic number 30 oxide nanoparticles and nanorods. The hydrothermal process is an environmentally friendly process and does not require a complex vacuum environment. The hydrothermal process is surface independent and provides good control over the morphology of the nanostructures. ZnO nanorods growth on glass substrates by thermal decomposition of hexamethylenete tramine (HMT) and zinc nitrate is reported by Baruah et al. Thermal degradation of HMT releases hydroxyl ions which react with Zn2+ ions from ZnO. The role of HMT is to supply the hydroxyl ions to drive the downfall reaction. Sugunan et al, have proposed that HMT being a long chain polymer and a nonpolar chelating agent, gets preferentially attached to the non polar facets of the zincitie crystal thereby cutting off the access of Zn2+ ions to them leaving only the polar (001) face for epitaxial growth.Metal oxide nanostructure based conductometric gas sensorsZinc oxideCharacterization of gas sensing properties of ZnO nanowires is reported by Ahn et.al. ZnO nanowires were fabricated by a selective growth method on imitate Au catalysts forming a nanobridge between two Pt towboat electrodes. The gas sensing properties were demonstrated using NO2 gas. The response as a function of temperature is shown to be highest at 225 C and linearly increased with the concentration of NO2 in the range of 0.5 3ppm and saturated beyond this range. The sensor performance is also compared with ZnO nanocrystals, Sn and In doped ZnO thin film. Also the nanobridge structure is shown to have fast recovery behaviour because the desorbed gas molecules can be easily removed off from the nanowires surfaces.Lupan et.al demonstrated the gas sensing behaviour of Al doped ZnO films synthesized by successive chemical deposition method. Successive chemical solution deposition method was reported to be simple and requires non-sophisticated equipment to produce nanostructures with high efficiency. Nanostructured ZnO films doped with Al showed a high sensitivity to CO2 than undoped ZnO films.Characterization and gas sensing properties of ZnO hollow spheres is reported by Zhang et.al. Different concentrations of NH3 and NO2 at different temperatures were used to test the gas sensor. ZnO hollow sphere sensor exhibited extremely different sensing behaviors to NH3 and NO2. The optimum operating te mperature of the sensor was 200 C for NH3 and 240 C for NO2 respectively. The gas sensor exhibited much higher response to NO2 than to other gases at 240 C implying good selectivity and potential application of the sensor for detecting NO2.Tin oxideLaw et.al, analyzed room temperature sensing properties of a single crystalline tin oxide nanowire sensor towards nitrogen dioxide. NO2 chemisorb strongly on SnO2 surface and at room temperature desorption is not complete when the NO2 is removed. UV light was used to activate both the adsorption and desorption process. In the dark, oxygen adsorbs on the surface capturing electrons from the semiconductor and creates a depletion layer. When exposed to UV, photo-generated holes migrate to the surface and recombine with electrons releasing oxygen ions, with an increase in conductance. The detection limit was 2 10 ppm of nitrogen dioxide.Kolmakov et.al studied the effect of catalysis in tin oxide single wire FET structures. The sensing capabi lities of SnO2 single nano-wires and nanobelts in a FET configuration before and after functionalization with Pd catalyst was analysed. The improvement in the sensing performance after catalysation was reported to be the combined effect of spill-over of atomic oxygen formed catalytically on Pd clusters and migrating on SnO2 surface and also to the back spill-over effect in which weakly bound molecular oxygen migrates to Pd clusters and are catalytically dissociated.Indium oxideIndium oxide nanowires have been tested towards ethanol by Xiangfeng et.al. A mixture of In2O3 nanowire and polyvinyl alcohol solution was coated on aluminum oxide tubes with two gold contacts at the end a heating wire was inserted in the tube to operate in the temperature range 100 500C. The resistance of the nanowires was monitored in presence of air, ethanol and other gases. The highest response was obtained with ethanol, the detection limit was estimated to be equal to 100 ppm.Molybdenum oxideMolybdenum o xide nanorods based gas sensing was reported. The MoO3 nanorods were characterized by high response to ethanol and CO at temperatures in the range of 100 C. The response of thin films with the same structure was comparatively studied and nanorods based sensor resulted in one order of magnitude more sensitive due to the high surface to volume ratio and reduced lateral dimensions of the nanorods.Other metal oxidesSawicka et.al. presented the nitrogen sensing properties of tungsten oxide nanowires disposed(p) with electrospinning. The effect of processing parameter variations was studied and a comparison with thin films prepared by sol-gel was also presented. WO3 nanowires showed give out NO2 sensing performances compared to sol-gel processed films due to increase in surface reach of nanowires.A large amount of literature is available on the gas sensing properties of carbon nanotubes. Only little attention is put in the studies of gas sensing properties of metal oxide based tubular s tructures. Varghese et.al. studied the atomic number 1 sensing properties of titania nano-tubes. The tests were performed in nitrogen atmosphere and 1% H2. The response time increased with temperature and the response time was 2-3 min.NO2 gas sensors based on ZnO nanostructuresLiu et.al reported the NO2 gas sensing properties of vertically aligned ZnO nanorod arrays prepared by hydrothermal method with zinc acetate and hexamethylenetetramine. The seed layer was deposited by inaudible spary pyrolysis. The aqueous hydrothermal solution was prepared by mixing equimolar ratio of zinc acetate dehydrate and HMT. The hydrothermal growth was carried out in a Teflon-lined unspotted container. The substrate was put in the solution with the seeded face down and the container was sealed and kept at 110C for three hours. The nanorod sensor shows a higher sensitivity than the ZnO film based sensor prepared by unhearable spray pyrolysis. The enhanced sensitivity is attributed to the higher re flection ratio of the nanorod structure and the sensitivity increases with the length of the nanorod. The relative response of the sensor is linearly proportional to NO2 concentration in the 0.2 5 ppm range.The NO2 gas sensing properties of semiconducting type gas sensors with channels composed of non-agglomerated, necked ZnO nanoparticles were investigated by Jun et.al. The heat treatment of the nanoparticles at 400C led to their neck and coarsening. The slight necking of the nanoparticles with their neighbors also enhanced the conductivity of the channels, due to the grave of the potential barrier. The response of the necked nanoparticle based sensor was reported to be as high as 100 when exposed to 0.2 ppm of NO2 at 200 C.NO2 gas sensor based on ZnO nanorods grown by ultrasonic irradiation was reported to very high sensitivity with a very low detection limit of 10 ppb at 250C. Sonochemical route was employed for the fabrication of vertically aligned nanorods on a Pt electrode patterned alumina substrate. The total time requir

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