Nanoparticle films for gas sensing applications: greener approaches

J. Environ. Res. Develop. Journal of Environmental Research And Development Vol. 9 No. 01, July-September 2014

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Review paper (NS)

NANOPARTICLE FILMS FOR GAS SENSING APPLICA-TIONS : GREENER APPROACHES Pethkar Padmashri A.1 and Pethkar Aniroodha V.*2

1. Department of Applied Sciences, R.H. Sapat College of Engineering, Management and Research, Nashik, Maharashtra (INDIA)

2. Department of Microbiology, Institute of Science, Aurangabad, Maharashtra (INDIA)

Received December 10, 2013 Accepted June 25, 2014

ABSTRACT Gas sensing technology is concerned with development of devices for detection of gases in industries, homes or the environment. Most popular designs employ layer/s of gas sensing materials coated on substrates and connected through electronics to amplifiers. The properties of gas sensing chemicals, the methods to produce them and technologies to assemble them are important factors for manufacturing highly sensitive gas sensors. Rapid developments in materials science research, especially the cross-disciplinary approach of nanotechnology would contribute greatly in this area. This article describes essentials of gas sensing technology, types of sensors, materials used and highlights the possibility of using green chemicals from natural sources for enhanced stability/efficiency of gas sensing materials.

Key Words : Gas sensing, Green technology, Semiconductors, Screen printing, Metal oxide,

INTRODUCTION Detection of gases is an important area of research due to the widespread applications in various areas. Some important applications are : (i) industrial production, e.g. methane detection in mines1, (ii) automotive industry, e.g. detection of polluting gases from vehicles2, (iii) medical applications, e.g. electronic noses simulating the human olfactory system3, (iv) indoor air quality monitoring, e.g., detection of carbon monoxide4

and (v) environmental studies, e.g. greenhouse gas monitoring5, etc. Gas detection instruments are indispensible for industrial health and safety, environmental monitoring and process control. In the modern world of smart materials, gas sensing devices are receiving increasing attention in homes, industry and academia. Atmospheric pollution caused by gaseous chemicals is a matter of serious concern. Many gases, even in low concentrations are toxic to living organisms including humans. Gases spread easily through the air are invisible and difficult to control if released accidentally. Most of the toxic gases released in the environment are not noticed prior to their ill-effects which may range from

unpleasant odors, irritability of the respiratory track and breathing discomfort to nausea, unconsciousness and even death of humans and animals that inhale such gases. Leakage of inflammable gases in the environment poses the dual risk of toxicity and fire hazard. Gas sensing technology is concerned with the development of suitable methods to detect hazardous gases in the environment. High sensitivity lies at the core of any such technology that aims at early detection of leakage and controlling the spread of gases in the environment to avert potential wide-spread hazards. The sensing technology itself has various branches, such as the investigation of different kinds of sensors, the sensing principles, generation of sensing materials and fabrication of sensing devices.6 Depending on the detection principle, materials used sensitivity, selectivity and versatility, the gas sensing devices or sensors are categorized into various types such as those measuring changes in electrical, optical, thermal and acoustic properties. Irrespective of the type, sensors essentially consist of layer/s of gas sensitive material coated or deposited onto an inert surface. The most sensitive *Author for correspondence

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devices make use of thin films of advanced materials such as nanocomposites with alternating or non-alternating patterns. High cost of production of these materials and subsequent layering is a major deterrent in the production of sensors. Among the variety of sensing materials, metal oxides have attracted particular attention due to easy availability, low cost and ease of use. Oxygen ions adsorbed onto the material’s surface, remove electrons from the bulk and create a potential barrier that limits electron movement and conductivity. Reactive gases combine with this oxygen to reduce the barrier and increase conductivity, a phenomenon which is directly related to the amount of a specific gas present in the environment. This allows quantitative determination of gas present in the environment. Recent spurt in the synthesis of nanoparticle metal oxides that are stabilized by naturally occurring green chemicals could offer revolutionary new sensing devices.

DISCUSSION Film deposition techniques A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to few tens of nanometers, while thick films constitute layers of 5-20 micrometers thickness. Electronic semiconductor devices and optical coatings are the main applications benefiting from thin film construction. One of the common applications of thin films is the household mirror, which has a thin metal coating on the back of a sheet of glass forming a reflective interface. A very thin film coating (less than a nanometer thick) is used to produce two-way mirrors. The performance of optical coatings (e.g. antireflective or AR coatings) are typically enhanced when the thin film coating consists of multiple layers having varying thicknesses and refractive indices. A periodic structure of alternating thin films of different materials may collectively form a so-called superlattice which exploits the phenol-menon of quantum confinement by restricting electronic phenomena to two-dimensions. Other applications worth mentioning pertain to use of ferromagnetic and ferroelectric thin films in computer memory devices, thin film drug delivery systems, thin-film batteries, transparent transistors, dye-sensitized solar cells and the

protective ceramic thin films. Thick films are mainly used for protective coatings, photovoltaic cells and sensing devices. In some processes, the thickness is not important for example, the purification of copper by electroplating and the deposition of silicon and enriched uranium by a CVD-like process after gas-phase processing. Film deposition techniques for thick and thin films are carefully controlled processes that ensure uniform layering of material on a surface. These fall in two major types, viz. chemical deposition and physical deposition techniques and are described in detail in the literature.7,8 A brief account of these techniques is given below : Chemical deposition In this technique, a fluid precursor undergoes a chemical change at a solid surface, leaving a solid layer. Since the fluid surrounds the solid object, deposition happens on every surface, with little regard to direction, thin films from chemical deposition techniques tend to be conformal, rather than directional. Based on the phase of the precursor and technique involved, chemical deposition is further categorized into various types as described below : (i) A liquid precursor, often a solution of

metal salt in water, e.g. electroplating of noble metals is used. The technique became popular in the semiconductor field when the use of chemical-mechanical polishing techniques became common.

(ii) Chemical Solution Deposition (CSD) or Chemical Bath Deposition (CBD) makes use of a liquid precursor, e.g. solution of organometallic powders dissolved in an organic solvent. The method, also called sol-gel method is quantitative, simple and inexpensive.

(iii) Spin coating/casting makes use of a liquid precursor or sol-gel precursor deposited onto a smooth, flat substrate. When spun at a high velocity, the solution spreads over the substrate. The spinning speed and viscocity of sol determine the thickness of the deposited film. Thickness can be increased by repeated such depositions. Thermal treatment is often


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carried out in order to crystallize the amorphous spin coated film to achieve preferred orientations.

(iv) Chemical Vapor Deposition (CVD) generally uses a gas-phase precursor at a high temperature, often a halide or hydride of the element to be deposited.

(v) Plasma Enhanced CVD (PECVD) uses an ionized vapor or plasma, as a precursor. Plasma is produced by chemical or physical means (electric current, micro-wave excitation).

(vi) Atomic Layer Deposition (ALD) uses gaseous precursor to deposit conformal thin films one layer at a time. One reactant is deposited first and then the second reactant is deposited, during which a chemical reaction occurs on the substrate, forming the film with desired composition. The process is slower than CVD, but can be run at low temperatures.

Physical deposition The physical deposition techniques involve mechanical, electromechanical or thermo-dynamic means to generate a film. The material to be deposited (source) is placed in an energetic, entropic environment, allowing particles from its surface to escape and form a thin solid layer on a cooler surface (substrate) facing the source. The system is kept under vacuum so that the particles travel freely contamination is avoided. Films deposited by physical means are commonly directional, rather than conformal. Physical deposition technique is categorized into various types as follows : (i) Sputtering relies on a plasma (usually a

noble gas, such as argon) to remove material a few atoms at a time from a target that is kept at a lower temperature. It is especially useful for compounds or mixtures, where different components would otherwise tend to evaporate at different rates. The surface coverage is conformal. The method is rapid with good control over thickness and is widely used to manufacture optical media such as CDs and DVDs.

(ii) Pulsed laser deposition (laser ablation process) uses pulses of high intensity

focused laser light to vaporize the target surface. The target surface is converted to a plasma which reverts to a gas before reaching the substrate to form a film.

(iii) Cathodic arc deposition (arc-PVD) make use of an electrical arc that blasts ions from the cathode. The arc has an extremely high power density resulting in a high level of ionization (30–100%), multiply charged ions, neutral particles, clusters and macro-particles (droplets). Use of reactive gas during evaporation process causes dissociation, ionization and excitation resulting in compound film deposition.

(iv) Electrohy drodynamic deposition (electro-spray deposition) is a method that utilizes a liquid to be deposited (nano-particle suspension or a solution) that is fed to a small capillary nozzle (usually metallic) which is connected to a high power source. The substrate on which the film has to be deposited is connected to the ground terminal of the power source. Through the influence of electric field, the liquid coming out of the nozzle takes a conical shape (Taylor cone) and at the apex of the cone a thin jet emanates which disintegrates into very fine and small positively charged droplets. The droplets are deposited on the substrate as a uniform thin layer.

(v) Molecular beam epitaxy allows a single layer of atoms to be deposited at a time, especially useful for reflective, anti-reflective coatings or self-cleaning glass (optical devices), layers of insulators, semiconductors and conductors (electr-onic devices), aluminium-coated films (packaging applications) etc.

(vi) Screen printing technology uses conductive, resistive and insulating pastes containing glass frit, deposited in patterns defined by screen printing and fused at high temperature onto a ceramic substrate. The films typically possess thickness in the range 5–20µm (thick films), and resistivities of 10Ω/square-10MΩ/square. The method offers considerable possibilities for building multi-layer structures.


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The inventory requirements and critical parameters for physical and chemical deposition make the processes rather unattractive at commercial scales. Most physical methods consume high amounts of energy making the processes costly. The screen printing technique proffers unique advantages such as simplicity and low inventory requirements and is in common use. Screen printing process Screen printing technique is a simple, low-cost, thick film deposition process.7,8 wherein, multiple layers (including the sensor material itself) are deposited onto rigid substrates and heated to a relatively low temperature for densifying the sensor film. The substrates are then integrated into electronic devices for application in gas detection instruments. Calcined powder of sensing material is mixed thoroughly with glass frit and ethyl cellulose as binders. Butyl carbitol acetate is added as a vehicle and the paste is deposited on alumina substrate using standard screen printing technique (using 140s mesh no. 355). Films are dried under IR lamp and fired at temperatures of 7000C, 800 0C, 9000C for 2 h in muffle furnace.9 The method does not require skilled manpower and energy. The technique however cannot generate thin films, thus imposing limitations on the efficiency of sensors. Use of nanocrystalline materials with enhanced surfaces may overcome the limitations and therefore forms a major part of the discussion in this paper. Importance of nanomaterials as sensing materials Considerable research into new sensors is underway, including efforts to enhance the performance of traditional devices, such as resistive metal oxide sensors, through nanoengineering. Metal oxide sensors have been utilized for several decades as low cost methods for detecting combustible and toxic gases. However, issues with sensitivity, selectivity and stability have limited their use, often in favor of more expensive approaches. Recent advances in nanomaterials provide opportunity to drama-tically increase the response of these materials, as their performance is directly related to exposed surface area.10 The recent availability of various metal oxide materials in high-surface-area nanopowder form, as well as implementation

of newly developed nano fabrication techniques, offers tremendous opportunities for sensor manufacturers. Generation of thick films by screen printing The development of thick films by screen printing essentially requires inks that are deposited on suitable substrates using screen and squeegee that allow patterning of the deposited material.11 (i) Substrates mixed with glassy binders

(fluxes) these are usually abrasive, brittle and easy to mark, e.g. alumina (particle size 3-5µm) and as fired ceramics for thick films. For thin films that require a much smoother surface finish, polished ceramics or glass are used. Newer materials such as porcelainised steel, organic materials viz. epoxies, flexible substrates and even synthetic diamond are also used as substrates.

(ii) Inks various components are produced on the substrate by applying inks (or ‘pastes’) in sequential manner to produce the required patterns. Different formulations of the ink are used to produce conductors, resistors and dielectrics. Each ink contains a binder, a glassy frit (carrier), organic solvent systems and plasticizers (materials to be deposited, typically metals, alloys and metal oxides). A glassy phase usually binds he metal particles together and to the substrate.

(iii) Screens made of tensioned stainless steel or polyester mesh that has a transparency (open area) of about 40% and allows the ink to pass through it (100-300 wires per linear inch). High mesh counts enable finer detail to be resolved but give thinner prints. Selection of mesh type is based on the ink particle size and 200 mesh and 325 mesh are commonly used.

Classification of gas sensing methods Sensing technology is based on the physical properties of the sensing materials and alterations in these properties when exposed to gases. Some of the physical properties that have been widely investigated are the electrical, optical, calorimetric and acoustic properties.12 Table 1 gives a list of these methods with their advantages, disadvantages and applications.


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Table 1 : Summary of gas sensing methods- their advantages, disadvantages and applications

Materials Advantages Disadvantages Target gases and application fields

Metal oxide semiconductor

Low cost Short response time Target gases- wide

range Long lifetime

Low sensitivity and selectivity Sensitive to environmental factors High energy consumption

Industrial applications and civil use

Polymer

High sensitivity Short response time Low cost of fabrication Simple, portable structure (Low energy consumption

Unstable over long periods Irreversibility Poor selectivity

Indoor air monitoring Storage place of synthetic products (paints, wax or fuels) Workplaces, viz. chemical industries

Carbon nanotubes

Ultra-sensitive High adsorptive capacity High surface area Quick response time Low weight.

Difficulties in fabrication and repeatability High cost

Detection of Partial Discharge (PD)

Moisture absorbing material

Low cost Low weight High selectivity to water vapor

Vulnerable to friction Potential irreversibility in high humidity

Humidity monitoring

Optical methods

High sensitivity, selectivity and stability long lifetime insensitive to environment change

Difficulty in miniaturization high cost

Remote air quality monitoring Gas leak detection systems High-end market applications

Calorimetric methods

Stable at ambient temp. low cost ppt level sensitivity for industrial detection

Risk of catalyst poisoning and explosion Intrinsic deficiencies in selectivity

Combustible gases in industrial environment Petrochemical plants Mine tunnels Kitchens

Gas chromatography

Excellent separation high sensitivity selectivity

High cost Difficulty in miniaturization for portable applications.

Typical laboratory analysis

Acoustic methods

Long lifetime No secondary pollution

Low sensitivity Sensitive to environment

Components of wireless sensor networks

Gas sensing by metal oxides based on alterations in electrical properties Devices based on the variations in electrical properties are the most popular on account of the good sensitivity, simplicity and ease of

construction. A wide array of sensing materials is available which includes metal oxide semiconductors, polymers, carbon nanotubes, and moisture absorbing materials, among others. A variety of metal oxides, both single component


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(ZnO, SnO2, WO3, TiO2, and Fe2O3) and multi-component oxides (BiFeO3, MgAl2O4, SrTiO3) have been reported in the literature for sensor applications13. The mechanism for gas detection in these materials is based largely on reactions that occur at the sensor surface, resulting in a change in the concentration of adsorbed oxygen. Oxygen ions adsorb onto the material’s surface, removing electrons from the bulk and creating a potential barrier that limits electron movement and conductivity. When reactive gases combine with this oxygen, the height of the barrier is reduced, increasing conductivity. This change in conductivity is directly related to the amount of a specific gas present in the environment, resulting in a quantitative determination of gas present. Generally, metal oxides can be classified into two types : non-transition and transition. The former (e.g., Al2O3) contains elements with only one oxidation state since much more energy is required to form other oxidation states, while the latter (e.g., Fe2O3) contains more oxidation states. Therefore, transition-metal oxides could form various oxidation states on the surface, which is utilized by metal oxide semiconductors as sensing materials, compared to the non-transition ones. More precisely, transition-metal oxides with d0 and d10 electronic configurations can be used in gas sensing applications.14 The d0 configuration is found in transition metal oxides (e.g., TiO2, V2O5, WO3) and d10 appears in post-transition-metal oxides (e.g., SnO2 and ZnO). Although most common metal oxide semiconductors sensitive to gas concentration are n-type semiconductors, there are also a few kinds of p-type semiconductors like NiOx (usually doped with n-type semiconductor like TiO2) that could be used as gas sensor. It is shown that 10% wt. NiOx content is needed to convert n-type conductivity into p-type. The main difference between n-type and p-type NiOx doped TiO2 film is that as temperature increases, the sensitivity of n-type towards reducing gases is increased, while that of the p-type is decreased.15 Therefore p-type semiconductors have relatively lower operating temperatures than n-type ones. Sensors based on metal oxide semiconductors are mainly applied to detect target gases through redox reactions between the target gases and the oxide surface.16

This process includes two steps, (1) redox reactions, during which O− distributed on the surface of the materials would react with molecules of target gases, leading to an electronic variation of the oxide surface; and (2) this variation is transduced into an electrical resistance variation of the sensors.17 The variation in resistance could be detected by measuring the change of capacitance, work function, mass, optical characteristics or reaction energy.14 Metal oxides, such as SnO2, CuO, Cr2O3, V2O5, WO3 and TiO2 can be utilized to detect combustible, reducing or oxidizing gases with sensors which are mainly based on the resistance change responses to the target gases.18 Tin dioxide (SnO2) is the commonly used gas sensing material. It is an n-type granular material whose electrical conductivity is dependent on the density of pre-adsorbed oxygen ions on its surface. The resistance of tin dioxide changes according to the variation in the concentration of gases such as Liquefied Petroleum Gas (LPG), Methane (CH4), Carbon Monoxide (CO) and other reducing gases; while the relationship between resistance and target gas concentration is nonlinear..19 Other metal oxide semiconductors (e.g., tungsten trioxide, WO3) are also widely used for gas sensing. Anodic tungsten oxide applying electrochemical etching of tungsten shows excellent responses towards molecular hydrogen and nitrogen oxide.20 However, the response of pure WO3 to NH3 is rather poor, and because of the interference from NOx, the selectivity of WO3 sensors for NH3 is low. In order to implement WO3 in gas sensing, WO3 should be decorated with copper and vanadium as catalytic additives to improve the response, and the abnormal behavior of sensors should be eliminated.21 Others like titanium dioxide (TiO2) are also used as sensitive layers for their sensitivity in terms of dielectric permittivity to gas adsorption.22 Several influencing factors, such as the characteristics and structure of the sensing layer, affect the redox reactions and thus decide the sensitivity of metal oxides as gas sensing materials. Among all sensors based on metal oxide semiconductors, the sensitivity of SnO2-based ones is relatively high, leading to its greater popularity. However, this high sensitivity


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is mainly based on the high working temperature, which is often realized through a heated filament. For most metal oxide gas sensors, the sensitive layer has to be preheated to an elevated temperature in order to increase the probability of gas molecule adsorption on the layer surface which would consume ions of the sensing materials. As the ions are consumed, the conductivity of the film will increase to realize the sensing function. Besides the heated filament, the micro-hotplate is another choice for keeping the sensing materials at an elevated temperature.23 Apart from the heating methods, there are also other methods like pre-concentration techn-ology that could be applied to improve the sensitivity of gas sensors.24 The main advantage of such methods is that they do not affect the characteristics of sensing materials as in the case of heating. Methods based on change in materials’ characteristics make use of composite materials such as SnO2-ZnO or FeO3-ZnO. Due to synergistic effect between the two components, such sensors often display improved sensitivity. Optimization of the sensitivity can be achieved by changing the proportions of each material in the composite. The working temperature of SnO2-based sensors is from 25°C to 500°C and the best sensing temperatures for various gases are different. This could cause potential selectivity problems in applications, because if the temperature deviates too much from the optimal value, other gas components may be more reactive towards SnO2, leading to poor selectivity. However, if the difference between these two temperatures is large, a single sensor could also be designed to detect two kinds of target gases at the same time. For example, the optimal sensing temperature of SnO2 to CH4 is 400°C while that for CO is 90°C, which requires a thermostatic cycle of the sensitive element at the two temperature values so that both gases could be detected by measuring the resistivity of the sensing element during each gas period.25 The best sensing temperature of another common aggressive gas, Hydrogen Fluoride (HF), is different from the above (380°C), but oxygen is more reactive toward SnO2 than HF if the temperature is higher than 380°C. While the sensitivity of SnO2 gas

sensors could be controlled mainly by temperature, there are methods that could improve the selectivity of the sensing materials such as doping the sensing film surface with a suitable catalyst material18 and the use of sensor arrays consisting of two or more sensing elements that detect different gases. Sensors with multiple elements have a gas recognition circuit to enhance selectivity.26 Methods for improving the selectivity also include the use of catalytic filtering technology for combustible gases27 and the compositional control of sensing materials (like TiO2-SnO2 composite), which also shows good sensitivity.28 Metal oxides for gas sensors Although a variety of inks such as polymers, carbon nanotubes, moisture absorbing mate-rials etc. have been successfully tested for gas sensing applications, conductor metric semi-conducting metal oxide gas sensors have been more thoroughly investigated and widely used in gas sensing applications. Investigations have indicated that the gas sensing process is strongly related to surface reactions, so one of the important parameters of gas sensors, the sensitivity of the metal oxide based materials, will change with the factors influencing the surface reactions, such as chemical components, surface - modification and microstructures of sensing layers, temperature and humidity. Many metal oxides are suitable for detecting combustible, reducing, or oxidizing gases by conductivity measure-ments.20, for example Cr2O3, Mn2O3, Co3O4, NiO, CuO, SrO, In2O3, WO3, TiO2, V2O3, Fe2O3, GeO2, Nb2O5, MoO3, Ta2O5, La2O3, CeO2 and Nd2O3. Due to the wide range of electronic structures of oxides, the metal oxides are divided into two categories, viz. (i) Transition-metal oxides (Fe2O3, NiO, Cr2O3, etc.) and (ii) Non-transition-metal oxides, which include pre-transition-metal oxides (Al2O3, MgO, etc.) and post-transition-metal oxides (ZnO, SnO2, etc.)28. Pre-transition-metal oxides are expected to be quite inert on account of their large band gaps and they lack electrons and holes. They are seldom used as gas sensor materials due to their difficulties in electrical conductivity measurements.14


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Fine metal ferrite particles are being extensively studies on account of their utility in a wide range of applications such as the production of catalysts, ferrofluids, adsorbents and sensors. Iron oxides are used as pigments, semiconductors, recording materials and memory devices. The physical properties such as stability and electronic behavior, e.g. semiconducting property of hematite i.e. α-Fe2O3 allows its use as photocatalyst29, while maghemite i.e. -Fe2O3, is useful as a magnetic recording material.30,31 α-geothite32 i.e. α-FeOOH is the precursor of -Fe2O3 and the synthesis of -Fe2O3 occurs in steps viz. α-FeOOH--α-Fe2O3--Fe3O4---Fe2O3. Preparation of nanoparticulate form of maghemite and alteration of its chemical structure by using doping has opened up new areas of applications and hence received greater attention.33 The ultra fine particles of ferrites are found to alter the electrical, magnetic, electro-optical and chemical properties,34,35 indicating their usefulness in gas-sensing technology. New synthetic routes for the prep-aration of ultra fine -Fe2O3 are being invest-igated, for example, the new combustion route described by Venkataramana et. al. 2001.36 In another novel sol-gel method37, varied amounts of pyrrole monomer were added to a solution containing iron nitrate as precursor and 2-methoxy ethanol as solvent. Matijevic and Scheiner,synthesized hematite particles by dissolving ferric salts in hydrochloric acid and heating at 100°C.38 Green nanotechnology for gas sensing Considerable research into new sensors is underway, including efforts to enhance the performance of traditional devices, such as resistive metal oxide sensors, through nanoengineering. Metal oxide sensors have been utilized for several decades as low cost methods for detecting combustible and toxic gases. However, issues with sensitivity, selectivity, and stability have limited their use, often in favor of more expensive approaches. Recent advances in nanomaterials provide opportunity to dramatically increase the response of these materials, as their performance is directly related to exposed surface area. The recent availability of various metal oxide materials in high-surface-area nanopowder form, as well as implementation of newly developed nanofabrication techniques, offers tremendous

opportunities for sensor manufacturers. Maximization of surface reactions requires a high ratio of surface area to volume of the material. There exists an inverse relationship between surface area and particle size. Owing to the fact that in nanomaterials, the surface atoms become more dominant than the core atoms, all surface dependent properties are enhanced. Hence, nanosized particles are desirable for the application of semiconducting metal oxide as a sensing layer. Rella and co-workers demonstrated good response to NO2 and CO when the SnO2 grain size was controlled below 10 nm.39 Ferroni and co-workers demonstrated good response to NO2 for solid solutions of TiO2 and WO3 when grain size was held at near 60 nm.40 Chung and co-workers demonstrated that increasing the firing temperature (which increases grain size) significantly reduces the response of WO3 sensors to NOx.41 Chiorino and co-workers also demonstrated that firing temperature plays a key role in the response of SnO2 sensors, with films treated to 650°C showing nearly twice the response to NO2 as films treated at 850°C.42 Proper control of grain size is an important challenge for high sensor performance. Thick film devices are heated to high temperatures for densification so that there is interconnectivity between individual grains. However, at very high processing temperatures, substantial grain growth can occur leading to reduced porosity and marked decrease in sensor response. Semiconductor nanoparticles stabilized by capping chemical or polymers may alleviate the problem by restricting the size of grains. However, there are concerns regarding the toxicity of the organic chemicals to various forms of living organisms including humans. Use of biologically derived chemicals and biopolymers as capping agents has been recently highlighted in the nanotechnology literature. Numerous reports describe enhanced surface properties of the nanomaterials capped with biopolymers. A case in point is the enhanced stability and reductive degradation of Acid Black 1 dye using FeS nanoparticles capped by a plant biopolymer.43 Nagpal et al. reported FeS nanoparticles stabilized by a fungal biopolymer for reductive dechlorination of lindane an


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organochlor pesticide.44 Hydroxyapatite (HAp) is widely used in biomedical applications due to its superb bioactive behavior. Moreover, it is used in varied fields like chromatography, catalyst, fuel cell and chemical gas sensor. Mene et al. deposited hydroxyapatite thick films on insulating substrate using screen printing technique.45 Biological synthesis of hydr-oxyapatite nanoparticles could have prospects in the gas sensing technology. Similar approaches using biogenic materials for synthesis and/or stabilization of gas sensing nanoparticles preparations might pave the way for green fabrication of sensors with enhanced capabilities.46-53 Work in our own laboratory has pointed to the potential application of

Biopolymer (BP1) stabilized magnetic iron oxide nanoparticles as gas sensing material. This is exemplified in the table given below (Table 2). Gasses such as NO2, Cl2 and HCl were passed over the thick film sensor and difference in voltage was measured as a response of the sensor. Sensitivity of the sensor film was determined by using the formula (Vg-Va)/Va, where Vg is voltage in gas environment and Va is the voltage without gas. It was found that the use of biopolymer capping agent resulted in enhancement of the sensitivity to 8-fold and 30-fold for NO2 and Cl2, respectively. Further research is being carried out to design sensors with increased sensitivity.

Table 2 : Sensitivity of iron oxide gas sensor with BP1 stabilized (capped) nanoparticles

Gas Operating

temperature (˚C)

Difference in voltage (mV)

Sensitivity (Vg-Va/Va)

Increase in sensitivity

(fold)

Uncapped MagNPs

MagNPs capped

with BP1

Uncapped MagNPs

MagNPs capped

with BP1

NO2 21-24 0.27 5.51 0.182 1.51 8.296 Cl2 24-27 0.24 7.05 0.137 4.147 30.270 HCl 27-30 1.65 2.3 0.882 0.793 0.899

CONCLUSION While addressing the sensitivity and specificity issues in the development of gas sensors, it is also necessary to fabricate devices using green materials. Polymeric materials of natural origin such as those derived from plants and/or microorganisms have the potential of reducing the hazards of toxic chemicals used in sensors. The natural polymers also proffer the advantage of improved sensitivity gas sensing devices.

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Nanoparticle films for gas sensing applications: greener approaches

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