Nanostructured CdFe [Sensors & Transducers (Canada)]
(Sensors & Transducers (Canada) Via Acquire Media NewsEdge) Abstract: Semiconductive nanoparticles of CdFe2 O4 were synthesized by a solution combustion technique. This process is a convenient, environment friendly, inexpensive and efficient for the preparation of CdFe2 O4 nanomaterial. The synthesized material is characterized by Thermo gravimetric Differential analysis (TG/DTA), X-ray Diffraction studied (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) techniques. Conductance response of the nanocrystalline CdFe2 O4 thick film is measured by exposing the film to reducing gases like ethanol, acetone, ammonia, H2 S and hydrogen. The sensor exhibited a fast response and a good recovery. The results demonstrated that CdFe2 O4 can be used as a gas-sensing material which has a high sensitivity and good selectivity to ethanol gas at 200 ppm. Copyright © 2012 IFSA.
Keywords: CdFe2 O4, TEM, Ethanol gas sensor, Thick film, Gas response.
(ProQuest: ... denotes formulae omitted.)
Semiconductor metal oxides as gas sensing materials have been extensively studied for a long time due to their advantageous features, such as good sensitivity to the ambient conditions and simplicity in fabrication [1-10]. Nevertheless, there are still some critical limitations to be overcome for the commercial sensors based on particulate or thin-film semiconductor metal oxides, such as limited maximum sensitivity, high working temperatures and lack of long-term stability. Recently, several groups reported the sensors based on semiconductor nanowires and nanoribbons. Ethanol-sensing material has been widely and deeply studied. Conventional ethanol sensors, mostly based on SnO2 , ZnO , T1O2 , and Fe2C>3 , and usually suffering from cross-response to other gases, need a high working temperature, or have low long-term stability, although they have rather high response to ethanol. So the research for new ethanol-sensing materials and developing the properties of conventional ethanol sensing materials has become an active research field. New ethanol sensors are based on ln203 , V205 , and complex oxide [17-21]. Their properties still need further investigation. Ethanol is explosively utilized for beverages, industrial and scientific sectors. Ethanol is a hypnotic  gas having toxic nature. Heavy exposure and/or consumption of alcoholic beverages, particularly by smokers, increase the risk of cancer [23-24] of the upper respiratory and digestive tracks. Alcoholic cirrhosis leads to liver cancer. Amongst the women, the chance of breast cancer increase with alcoholic consumption or exposure. Those working on ethanol synthesis have great chances of being victims of respiratory and digestive track cancer. So there is a great demand and emerging challenges for monitoring ethanol gas at trace level.
In this paper, we have synthesized CdFe204 nanoparticles by novel combustion reaction. One of our aims is to develop a general synthesis method and explore the gas sensing properties of the CdFe204 nanopowder. We found that the process is a convenient, environment friendly, inexpensive and efficient. Furthermore, the CdFe204obtained possesses excellent gas-sensing responses to reducing gas and grain size is about 50-100 run. This discovery could aid in the synthesis of low cost and CdFe204 is outstanding in promoting the sensing properties of C2H5OH in air.
2. Materials and Methods
All the reagents are of analytical grade and are used as received without further purification. Cadmium nitrate [Cd(NOs)26H20], iron nitrate [Fe(N03)26H20], citric acid are purchased from Sigma-Aldrich chemical reagents Co. (USA).
For the present study, polycrystalline CdFe2C>4 powder was prepared by combustion route using Citric acid as fuel. The materials used as precursors were Cadmium nitrate hexahydrate Cd (NÜ3)2 6H2O, Fe ( 3)20 2 Iron nitrate hexahydrate (all these were procured from A.R. Grade of Qualligen) and Citric acid (Nuclear band). Citric acid possesses a high heat of combustion. It is an organic fuel and provides a platform for redox reactions during the course of combustion. Initially the Cadmium nitrates, Iron nitrates and Citric acid are taken in the 1:1:4 stoichiometric amount and dissolved in a 250 ml beaker then slowly string by using glass rod then clear solution was obtained. Solution formed was evaporated on hot plate in temperature range 70 °C to 80 °C gives thick gel. The gel was kept on a hot plate for auto combustion and heated in the temperature range 170 °C to 180 °C. The nanocrystalline CdFe204 powder was formed within 40-50 minutes. And then sintered at about 800 °C for about 4 hours then we got brown colour shining powder of nanocrystalline CdFe204.
2.3. Thick Film Formation
Cadmium ferrites powder was ground in an agate pastel-moter to ensure sufficiently fine particle size. The fine powder was calcined at 800 °C for 24 h in air and re-ground. The thixotropic paste was formulated by mixing the resulting CdFe204 fine powder with a solution of ethyl cellulose (a temporary binder) in a mixture of organic solvents such as butyl carbitol acetate, and turpineol. The ratio of inorganic and organic path was kept as 75:25 in formulating the paste. The paste was then used to prepare thick films. The thixotropic paste was screen printed on a glass substrate in desired patterns. The films prepared were fired at 500 °C for 24 h.
2.4. Characterization Techniques
The as -prepared samples were characterized by TG/DTA thermal analyzer (SDT Q600 V 20.9 Build 20), XRD Philips Analytic X-ray B.V. (PW-3710 Based Model diffraction analysis using Cu-Ka radiation), scanning electron microscope (SEM, JEOL JED 2300) A JEOL JEM-200 CX transmission electron microscope operating at 200 kV analysis.
2.5. Fabrication and Analysis of Gas Sensors
The sensing performance of the sensors was examined using a "static gas-sensing system. There were electrical feeds through the base plate. The heating was constant on the base plate to heat the sample under test up to required operating temperatures. The current passing through the heating element was monitored using a relay with adjustable ON/OFF time intervals. A Cr-Al thermocouple was used to sense the operating temperature of the sensors. The output of the thermocouple was connected to digital temperature indicators. A gas inlet valve was fitted at one port of the base plate. The required gas concentration inside the static system was achieved by injecting a known volume of test gas using a gas-injecting syringe. A constant voltage was applied to the sensors, and current was measured by a digital Pico-ammeter. Air was allowed to pass into the glass dome after every Gases exposure cycle as shown in Fig. 1.
3. Result and Discussion
3.1. TG-DTA Analysis
The TG curve recorded for thermal decomposition of CdFe2Ü4 is shown in Fig. 2. The curve indicates that the slight weight loss in CdFe2Ü4 powder due to little loss of moisture, carbon dioxide and nitrogen gas. The DTA curve of CdFe204 recorded in static air, curve shows that CdFe204 did not decompose, but weight loss was due to dehydrogenation decarboxylation and denization and yield final product at 775 °C. This weight loss and weight gained was very negligible. This weight change was in the range of 0.02 % only. These indicate that the synthesized powder was almost stable from the beginning.
3.2. X-Ray Diffraction Studies
The XRD pattern shown in the Fig. 3. The XRD pattern shows that the product is pure spinel oxide CdFe204 with a cubic structure. The diffraction data are good agreement with JCPD card of CdFe204 (JCPDS No.24-1016) The average crystalline size of CdFe2C>4 spinel powder was estimated with the help of Scherrer's equation t =0.9 / ß cosO [25-28] where t is the thickness of the crystals (in angstroms), is the X-ray wavelength and is the Bragg angle ß is the integral breath that depends on the width of the most predominant peak at 100% intensity =1.54056 A0. The average particle size of nanocrystalline CdFe204 was ~ 48 nm. CdFe204 nanocrystals are more attractive in the field of gas sensing application.
3.3. Particle Size Analysis
Particle size distribution studies have been carried out by using dynamic light scattering techniques (DLS) Via Laser input energy of 632 nm). It was observed that cadmium iron oxide nanoparticles have narrow size distribution within the range of about 30-50 nm (Fig. 4), which are similar to the values calculated from Scherrer equation.
3.4. SEM Analysis
The microstructure of the 800 °C sintered samples can be visualized from scanning electron microscope (SEM) tool. Fig. 5 depicts SEM images of CdFe2Ü4 powder it shown the particle morphology of high resolution the particle are most irregular in shape with a nanosized range of 80-150 nm some particles are found as agglomerations containing very fine particles. It can be observed that CdFe204have uniformed size. It seems that surfaces are smooth, spongy and pores are seen in the micrograph.
3.5. TEM Analysis
Fig. 6 (a, b) show TEM micrographs of the specimens calcined at 800 °C, respectively. It can be seen that calcining temperature has an obvious influence on morphology for samples. The 800 °C sample consists of spheroidic grains with narrow size distribution (60 ± 100 nm), where as 800 °C sample is composed of small grains with various shapes due to the coexistence of multi-phases. According to the result from XRD, however, we consider that the spheroidic particles (~ 50 nm in size) mainly distributed in the left and right sides of Fig. 6(c) belong to the phase of CdO; whereas the aggregates with smaller grain size located in the centre of Fig. 3 (a) is a mixture of CdFe204. The selected area electron diffraction (SAED) pattern Fig. 6 (d) shows the spot type pattern which is indicative of the presence of single crystallite particles. No evidence was found for more than one pattern, suggesting the single phase native of the material.
3.6. Thickness Measurement
The thicknesses of the films were observed to be in the range from 25-35µ . The reproducibility of the film thickness was achieved by maintaining the proper rheology and thixotropy of the paste.
4. Electrical Properties of Sensor
4.1. 1-V Characteristics
Fig. 7 depicts I-V characteristics of CdFe204 films. It is clear from the symmetrical I-V characteristics that the silver contacts on the films were ohmic in nature. The voltage applied was in the range 1-30 V.\
4.2. Electrical Conductivity
The variation of resistivity CdFe204 with temperature is shown in Fig. 8 and it was found that the resistivity of CdFe2Ü4 sample decreases with operating temperature. The decreases in resistivity with increasing temperature could be attributed to negative temperature coefficient of resistance and semiconducting nature of CdFe2C>4. It is observed from Fig. 8 that the electrical resistivity of the CdFe204 films is nearly linear in the temperature range from 50-400 °C in ambient air.
5. Sensing Performance of Sensor
5.1. Gas Response, Selectivity, Response and Recovery Time
The relative response (S) to a target gas is defined as the ratio of the change in conductance of a sample upon exposure of the gas to the original conductance in air, which can be calculated by fallowing equation.
where Ga is the conductance in air, and Gg is the conductance in a sample gas. Specificity or selectivity is defined as the ability of a sensor to respond to a certain gas in the presence of different gases. Response time (RST) is defined as the time requires for a sensor to attain 90 % of the maximum increases in conductance on exposure to the target gas. Recovery time (RCT) is the time taken to get back 90 % of the original conductance in air.
5.2. Sensing Performance of CdFe204 Thick Films
5.2.1. Response of Sensors to Various Gases
The response of CdFe204 sample variation for different gases with operating temperature is represented in Fig. 9. It is clear from the figure that the gas responses goes on increasing and attain to their respective maxima and decreased further with increase in operating temperatures. It is clear from the figure that the CdFe204 shows the largest response to ethanol vapours at 350 °C.
5.2.2. Active Region of Sensor
Fig. 10. depicts the variation of gas response of CdFe204 sample with ethanol vapour concentrations. It is clear from the figure that the gas response goes on increasing linearly with gas concentration up to 200 ppm and saturated beyond it. The rate of increase in gas response was relatively larger up to 200 ppm. The monolayer of the gas molecules formed on the surface could cover the whole surface of the film. The gas molecules from that layer would reach the surface active sites of the film. The excess gas molecules would remain idle and would not reach surface active site of the sensor. So, the gas response at higher concentrations of the gas is not expected to increase further in large extent. Thus, the active region of the sensor would be up to 200 ppm.
5.2.3. Selectivity Factors of CdFe204 for Various Gases
It is observed from Fig. 1 1 . That the CdFe204 sensors give maximum response to ethanol vapours (200 ppm) at 350 °C. The sensors showed highest selectivity for ethanol against all other tested gases Acetone, H2S, H2 and NH3.
5.2.4. Response and Recovery Time
Fig. 12 depicts the response and recovery of CdFe2C>4 sensor are the response was quick (~ 40 s) to 200 ppm of ethanol, while the recovery was fast (~ 50 s) .The quick response may be due to faster oxidation of gas. The negligible quantity of the surface reaction product and its high volatility explains its quick response and fast recovery to its initial chemical status.
The working principle of the thick film semiconducting gas sensors is based on the change of the electronic conductivity of the semiconducting material upon exposure to ethanol vapours. The interaction of ethanol gas molecules with surface of thick film causes the transfer of electrons between semiconducting surface and adsorbents. The atmospheric oxygen molecule O2 is adsorbed on the surface of the thick film. They capture the electrons from conduction band of the thick film material as
It results in decreasing electronic conductivity of the film. The CdFe204 sample was not as per the stoichiometric proportion and all samples were observed to be oxygen-deficient. This deficiency gets reduced duo to adsorption of atmospheric molecular oxygen. This helps to decrease electronic conductivity of the film. Upon exposure, ethanol molecules get oxidized with the adsorbed oxygen ions, by following a series of intermediate stages, producing CO2 and H2O. This results in evolving oxygen as electrically neutral atoms trapping behind the negative charges on the film surface. Upon exposure of ethanol gas, the energy released in decomposition of ethanol molecule would be sufficient for trapped electron to jump increase in the conductivity of the thick film of CdFe204. These generated electrons and donor level in the energy band gap of CdFe204will contribute to increase in conductivity. When ethanol reacts with oxygen a complex series of reactions take place, ultimately converting the ethanol to carbon dioxide and water as fallows
This shows -type conduction mechanism. Thus on oxidation, single molecule of ethanol liberates twelve electrons (1) in conduction band and results in increase in conductivity of the sensors. Increase in operating temperature causes oxidation of large number of ethanol molecules, thus producing very large number of electron resulting conductivity increases to a large extent. This may be the reason why the gas response increases with operating temperature. The thermal energy (temperature) at which the gas response is maximum, is the actual thermal energy needed to activate the material for progress in reaction. However, the response decreases at higher operating temperature, as the oxygen adsorbates are desorbed from the surface of the sensor. Also, at higher temperature, the carrier concentration increases due to intrinsic thermal excitation and the Debye length decreases. This may be one of the reasons for decreased gas response at higher temperature.
From the study, following conclusions can be made for the sensing performance of the sensors.
(I) Nanocrystalline CdFe204has synthesized by self combustion route. This synthesis route may be used for the synthesis of other metal oxide.
(II) The phage formation of the CdFe204is investigated by TG-DTA, XRD, TEM techniques. The synthesized product shows single phage of inverse spinel structure with an average diameter 50-100 nm.
(IV) Among all other additives tested, CdFe2C>4 is outstanding in promoting the ethanol gas sensing mechanism.
(V) CdFe204 to be optimum and showed highest response to ethanol vapours at 350 °C.
(VI) The sensor showed very rapid response (~ 40s) and gives recovery (~ 50s) to ethanol vapours.
(VII) Sensing mechanism of CdFe204was the substitution of lattice oxygen by ethanol gas. Material gains electrons in this substitution.
(VIII) The sensor has good selectivity to ethanol against, Acetone, H2S, NH3 and H2,
The author S. V. Bangale is thankful to director IIT Bombay for providing the TEM facility.
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2012 Copyright ©, International Frequency Sensor Association (IFSA). All rights reserved.
* S. V. BANGALE, R. D. PRAKSHALE, S. R. BAMANE
Metal Oxide Research, Laboratory, Department of Chemistry,
Dr. Patangrao Kadam Mahavidyalaya, S angli, 416416 (M.S.) India
Tel.: 0233-2535993, fax: 0233-2535993
Received: 6 April 2012 /Accepted: 23 November 2012 /Published: 30 November 2012
(c) 2012 International Frequency Sensor Association
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