Talanta 78 (2009) 1136–1140 Contents lists available at ScienceDirect Talanta journal homepage: www.elsevier.com/locate/talanta Sol–gel synthesized semiconducting LaCo 0.8 Fe 0.2 O 3 -based powder for thick film NH 3 gas sensor G.N. Chaudhari a,∗ , S.V. Jagtap a , N.N. Gedam a , M.J. Pawar a , V.S. Sangawar b a Nano Technology Research Laboratory, Department of Chemistry, Shri Shivaji Science College, Amravati 444602, M.S., India b P.G. Department of Physics, G.V.I.S.H., Amravati 444604, M.S., India article info Article history: Received 12 October 2008 Received in revised form 15 January 2009 Accepted 16 January 2009 Available online 24 January 2009 Keywords: LaCo 0.8 Fe 0.2 O 3 Electrical and sensing properties NH 3 sensor Selectivity abstract Perovskite type LaCo x Fe 1−x O 3 nanoparticles was synthesized by a sol–gel citrate method. The structural, electrical and sensing characteristics of the LaCo x Fe 1−x O 3 system were investigated. The structural char- acteristics were performed by using X-ray diffraction (XRD) and transmission electron microscopy (TEM) to examine the phase and morphology of the resultant powder. The XRD pattern shows nanocrystalline solid solution of LaCo x Fe 1−x O 3 with perovskite phase. Electrical properties of synthesized nanoparticles are studied by DC conductivity measurement. The sensor shows high response towards ammonia gas in spite of other reducing gases when x = 0.8. The effect of 0.3 wt.% Pd-doped LaCo 0.8 Fe 0.2 O 3 on the response and a recovery time was also addressed. © 2009 Published by Elsevier B.V. 1. Introduction There is a need for ammonia (NH 3 ) sensors in many situations including leak-detection in air-conditioning systems [1], environ- mental sensing of trace amounts ambient NH 3 in air [2], breath analysis for medical diagnoses [3], animal housing [2], and more. Generally, because it is toxic, it is required to be able to sense low levels (∼ppm)ofNH 3 , but it should also be sensitive too much higher levels. NH 3 gas can be quiet corrosive, often causing NH 3 sensors to have short lifetimes. The gas-sensing mechanism of metal oxide materials is based on the reaction between the adsorbed oxygen on the surface of the materials and the gas molecules to be detected. The state and the amount of oxygen on the surface of materials are strongly dependent on the microstructure of the materials, namely, specific area, particle size, as well as the film thickness of the sens- ing film. In order to obtain gas sensors with good performance, the recent research works [4–6] were devoted to nano-materials because they have high specific area and contain more grain boundaries. Some well-known materials for NH 3 gas sensing are ZnO [7], modified-ZnO (viz. Fe–ZnO and Ru–ZnO) [8,9], indium oxide [10], molybdenum oxide [11], polyaniline [12–14], polypyrrole [15],Au and MoO 3 -modified WO 3 [16,17], Pt- and SiO 2 -doped SnO 2 [18], etc. Various ammonia sensors reported basically work at higher ∗ Corresponding author. E-mail address: nano.d@rediffmail.com (G.N. Chaudhari). temperature such as 350 ◦ C, but it is not convenient to work at such high temperature while sensing. There has been much interest in perovskite-structured com- pounds (general formula ABO 3 ) because of their unique catalytic action [19] and gas-sensing properties [20–24]. Their sensitive and selective characteristics can be controlled by selecting suitable chemical dopants. In recent studies undoped and doped LaFeO 3 perovskite powder is reported to have good sensitivity to CO [21], ethanol [25] and H 2 S gas [26]. We did some further studies by replace Fe 3+ with Co 2+ . The present work was undertaken to investigate the gas-sensing behav- ior of nanosized LaCo x Fe 1−x O 3 thick films prepared by a sol–gel citrate method. The aim of the present work is to study the evolution of structural and electrical properties of LaCo x Fe 1−x O 3 nanocrys- talline powder in order to understand the sensor response to NH 3 gas sensor based on effect of Co doping in LaFeO 3 . These studies show that in perovskite structure different B-site cations exhibit significant variation in sensing properties. The structural charac- teristic of the material was studied by using X-ray diffraction (XRD) and transmission electron microscopy (TEM). 2. Experimental details 2.1. Preparation of LaCo x Fe 1−x O 3 LaCo x Fe 1−x O 3 nanocrystalline powders used in this work were synthesized by a sol–gel citrate method. The stoichiometric amount of starting material, such as lanthanum nitrate hexahydrate [La(NO 3 ) 2 ·6H 2 O], cobalt nitrate hexahydrate [Co(NO 3 ) 2 ·6H 2 O], 0039-9140/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.talanta.2009.01.030 G.N. Chaudhari et al. / Talanta 78 (2009) 1136–1140 1137 ferric nitrate polyhydrate [Fe(NO 3 ) 2 ·9H 2 O], and citric acid were weighed and dissolved in ethylene glycol. The mixture was mag- netically stirred at 80 ◦ C for 2 h to get homogeneous mixture. Then this mixture was heated at 130 ◦ C for 12 h in a closed vessel to form gel precursor. The resulting material was further heated for 3 h at 350 ◦ C and subsequently calcined at 650 ◦ C for 6 h to obtain the nanoparticles. Pd was incorporated in the nanoparticles by the process of impregnation. Appropriate quantities of PdCl 2 and samples were dissolved in deionized water. This mixture was vigorously stirred and slowly dried on a water bath. The dried compound was ground to a fine powder and calcined at 200 ◦ Cfor 1 h to decompose the chloride. 2.2. Characterization techniques The synthesized samples were characterized for their struc- ture and morphology by X-ray powder diffraction (XRD; Siemens D5000) and transmission electron microscopy (TEM; Hitachi-800). The X-ray diffraction data were recorded by using Cu K␣ radia- tion (1.5406 A ◦ ). The intensity data were collected over a 2Â range of 10–70 ◦ . The average crystallite size of the samples was esti- mated with the help of Scherrer equation using the diffraction broadening of all prominent lines. The DC characteristics of the LaCo x Fe 1−x O 3 was studied in the range of 30–350 ◦ C with a step of 5 ◦ C. 2.3. Sensor fabrication The LaCo x Fe 1−x O 3 powders prepared were mixed with a suit- able amount of adhesive and then ground into paste. After that the paste was packed into a ceramic tube on which two electrodes had been installed at each end. The ceramic tube was about 10 mm in length, 2 mm in external diameter, and 1.7 mm in internal diameter. In order to improve their stability and repeatability, the gas sensors were calcined at 400 ◦ C for 2 h. The gas-sensing properties were measured in a temperature range of 50–350 ◦ C. The resistance of a sensor was measured in air and in a sample gas. Fresh air was used as a carrier gas. The gas sensitivity (S) is defined as the ratio of the change of resistance in presence of gas (R g ) to that in air (R a ) [27]. For the p-type semiconducting material following formula is used for sen- sitivity determination. S = R g − R a R a = R R a (1) The gas-sensing properties were studied for reducing gases such as hydrogen sulphides (H 2 S), ammonia (NH 3 ), liquefied petroleum gas (LPG), carbon monoxide (CO), and ethanol (C 2 H 5 OH), whose concentration were fixed at 3000 ppm in air. 2.4. Pellet fabrication In order to measure the electrical properties of the compound, the pellet was fabricated as: the crystalline powder was crushed into fine powder using a pestle and mortar. A quantity of 2 g pow- der was taken with 0.05 ml (25 mg) of an organic binder, poly vinyl alcohol (PVA) and then it was again ground to mix PVA uni- formly in the powder. This powder was then used to make a dense pellet with a die of diameter 10 mm. A hand press machine was used to apply a pressure of about 8 tones on the die. The pellets prepared were then fired at 250 ◦ C for 1 h to remove the organic binder. Silver paint was applied at two different points on the same surface of a pellet from which contacts were drawn with very thin flexible copper wires and then it was used for electrical studies. 3. Results and discussion 3.1. X-ray diffraction Fig. 1 shows the X-ray diffraction patterns of (a) LaCo 0.8 Fe 0.2 O 3 and (b) LaFeO 3 , calcined at 650 ◦ C. Definite line broadening of the diffraction peaks is an indication that the synthesized materials are in nanometer range. This is an advantage for improving the gas-sensing properties for oxide semiconductor materials. The pat- terns indicate that both samples have an orthorhombic distorted perovskite structure. The average particle size D was estimated by means of Scherrer formula through measuring the half-peak widths of the diffraction lines. The obtained D values were about 30 nm and 40 nm for LaCo 0.8 Fe 0.2 O 3 and LaFeO 3 , respectively, showing that the partial substitution of Co 2+ for Fe 3+ in the LaFeO 3 lattice. The lattice parameters calculated were also in accordance with the reported value. 3.2. Transmission electron microscopy The morphology of the powder sample has b een observed by TEM. Fig. 2 shows the TEM micrograph of the LaCo 0.8 Fe 0.2 O 3 pow- Fig. 1. X-ray diffraction patterns of (a) LaCo 0.8 Fe 0.2 O 3 and (b) LaFeO 3 , calcined at 650 ◦ C. Fig. 2. TEM image of LaCo 0.8 Fe 0.2 O 3 calcined at 650 ◦ C. 1138 G.N. Chaudhari et al. / Talanta 78 (2009) 1136–1140 Fig. 3. Electrical conductivity of (a) LaFeO 3 and (b) LaCo 0.8 Fe 0.2 O 3 in air. der with uniform grain size distribution having a small tendency of agglomerates formation. Due to the formation of polycrystalline material particle size are formed in the range of 30–40 nm. 3.3. DC conductivity Fig. 3 shows the log versus 1/T Arrhenius plot for the LaFeO 3 and LaCo 0.8 Fe 0.2 O 3 nanoparticles. As seen from the figure, the con- ductivity of the sample increased with an increase in temperature, following a linear dependence, as expected for a typical semicon- ducting material. A linear relationship of both materials could be due no change in semiconducting properties, except enhancement of conductivity. The activation energy of LaCo 0.8 Fe 0.2 O 3 calculated from the reciprocal of slope was about 0.5 eV. 3.4. Resistivity Fig. 4 shows the resistance behavior against temperature of (a) LaFeO 3 and (b) LaCo 0.8 Fe 0.2 O 3 , respectively. It is very interesting to note that the resistance changes very sufficiently with temperature up to 200 ◦ C and beyond this temperature the change is small. Due to incorporation of Co in LaFeO 3 (LaCo 0.8 Fe 0.2 O 3 ), the resistance is considerably reduced. This reduction in resistivity may be due to rise in Fermi level by the interaction with Co to LaFeO 3 . 3.5. Gas-sensing characteristics Fig. 5 shows the NH 3 gas response of undoped LaFeO 3, calcined at 650 ◦ C for various operating temperature. In general at low tem- perature, the gas response is restricted by the speed of chemical Fig. 4. Resistance behavior against temperature of (a) LaFeO 3 and (b) LaCo 0.8 Fe 0.2 O 3 . Fig. 5. Gas-sensing characteristics of undoped LaFeO 3 for NH 3 gas as a function of operating temperature. reaction, and at higher temperature, it is restricted by the speed of diffusion of gas molecules. At some intermediate temperature, the speed value of the two processes becomes equal and at that point the sensor response reaches to its maximum. Here, the response increases with increase in operating temperature to attain maxi- mumat300 ◦ C for NH 3 , and then decrease with further increase in operating temperature. The samples with the composition LaCo x Fe 1−x O 3 (x = 0.2, 0.4, 0.6 and 0.8) were studied as a NH 3 gas sensor. It was found that response mainly depends upon operating temperature and Co con- tent. Fig. 6 shows the gas response for different amount of x (x = 0.2, 0.4, 0.6 and 0.8) at various operating temperatures. From figure it is seen that there is increase in response for LaCo 0.8 Fe 0.2 O 3 to maximum value at an operating temperature 260 ◦ C , whereas the other samples show lower response to NH 3 gas. Compared with LaFeO 3 , LaCo 0.8 Fe 0.2 O 3 showed the large response to NH 3 gas. The reason may be that the partial replacement of Fe 3+ ions by Co 2+ ions at the B-sites is advantageous to adsorption and oxidization for NH 3 gas. Studies showed that LaFeO 3 is a p-type semiconduc- tor, its charge carries are holes (h). When Fe 3+ in LaFeO 3 is replaced by Co 2+ , the carrier’s concentration will depend on the holes pro- duced by ionization of [Co Fex ]. In this formula, Co Fex mean the point defect, which is produced when Co 2+ occupies the sites of Fe 3+ in the crystal. Upon the addition of Co 2+ , holes will be generated based on this equation. So the concentration of ‘h’ increases, which results in the conductivity of Co-doping samples is considerably higher than that of LaFeO 3 . Fig. 6. Response to NH 3 gas of 200 ppm of LaCo x Fe 1−x O 3 doped with different amount of Co as a function of operating temperature (X = 0.2, 0.4, 0.6, 0.8). G.N. Chaudhari et al. / Talanta 78 (2009) 1136–1140 1139 Fig. 7. Gas-sensing characteristics of LaCo 0.8 Fe 0.2 O 3 for various reducing gases. Iron ion are very stable in the +3 valence state, where as Co ions tend to stabilize in the +2 valence state, thus influenc- ing the stability range of the A 3+ B 3+ O 3 perovskite structure. It seems that, they are relatively stable under the reducing atmo- sphere, when Fe and Co ions exist in trivalent and divalent state, respectively [28]. Selectivity is the ability that a gas sensor to distinguishes between different kinds of gases. Fig. 7 shows the selectivity of LaCo 0.8 Fe 0.2 O 3 sensor to various test gases such as NH 3 , LPG, H 2 S, CO and ethanol. The sensor shows high degree ofselectivity towards NH 3 gas than other reducing gases. In order to modify and/or control the surface properties of the LaCo 0.8 Fe 0.2 O 3 , introduction of Pd additives is usually performed. The most important effects of noble metal addition are the increase of the maximum sensitivity and the rate of response, as well as the lowering of the temperature of maximum sensitivity. All these effects arise as a consequence of the promoting catalytic activ- ity when loading with noble metals. A great amount of additives have been studied being Pd and Pt the most used. Here Pd was incorporated into the LaCo 0.8 Fe 0.2 O 3 by adopting the chemical wet method. Fig. 8 shows the response of LaCo 0.8 Fe 0.2 O 3 for ammo- nia gas as a function of different wt.% Pd-doped. 0.3 wt.% Pd-doped LaCo 0.8 Fe 0.2 O 3 shows higher response to ammonia gas as compare to other. Fig. 9 shows the response of 0.3 wt.% Pd-doped LaCo 0.8 Fe 0.2 O 3 as a function of operating temperature. Incorporation of Pd is seen to improve the response of the sensor to NH 3 gas and also reduce Fig. 8. Effect of Pd doping of LaCo 0.8 Fe 0.2 O 3 on the response to NH 3 gasof200ppm. Fig. 9. Response of 0.3 wt.% Pd-doped LaCo 0.8 Fe 0.2 O 3 as a function of operating tem- perature. Fig. 10. Response of 0.3 wt.% Pd-doped LaCo 0.8 Fe 0.2 O 3 to NH 3 of different Concen- tration (in ppm). the operating temperature. The sensor reaches its maximum value of response at an operating temperature 200 ◦ C. The variation of gas response of 0.3 wt.% Pd-doped LaCo 0.8 Fe 0.2 O 3 sample with NH 3 gas concentration at 200 ◦ C is shown in Fig. 10. Fig. 10 shows that the response values were observed to increases continuously with increasing the gas con- centration up to 200 ppm at an operating temperature 200 ◦ C. The rate of increase in response was relatively larger up to 200 ppm, Fig. 11. Response characteristics of (a) 0.3 wt.% Pd-doped LaCo 0.8 Fe 0.2 O 3 and (b) LaCo 0.8 Fe 0.2 O 3 . 114 0 G.N. Chaudhari et al. / Talanta 78 (2009) 1136–1140 but smaller during 200–1000 ppm. Thus the active region of the sensors would be up to 200 ppm. Response and recovery times, define d as the time reaching 90% of final signal, are the important parameters of the gas sensor. Fig. 11 shows the response characteristics for the LaCo 0.8 Fe 0.2 O 3 at 260 ◦ C and 0.3 wt.% Pd-doped LaCo 0.8 Fe 0.2 O 3 to 200 ppm NH 3 gasat200 ◦ C. The 0.3 wt.% Pd-doped LaCo 0.8 Fe 0.2 O 3 sensor exhib- ited shorter response and recovery times (8 and 20 s, respectively) than LaCo 0.8 Fe 0.2 O 3 . From this observation we can infer that the 0.3 wt.% Pd-doped LaCo 0.8 Fe 0.2 O 3 responds more efficiently than LaCo 0.8 Fe 0.2 O 3 . 4. Conclusion In the present investigation we have presented the method of synthesis and a systematic improvement in the sensing properties of LaFeO 3 using two significant strategies, namely surface modifica- tion and doping with cobalt on B-site of the perovskite. 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Solid State Chem. 118 (1995) 117–124. . homepage: www.elsevier.com/locate/talanta Sol–gel synthesized semiconducting LaCo 0.8 Fe 0.2 O 3 -based powder for thick film NH 3 gas sensor G.N. Chaudhari a,∗ ,. a fine powder and calcined at 200 ◦ Cfor 1 h to decompose the chloride. 2.2. Characterization techniques The synthesized samples were characterized for their