Chlorine gas sensing performance of palladium doped nickel ferrite thin films

Journal of Magnetism and Magnetic Materials 405 (2016) 219–224

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Current perspectives

Chlorine gas sensing performance of palladium doped nickel ferrite thin films Pratibha Rao, R.V. Godbole, Sunita Bhagwat n Department of Physics, Abasaheb Garware College, Karve Road, Pune 411 004, India

art ic l e i nf o

a b s t r a c t

Article history: Received 28 September 2015 Received in revised form 17 December 2015 Accepted 19 December 2015 Available online 21 December 2015

NiFe2O4 and Pd:NiFe2O4 (Pd¼ 1 w/o, 3 w/o and 5 w/o) thin films, p-type semiconducting oxides with an inverse spinel structure have been used as a gas sensor to detect chlorine. These films were prepared by spray pyrolysis technique and XRD was used to confirm the structure. The surface morphology was studied using SEM. Magnetization measurements were carried out at room temperature using SQUID VSM, which shows ferrimagnetic behavior of the samples. The reduction in optimum operating temperature and enhancement in response was observed on Pd-incorporation in nickel ferrite thin films. Faster response and recovery characteristic is observed Pd-incorporated nickel ferrite thin films. The long-term stability is evaluated over a period of six months. This feature may be regarded as a significant facet towards their practical application as gas sensors. & 2015 Elsevier B.V. All rights reserved.

Keywords: Thin films Spray pyrolysis Magnetic properties Gas sensing

1. Introduction Now-a-days, it has become necessary to monitor toxic and harmful gases such as CO, NOx, SOx, Cl2 and H2S at lower level of concentration. Human exposures to toxic levels of chlorine are generally accidental as the release of high levels of chlorine is virtually always unintentional. Chlorine plays a major role in the most serious environmental problems which we face today; depletion of the ozone layer, global warming and acid rain. The pollution caused by its widespread use has been linked to a variety of serious health effects; poisonings have occurred in the chlorine industry since its inception and chlorine compounds have accumulated in the body fat of animals and humans. Chlorine inhalation toxicity can occur during routine attendance at swimming pools, and in higher-level exposures at swimming pools when accidents occur with systems used for water purification [1,2] during military exposures, following transportation accidents, upon industrial exposure, with misuse of domestic cleaners, and, more recently, as a result of chemical terrorism. The forms of chlorine involved in respiratory toxicity are not limited to chlorine gas, but also can include hypochlorous acid, chlorine dioxide, and chloramine. Chlorine exposure can result in injury to the eyes, skin and upper airways as well. The airway is especially affected from the nose to the level of the bronchi [3]. Repeated exposure to chlorine in the pool has been postulated to n

Corresponding author. E-mail address: [email protected] (S. Bhagwat).

http://dx.doi.org/10.1016/j.jmmm.2015.12.065 0304-8853/& 2015 Elsevier B.V. All rights reserved.

be a significant risk factor for an excess of asthma among swimmers [4]. So there is tremendous need of chlorine gas sensor which can detect the gas at very low level of concentration. Sensors based on semiconducting oxide like SnO2, ZnO2 and WO3 have been widely studied as gas sensors, however selectivity remains the main challenge for such materials. Hence, there is always a search of new gas sensor material. Apart from magnetic and electronic uses now ferrites are the subject of interest for gas and humidity sensing applications [5,6]. Nickel ferrite nanoparticles are reported as gas sensor for various gases like H2S, Cl2, LPG whereas nickel ferrite thin films as gas sensors are not explored much. Thin film has advantage like high surface area, fast recovery, lower energy input, device compatibility, miniaturization and overall cost effectiveness. It has been shown that the gas sensing properties can be much improved by doping the ferrites with noble metals [7,8]. In the present work, we have deposited NiFe2O4 and Pd-doped NiFe2O4 thin films using spray pyrolysis technique. The effect of Pd doping in NiFe2O4 thin films for chlorine gas sensing properties is investigated.

2. Experimental 2.1. Synthesis of NiFe2O4 and Pd:NiFe2O4 thin films NiFe2O4 and Pd:NiFe2O4 (1, 3 and 5 w/o) thin films were deposited using automated spray pyrolysis system. 0.15 M aqueous ethanol solutions of NiCl2 and FeCl3 (mole ratio 1: 2) were used as

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the precursor for the deposition of NiFe2O4 thin films. Aqueous solution of PdCl2 (1, 3 and 5 w/o) was added to this solution to obtain Pd:NiFe2O4 thin films. These films were deposited on both Si (100) and alumina substrates. Si wafer was cleaned with HF:DI (1:20) and then washed thoroughly with DI water to remove the native oxide layer and any contamination in the oxide from the wafer surface. The alumina was cleaned using soap solution, and then washed with distilled water. Both the substrates were then ultrasonically cleaned with distilled water prior to deposition. The solution was sprayed by a spray gun and the resulting mist was deposited on to the Si (100) and alumina by compressed air at a flow rate of 15–17 lpm. The nozzle-substrate distance was kept fixed at 30 cm and the substrate temperature was maintained at 350 °C and during deposition. After deposition, the coated substrates were allowed to naturally cool down to room temperature. Deposited thin films were then air annealed at 650 °C for 3 h. The crystal structure of the nickel ferrite thin films deposited on Si (100) was investigated using Bruker AXS D8 diffractometer, with CuKα radiation. The surface morphology of all the samples was examined using JEOL, JSM 6360 A scanning electron microscope (SEM). Thickness of the ferrite films was measured using Talystep Profilometer and they were found to be about 3 mm for all the films. The magnetic properties of ferrite thin films deposited on Si (100) were investigated using LOT-Quantum Design MPMS SQUID VSM. 2.2. Gas sensitivity measurements The gas-sensing characteristics of NiFe2O4 and Pd:NiFe2O4 thin films deposited on alumina were studied using static setup. The gas response was measured after providing the ohmic contacts to the films using silver paste. The sensor material was kept in a steel chamber to perform sensitivity measurements towards Cl2 gas. The gas sensing characteristics at different temperatures were recorded using a Keithley 2400 source meter. Stability of thin films and response-recovery characteristics were also studied.

3. Results and discussion 3.1. Structural studies All the NiFe2O4 and Pd:NiFe2O4 thin films were analyzed using X–ray diffraction (XRD) technique. For XRD analysis, films deposited on Si (100) were used. The XRD patterns (Fig. 1) show

Table 1 Parameters calculated from XRD. Sample

Crystallite size from XRD (nm)

NiFe2O4 thin film 1 w/o Pd:NiFe2O4 thin film 3 w/o Pd:NiFe2O4 thin film 5 w/o Pd:NiFe2O4 thin film

46 38 35 30

Lattice constant (Å)

8.303 8.288 8.285 8.277

single cubic spinel phase of the NiFe2O4 according to JCPDS card # 74–2081 for all the samples. The additional phase PdO is observed with 3 w/o and 5 w/o Pd doping in NiFe2O4. A small contribution of SiO2 is observed which was formed during the deposition. Scherrer’s formula, given by Eq. (1) [9] was used to calculate the average crystallite size, t, which are tabulated in Table 1.

t=

0.9λ β cos θ b

(1)

where β is the angular line width at half maximum intensity and θb is the Bragg angle for the actual peak. XRD shows the variation in crystallite size between 30 and 46 nm. The incorporation of Pd show remarkable decrease in crystallite size which in turn increases the surface area. It is an established fact that the grain growth depends upon the grain boundary mobility [10]. A plausible reason for the decreasing trend of crystallite size (Table 1) is that the increasing concentration of Pd reduces the grain growth probably due to segregation on or near the grain boundaries which hampers its movement. The lattice constants of these films were calculated using indexing method [11] given by Eq. (2),

λ2 sin2 θ = = constant 2 N 4a

(2)

where N ¼n2(h2 þk2 þl2). These lattice constants are also tabulated in Table 1 which shows reduction with increase in Pd concentration. Pd2 þ (r ¼0.86 Å) ions enter into the tetrahedral lattice sites (A) of the spinel lattice. With the entering of Pd2 þ ions some of the Fe3 þ (r ¼0.67 Å) ions transform to Fe2 þ (r ¼ 0.83 Å) ions for maintaining the charge neutrality. An increase in the population of Pd2 þ cations and a decrease in the Fe3 þ cations in the A site contributes to the decrease in radius of tetrahedral site while an increase in the population of Fe2 þ cations in the A site increases the radius of tetrahedral site. Also to relax the strain some of the Fe2 þ ions will be migrated to the octahedral site. Because of all these changes the change in the lattice constant is observed. 3.2. Morphological studies

Fig. 1. XRD spectra of NiFe2O4 and Pd:NiFe2O4 thin films.

The spray deposited films had very good adherence to the substrates. SEM images of all the thin films deposited on Si (100) are shown in Fig. 2. It is observed that NiFe2O4 thin film exhibits petal like structure whereas Pd:NiFe2O4 thin films posses agglomerates of the petals. EDAX of Pd:NiFe2O4 thin films confirm the presence of small amount of Pd along with Ni, Fe and O are shown in Fig. 3. Various elemental ratio calculated from EDAX are tabulated in Table 2. It is confirmed that Pd/Ni ratio increases with increase in doping concentration of Pd. This increase in Pd concentration on the surface results in catalytic effect which in turn increases the sensitivity towards Cl2 gas (discussed later in Section 3.3).

P. Rao et al. / Journal of Magnetism and Magnetic Materials 405 (2016) 219–224

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Fig. 2. SEM of NiFe2O4 and Pd:NiFe2O4 thin films.

Fig. 3. EDAX of NiFe2O4 and Pd:NiFe2O4 thin films.

Fig. 4. SQUID VSM of NiFe2O4 and Pd:NiFe2O4 thin films.

Table 2 Parameters calculated from EDAX. Sample

Pd/Ni

Ni/O

Ni/Fe

NiFe2O4 thin film 1 w/o Pd–doped NiFe2O4 thin film 3 w/o Pd–doped NiFe2O4 thin film 5 w/o Pd–doped NiFe2O4 thin film

0.000 0.054 0.148 0.213

0.089 0.077 0.105 0.074

0.324 0.263 0.205 0.275

3.3. VSM studies Fig. 4 shows variation in magnetization as a function of magnetic field for all the ferrite thin films deposited on Si (100)

Table 3 Parameters calculated from SQUID VSM. Sample

NiFe2O4 thin film 1 w/o Pd:NiFe2O4 thin film 3 w/o Pd:NiFe2O4 thin film 5 w/o Pd:NiFe2O4 thin film

Saturation Magnetization Ms (emu/cc)

Remanence MR (emu/cc)

Coercivity Hc (Oe)

15.8 46.2

1.1 10.0

76.5 151.4

28.2

6.2

165.4

21.6

5.5

194.2

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substrate and air annealed. Hysteresis curves for these films were recorded at 300 K between 10 kOe and þ10 kOe. The saturation magnetization (Ms), remanence (MR) and coercivity (Hc) of all the ferrite thin films are tabulated in Table 3. Since Pd2 þ ion is paramagnetic and when occupies tetrahedral site (A-site), the transformation of Fe3 þ ions into Fe2 þ ions takes place. Due to this the magnetization of A-site decreases. Effectively, the appreciable increase in magnetization (B–A) is observed in 1 w/o Pd:NiFe2O4 thin film, which is consistent with XRD result. Agglomeration of 1 w/o Pd:NiFe2O4 particles (Fig. 2) could be responsible for an enhancement in saturation magnetization (Table 3). It is interesting to mention that further increase in Pd content (3 and 5 w/o), the decrease in magnetization is observed which could be attributed to the decrease in particle size as mentioned in Table 1 and clearly seen in SEM image (Fig. 2). Also with 3 and 5 w/o concentration of Pd, Pd2 þ ions tried to occupy B-site which in turn shifts Ni2 þ ions towards A–site. Ni2 þ ions are magnetic and increase the magnetization of A-site. This contributes towards lowering the magnetization (B – A) of 3 w/o and 5 w/o Pd-doped NiFe2O4 thin films. From Table 3, it is seen that coercivity increases with the increase of Pd concentration. Further it is confirmed that coercivity increases with decrease in particle size (Table 1). Coercivity depends on microstructure of the film. Smaller the grain size, more the number of grain boundaries. These boundaries act as pinning sites for domain walls which are considered to be responsible for high value of coercivity. Similar observation is reported by Sankpal et al. [12]. 3.4. Gas sensing characteristics 3.4.1. Sensitivity It is well known that depending upon the morphology and operating temperatures; the oxide surface hold various oxygen species, such as O  , O−2 , O2  . Their number and distribution also plays an important role in the gas sensing characteristics [13]. The adsorption and ionization of oxygen from air can be expressed as O2 (air)2O2 (ads) O2 (ads) þe  2O2- (ads) O2  (ads) þe  2O2  (ads) When a semiconductor sensor is exposed to a gas, the change in resistance is mainly due to the reaction between the gas and the oxygen species adsorbed on the surface of the semiconductor. The adsorption of gas, which depends on both the type of test gas and the sensor material, may affect the response characteristic. Better response would be expected if a large amount of gas is adsorbed and subsequently the reaction between the adsorbed gas and oxygen species is more favorable. Thin film has advantage like high surface area, fast recovery, lower energy input, device compatibility, miniaturization and overall cost effectiveness. The extent of adsorbed oxygen ions and existence of their different chemical forms (O  , O−2 , O2  etc.) on the sensor surface are controlled by the sensor operating temperature. Ferrites mostly sense gases at higher temperatures. The effect of doping Pd in NiFe2O4 thin films for gas sensing characteristic is investigated for Cl2 gas. The sensitivity of the ferrite thin films are calculated when they are exposed to 5 ppm of Cl2 gas. NiFe2O4 thin films are p-type semiconducting oxides as all films respond to Cl2 gas (oxidizing gas) by decrease in the electrical resistance. The basic mechanism of sensing in NiFe2O4 thin films to any oxidizing gas can be explained as follows [14]. At the operating temperature, when chlorine or any oxidizing gas comes in contact with this semiconductor surface, the gas gets reduced and draws electrons from

the semiconductor. This increases the hole concentration in the semiconductor, which are the majority carriers, and thereby the conductivity. Cl2 þ2e–- 2Cl– Cl– þ2O2  - ClO2 Since electrons are extracted the number of majority charge carriers (holes) in the material increases and hence electrical resistance decreases. The similar response was observed by Reddy et al. when NiFe2O4 was prepared using co-precipitation method to study its sensitivity towards Cl2 gas sensor [14]. The gas response or sensitivity (S) [15,16] for a given test gas is calculated using Eq. (3):

s=

|R a − R g | Rg

(3)

where Ra and Rg are the electrical resistances of the sensor in air and in test gas respectively. The results obtained during the gas-sensing measurements of the NiFe2O4, 1 w/o, 3 w/o and 5 w/o Pd:NiFe2O4 thin films towards Cl2 gas are shown in the Fig. 5. The measurements of I–V characteristics were performed for 5 ppm of Cl2 gas at different fixed temperatures between 200 °C to 400 °C with a step of 25 °C. The two-probe electrodes were made using silver paste on the films. The films were placed inside the chamber and exposed to the gas. Change in resistance was recorded using Keithley 2400 source meter. NiFe2O4 thin film shows optimum response at 375 °C, while on doping with Pd the maximum sensitivity shifts towards lower temperatures; more precisely, to 325 °C for 5 w/o Pd doping. Much higher sensitivity (S ¼6.91) is observed at lower temperature for 5 w/o Pd:NiFe2O4 thin film as compared to NiFe2O4 (S¼ 0.59), 1 w/ o Pd:NiFe2O4 (S¼ 0.7) and 3 w/o Pd:NiFe2O4 (S ¼5.41) thin films. This shift in sensitivity towards the lower operating temperatures on Pd incorporation may be originating due to the well known catalytic activity of palladium and increase in surface area due to decrease in crystallite size. Such a shift can be attributed to the process of availability of the free electrons at lower operating temperature resulting dissociative adsorption of oxygen on the surface results in a greater degree of electron withdrawal from the film at a lower temperature than the pure metal oxide film. Similar observations were reported by Darshane et al. [8] in case of Pd:MgFe2O4 nanoparticles. Also Pd atoms present on the surface follows a chemical mechanism that enhances the rate of

Fig. 5. Sensitivity of NiFe2O4 and Pd:NiFe2O4 thin films for Cl2 gas at 5 ppm.

P. Rao et al. / Journal of Magnetism and Magnetic Materials 405 (2016) 219–224

Fig. 6. Gas sensing characteristics of NiFe2O4 and Pd:NiFe2O4 thin films.

dissociation and diffusion of oxygen species on the surface of oxide species. Moreover the formation of PdO increases active oxygen species viz. O  , O2  , O−2 on the sensor surface which in turn helps in increasing the sensitivity [17]. Hence the combined effect of catalytic activity of Pd and PdO, the sensor improves gas sensitivity in between 3 w/o and 5 w/o Pd: NiFe2O4 thin films. Fig. 6 shows maximum sensitivities obtained for all the thin films. Reddy et al. [14] reported the sensitivity about 0.9 towards 1000 ppm of Cl2 gas for Pd doped NiFe2O4 nanoparticles. This sensor was observed sensitive at lower concentrations (less than 10 ppm) but with very little sensitivity. Mahanubhav et al. [18] reported CdIn2O4 thick film Cl2 gas sensor with sensitivity 9.24 at high optimum operating temperature (450 °C). Here for 5 w/o Pd:NiFe2O4 thin film, the sensitivity of 6.91 is observed for 5 ppm of Cl2 gas, which is much higher and comparatively the concentration is very low. Moreover it can sense lower concentration of 1 ppm (S ¼0.55) which is the toxic limit of Cl2 gas. Apparently there seems to be plausible correlation between magnetic behavior (saturation magnetization) and gas sensing property of thin films. It is observed that 1 w/o Pd:NiFe2O4 thin film has highest magnetization and lower gas sensitivity whereas 5 w/o Pd:NiFe2O4 thin film has lower magnetization and highest sensitivity. For higher doping (3 and 5 w/o Pd:NiFe2O4) Pd2 þ ions try to occupy octahedral site (B-site) and hence move towards surface rather than bulk. Due to the catalytic effect of Pd2 þ the adsorption of the gas is enhanced and hence the sensitivity increases. Fig. 7 shows sensitivity of 5 w/o Pd:NiFe2O4 thin film at different ppm level of Cl2 gas. This is very challenging result for future applications and device fabrication. 3.4.2. Response and recovery time The response and recovery characteristics are important criteria for efficient gas sensors. The response time is defined as the time taken by the sensor to reach 90% of the total current/resistance/ voltage change when the sensor is exposed to the gas while recovery time corresponds to the time required for removal of gas. The response and recovery characteristic curve of NiFe2O4 and 5 w/o Pd:NiFe2O4 thin films for 15 ppm of Cl2 gas is shown in Fig. 8 (Ra ¼25.38 MΩ and Rg ¼ 12.05 MΩ; and Ra ¼31.64 MΩ and Rg ¼1.66 MΩ for NiFe2O4 and 5 w/o Pd:NiFe2O4 thin film respectively). The response time and recovery time for NiFe2O4 thin film is found to be 3 s and 28 s while for 5 w/o Pd:NiFe2O4 thin film it is between 2 s and 6 s respectively. The relatively smaller response time and recovery time for 5 w/o Pd:NiFe2O4 thin film suggest that it is a better sensor. Sen et al. [19] reported SnO2/W18O49 thin film as

223

Fig. 7. Sensitivity of 5 w/o Pd:NiFe2O4 thin film for Cl2 gas at different ppm level.

Fig. 8. Response and recovery characteristics of NiFe2O4 and 5 w/o Pd:NiFe2O4 thin films for 15 ppm of Cl2 gas.

Cl2 gas sensor having sensitivity 11, however offers very slow response and recovery time about 4.6 min and 17 min respectively. Higher sensitivity (15.8) was obtained in ZnO-NW:PPy thin film when used as Cl2 gas sensor; and response and recovery time were much longer (55 s and 800 s respectively) [20] than the present studies. 3.4.3. Long term stability of thin films The long-term stability of the metal oxide gas sensor is one of the major features for its practical applications. To address this issue, the stability of NiFe2O4 and Pd:NiFe2O4 thin films were monitored over a period of six months (Fig. 9). It can be observed that the sensitivity is just slightly changed over the entire period of six months indicating that the NiFe2O4 and Pd:NiFe2O4 thin films displays excellent long-term stability.

4. Conclusions Nanocrystalline NiFe2O4 and Pd:NiFe2O4 thin films have been successfully deposited onto Si (100) and alumina using low cost spray pyrolysis deposition technique. These films are post annealed to obtain single phase spinel structure. Petal like morphology and agglomerates are observed in SEM images of these films. The SQUID VSM analyze reveals the ferrimagnetic nature of

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Grants Commission (UGC–42/778/2013(SR)), India and UGC–DAE CSR, Indore to avail VSM facility. Authors are also grateful to UGC– DAE CSR, Indore for providing SQUID VSM facility.

References

Fig. 9. The stability of NiFe2O4 and Pd:NiFe2O4 thin films.

the films. The combined effect of grain size and type of cations is observed in thin films from the VSM study. Cl2 sensing properties of these films suggest that Pd doping in NiFe2O4 thin film increases the sensitivity and decreases the operating temperature due to the catalytic activity of palladium and increase in surface area of the film. Long term stability of thin film gas sensors was confirmed. Response and recovery is faster in Pd:NiFe2O4 thin films as compared to pure NiFe2O4 thin film.

Acknowledgment Authors are grateful to funding agencies Department of Science and Technology (DST–SR/S2CMP–0035/2012), India and University

[1] T.T. Martinez, C. Long, Explosion risk from swimming pool chlorinators and review of chlorine toxicity, J. Toxicol. Clin. Toxicol. 33 (1995) 349–354. [2] R.V. Babu, V. Cardenas, G. Sharma, Acute respiratory distress syndrome from chlorine inhalation during a swimming pool accident: a case report and review of the literature, J. Intensive Care Med. 23 (2008) 275–280. [3] C. Winder, The toxicology of chlorine, Environ. Res. 85 (2001) 105–114. [4] A. Bernard, S. Carbonnelle, X. Dumont, M. Nickmilder, Infant swimming practice, pulmonary epithelium integrity, and the risk of allergic and respiratory diseases later in childhood, Pediatrics 119 (2007) 1095–1103. [5] S. Singh, B.C. Yadav, V.D. Gupta, P.K. Dwivedi, Investigation on effects of surface morphologies on response of LPG sensor based on nanostructured copper ferrite system, Mater. Res. Bull. 47 (2012) 3538–3547. [6] R.K. Kotnala, J. Shah, B. Singh, H. Kishan, S. Singh, S.K. Dhawan, A. Sengupta, Humidity Response of Li–Substituted Magnesium Ferrite, Sens. Actuators: B Chem. 129 (2008) 909–914. [7] Z. Jiao, M. Wu, Z. Qin, J. Gu, M. Lu, Nano zinc ferrite acetic acid gas sensor for smuggled drug detection in southern China, Sens. Transducers Mag. 37 (2003) 24–29. [8] S.L. Darshane, I.S. Mulla, Influence of palladium on gas–sensing performance of magnesium ferrite Nanoparticles, Mater. Chem. Phys. 119 (2010) 319–323. [9] H.P. Klug, L.E. Alexander, X–ray Diffraction Procedures for Polycrystalline and Amorphous Materials, John Wiley, New York, 1974. [10] V. Raghavan, 2010. Materials Science and Engineering: A First Course, 5th ed., PHI Learning Pvt. Ltd., pp. 231. [11] E.C. Subbarao, L.K. Singhal, D. Chakravorty, M.F. Merriam, V. Raghavan, Experiments in Materials Science, Tata Magraw Hill Publishing Co. Ltd., Noida, 1972. [12] A.M. Sankpal, S.S. Suryavanshi, S.V. Kakatkar, G.G. Tengshe, R.S. Patil, N. D. Chaudhari, R.S. Sawant, Magnetization studies on aluminium and chromium substituted Ni–Zn ferrites, J. Magn. Magn. Mater. 186 (1998) 349–356. [13] S.L. Darshane, S.S. Suryavanshi, I.S. Mulla, Nanostructured nickel ferrite: a liquid petroleum gas sensor, Ceram. Int. 35 (2009) 1793–1797. [14] C.V. Gopal Reddy, S.V. Manorama, V.J. Rao, Semiconducting gas sensor for chlorine based on inverse spinel nickel ferrite, Sens. Actuators B: Chem. 55 (1999) 90–95. [15] C.V. Gopal Reddy, S.V. Manorama, V.J. Rao, Preparation and characterization of ferrites as gas sensor materials, J. Mater. Sci. Lett. 19 (2000) 775–778. [16] N. Iftimie, E. Rezlescu, P.D. Popa, N. Rezlescu, On the possibility of the use of a nickel ferrite as a semiconducting gas sensor, J. Optoelectron. Adv. Mater. 7 (2005) 911–914. [17] M. Epifani, J. Arbiol, E. Pellicer, E. Comini, P. Siciliano, G. Faglia, J.R. Morante, Synthesis and gas-sensing properties of Pd-Doped SnO2 nanocrystals. A case study of a general methodology for doping metal oxide nanocrystals, Cryst. Growth Des. 8 (2008) 1774–1778. [18] M.D. Mahanubhav, L.A. Patil, Studies on gas sensing performance of CuOmodified CdIn2O4 thick film resistors, Sens. Actuators B 128 (2007) 186–192. [19] S. Sen, P. Kanitkar, A. Sharma, Growth of SnO2/W18O49 nanowire hierarchical heterostructure and their application as chemical sensor, Sens. Actuators B 147 (2010) 453–460. [20] A. Joshi, D.K. Aswal, S.K. Gupta, J.V. Yakhmi, S.A. Gangal, ZnO-nanowires modified polypyrrole films as highly selective sensitive chlorine sensors, Appl. Phys. Lett. 94 (1–3) (2009) 103115.