Influence of Ni²⁺ and Sn⁴⁺ substitution on gas sensing behaviour of zinc ferrite thick films

Abstract

Nanocrystalline ferrite powders of Ni _x Zn_{1 − x + y} Fe_2−2y Sn _y O₄ (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0 and y = 0.1, 0.2) were prepared by oxalate co-precipitation method and characterized by XRD, FT-IR and FE-SEM techniques. The ferrite thick films (FTFs) of all compositions were prepared by screen printing technique and tested for gas sensing behaviour for liquid petroleum gas (LPG), ethanol (C₂H₅OH) and chlorine (Cl₂). For LPG, the sensitivity decreases with an increase in Ni²⁺ up to x = 0.6. It increases slightly for further increment in Ni²⁺. For this gas, the sensitivity is higher for higher concentration of Sn⁴⁺ (y = 0.2) while the optimum temperatures are smaller than that for lower concentration of Sn⁴⁺ (y = 0.1). The response and recovery times increase with the increase in Ni²⁺ for lower concentration of Sn⁴⁺ (y = 0.1), while at higher concentration of Sn⁴⁺ (y = 0.2), there is increase (for Ni²⁺ up to x = 0.6) and decrease (for further increase in Ni²⁺ up to 1.0) in response and recovery times. For ethanol and Cl₂, the sensitivity of Zn FTFs decreases with an increase in Ni²⁺ and increases with an increase in Sn⁴⁺. For these gases, the optimum temperature is found to be higher for higher concentration of Sn⁴⁺. The response and recovery times of Zn FTFs for ethanol and Cl₂ increase with increasing Ni²⁺ and Sn⁴⁺.

Introduction

Soft ferrites are one of the most important materials used in the fabrication of electronic components. They possess good chemical and thermal stability under operating conditions. Nowadays, researchers have focused their attention on gas sensors of soft ferrites. Among the soft ferrites, Ni-Zn ferrites are particularly useful because of their high resistivity, low loss and high saturation magnetization [1]. By substituting divalent, trivalent and tetravalent impurities, the properties of the soft ferrites can be changed. Kadu et al. [2] synthesized Zn-Mn ferrite sensors and tested for LPG, CH₃, CO and C₂H₅OH. They reported that Zn-Mn ferrite sensor is sensitive to ethanol at an operating temperature of 300 °C. They found improved sensitivity, response and reduced operating temperature to 230 °C with the addition of palladium in Zn-Mn ferrite sensors. Vasambekar et al. [3] studied the gas sensing properties of Nd³⁺-substituted zinc ferrites for ethanol, Cl₂ and LPG. They reported that response and recovery decrease with an increase in Nd³⁺ content in zinc ferrite. Rezlescu et al. [4] reported the influence of partial replacement of Mg and Fe ions by Sn⁴⁺ and Mo⁶⁺ ions and studied their structural, electrical and gas sensing properties. These ferrites were more sensitive to acetone than ethanol. Doroftei et al. [5] studied the influence of Sn⁴⁺ and/or Mo⁶⁺ ions on the structure and the humidity sensitivity of Mg ferrite prepared by self-combustion method. The role of Sn⁴⁺ substitution in Mg ferrite is to enhance the humidity sensitivity. The improvement of resistivity, density and permeability with the addition of Sb in Ni-Zn ferrite is reported by Praveena and Srinath [6].

The study of gas sensing properties of ferrites showed very good surface reactivity. The ferrites have temperature-dependent surface morphologies [7–10]. Reddy et al. [7] prepared nanocrystalline nickel ferrite by micro-emulsion and hydrothermal methods and studied its gas sensing properties for LPG. They found that nickel ferrite prepared by the hydrothermal method shows high sensitivity at lower operating temperatures compared to that prepared by mitcelle technique. They also observed the significant reduction in operating temperature and faster response for palladium-substituted nickel ferrite. Gopal Reddy et al. [8] reported the gas sensing properties at lower concentrations of chlorine gas for nickel ferrite. Gadkari et al. [9] studied the gas sensing properties of Sm³⁺-added Mg-Cd ferrites. They reported lower response and recovery times for Sm³⁺-added Mg-Cd ferrites than that for Mg-Cd ferrites. They also observed that response-recovery times of Mg-Cd ferrites decrease with an increase in Cd²⁺ content.

Kamble and Mathe [10] showed that the nickel ferrite thick film (FTF) is an efficient gas sensor for chlorine compared to LPG and NH₃ at room temperature. Gawas et al. [11] prepared Mn_0.3Ni_0.3Zn_0.4Fe₂O₄ powder by autocatalytic thermal decomposition technique. They deposited thick films by the screen printing method. The sensors showed high sensitivity to NH₃ gas at room temperature and also high selectivity at 10 ppm against other toxic gases at higher concentrations. Bangale and Bamane [12] prepared Mg ferrite powders by solution combustion method and thick films by the screen printing method. They showed that these ferrites have high sensitivity and good selectivity to ethanol gas at 30 ppm compared to acetone, H₂S, CO₂ and LPG. By the similar method, zinc FTF was prepared. This nanocrystalline zinc FTF has high sensitivity and good selectivity to Cl₂ at 300 ppm compared to acetone, NH₃, H₂, H₂S and LPG [13]. They also found that zinc FTF has high sensitivity and good selectivity to H₂S gas at 1000 ppm [14].

NiO/TiO₂-added zinc ferrite gas sensor was tested by Arshak and Gaidan [15] for alcohols like ethanol, pentanol, propanol, butanol and hexanol. The sensor showed highest sensitivity to hexanol. It is observed that the addition of NiO/TiO₂ improved the response and recovery times. They also found that the response and recovery times are temperature dependent and decrease with an increase in operating temperature. Rezlescu et al. [16] prepared zinc ferrite by self-combustion method and tested its sensing properties for LPG, C₂H₅OH and CH₃COOH. They found that zinc ferrite is sensitive and selective to ethanol. Xiangfeng and Chenmou [17] prepared MFe₂O₄ (M = Zn, Cd, Mg and Cu) thick films by co-precipitation method and tested their sensing properties for H₂S, CH₃SH and (CH₃)₂S. They reported that CdFe₂O₄ is sensitive, even at 0.01 ppm concentration of CH₃SH.

Current status and future trends on solid-state potentiometric gas sensors have been reported by Pasierb and Rekas [18]. They reported that the potentiometric gas sensors may be used in a variety of areas including domestic applications, agriculture, industries, medicine, automobiles, mines, control of gas emissions, leak and fire. Mobius and Hartung [19] carried further research on potentiometric gas sensors by adding some oxygen sensors in reducing gases. Bahteeva et al. [20], Shalaeva et al. [21], Markov et al. [22] and Kharton et al. [23] synthesized strontium-substituted ferrites. They revealed that the variation in accepter doping level and oxygen vacancy concentration results in structural modifications that develop simultaneously with changes in ionic conductivity.

We reported the gas sensing properties of Y³⁺-added nanocrystallite Mg-Cd ferrite [24] and Ni-Zn ferrite thick films [25] for LPG, ethanol and chlorine. It is clear that thick films show good sensitivity as compared to the bulk material. No literature is found for the gas sensing properties of Sn⁴⁺-substituted Ni-Zn ferrites and Sn⁴⁺-substituted Ni-Zn FTFs. Therefore, it was decided to prepare Ni-Zn-Sn FTFs and study their gas sensing properties for LPG, ethanol and chlorine at various operating temperatures.

Experimental

Preparation of Ni-Zn-Sn ferrite powder

The nanosize ferrite powders of Ni _x Zn_{1 − x + y} Fe_2 − 2y Sn _y O₄ (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0 and y = 0.1, 0.2) system were prepared by oxalate co-precipitation method. The AR-grade ZnSO₄.6H₂O (Thomas Baker), NiSO₄ ^.6H₂O (Molychem), FeSO_4.7H₂O (Thomas Baker) and SnSO₄ (Thomas Baker) were dissolved in double-glass-distilled water in the required proportion. The pH of the solution was maintained at 4.0 by adding conc. H₂SO_4. The mixture was heated at 80 °C for 1 h to optimize the complete ionization of metal sulphates. AR-grade ammonium oxalate solution (Sd Fine-Chem. Ltd) was added with stirring until complete precipitation was obtained. The precipitate was filtered and washed several times with double-glass-distilled water in order to remove sulphate ions. The removal of sulphate ions was confirmed by barium chloride test [9, 24]. The precipitate was dried on a hot plate and pre-sintered at 200 °C for 1 h. The pre-sintered powders were milled in an agate mortar with acetone as a base and finally sintered at 450 °C for 2 h.

Ni-Zn-Sn ferrite thick film preparation

The FTFs of Ni _x Zn_{1 − x + y} Fe_2 − 2y Sn _y O₄ (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0; y = 0.1, 0.2) powders having an area of 10 × 10 mm² were prepared by screen printing technique. Ni-Zn-Sn ferrite powders of different compositions were deposited on a glass substrate. For deposition, the mixture of 90 wt% of Ni-Zn-Sn ferrite powders with an organic binder of 5 wt% ethyl cellulose and 5 wt% 2-(2-butoxyethoxy) ethyl acetate was used. The electrical contacts on both the ends of the thick films were made by using a silver conductive adhesive paste (AlfaAesar). The photograph of Ni-Zn-Sn FTFs (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0; y = 0.1) were shown in Fig. 1.

Fig. 1

Photograph of Ni-Zn-Sn FTFs (x = 0, 0.2, 0.4, 0.6, 0.8,1.0; y = 0.1)

The measurements of FTF thicknesses were carried on surface profiler Ambios Tech. XP-1 and were found in the range of 15–20 μm.

Characterization

The XRD patterns of Ni-Zn-Sn ferrite powder were recorded on (Bruker D2 phaser, USA) desktop X-ray powder diffractometer in the range of 10°–90° (2θ), a step size of 0.02° at 30 kV, 10 mA with CuKα radiation (λ = 1.54184 Å). The FT-IR spectra of ferrite powders were recorded in the range of 300–800 cm⁻¹ using a Perkin Elmer spectrum one spectrometer, USA, by KBr pellet technique. The surface morphology of the FTF was studied with the help of FE-SEM on Hitachi, Model: S-4700, Japan.

Gas sensing

The gas sensing behaviour of Ni _x Zn_{1 − x + y} Fe_2 − 2y Sn _y O₄ (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0 and y = 0.1, 0.2) FTFs was measured by using a lab-made gas sensor unit for the gases like chlorine, ethanol and LPG by using a two-probe method. Ohmic contacts of silver paste were made for the 10 × 10 mm² thick films on glass substrates. A 500-ppm test gas was inserted in the chamber with the help of a syringe. The resistance of the ferrite thick film sensor in air (R _a) and in the presence of various test gases (R _g) of LPG, C₂H₅OH and Cl₂ was calculated by measuring the current flowing through the film at various temperatures.

The sensitivity of all FTF sensor elements was calculated by using relations [9, 24–25].

$$ S\left(\%\right)=\frac{\varDelta R}{R_{\mathrm{a}}}\times 100=\frac{\left|{R}_{\mathrm{a}}-{R}_{\mathrm{g}}\right|}{R_{\mathrm{a}}}\times 100 $$

(1)

Results and discussion

Structural properties

The typical X-ray diffraction patterns of Ni-Zn-Sn (x = 0.8, 1.0; y = 0.1, 0.2) are presented in Fig. 2. The presence of (111), (220), (311), (222), (400), (422), (511), (440) and (533) planes in the figure confirms the formation of a single-phase cubic spinel structure. All the peaks in the diffraction patterns with JCPDS card number-52-0278 are in good agreement. From this figure, it is found that there is no extra peak present in the samples. The typical FE-SEM image of Ni-Zn-Sn (x = 0.6, y = 0.1) FTF is presented in Fig. 3. It is noticed that the average grain size of the samples under investigation lies in the range of 20–60 nm, highly porous nature and located in loosely packed agglomerates. The particle size of the samples decreases with an increase in Sn⁴⁺ content. The typical FT-IR spectrum of Ni-Zn-Sn (x = 0.8, 1.0; y = 0.1, 0.2) ferrite is presented in Fig. 4. The figure shows two major absorption bands near the high-frequency absorption band (υ ₁) in the range of 596 to 568 cm⁻¹ and low-frequency absorption bands (υ ₂) in the range of 481 to 469 cm⁻¹. These are corresponding to intrinsic lattice vibrations of octahedral and tetrahedral sites, respectively [26].

Fig. 2

Typical X-ray diffraction patterns of Ni-Zn-Sn (x = 0.8, 1.0; y = 0.1, 0.2) ferrite

Fig. 3

Typical FE-SEM images of Ni-Zn-Sn (x = 0.6, y = 0.1) FTF

Fig. 4

Typical FT-IR spectrum of Ni-Zn-Sn (x = 0.8, 1.0; y = 0.1, 0.2) ferrite

Gas sensing properties

The gas sensing properties of the Ni-Zn-Sn FTFs were measured by laboratory-made gas sensor unit for LPG, ethanol and chlorine at a concentration of 500 ppm. The variation of sensitivity with operating temperature of Ni-Zn-Sn FTFs for LPG, ethanol and chlorine is presented in Figs. 5, 11 and 15. From these figures, it is found that the sensitivity of each FTF initially increases, reaches to a maximum, and then decreases with an increase in temperature for each test gas. This type of behaviour of sensitivity is attributed to the rate of chemical reaction and diffusion of gas molecules. For maximum sensitivity, the rate of chemical reaction and diffusion of gas molecules becomes equal. The temperature at highest sensitivity is referred to as an optimum operating temperature [24]. Such type of behaviour of the sensitivity of FTFs is reported by several researchers [10, 12–17]. The gas sensing mechanism of FTFs under investigation is depicted by the flow diagram presented in Fig. 6.

Fig. 5

Variation of sensitivity with operating temperature of a Ni _x Zn_{1 − x + y} Fe_2 − 2y Sn _y O₄ (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0; y = 0.1) and b Ni _x Zn_{1 − x + y} Fe_2 − 2y Sn _y O₄ (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0; y = 0.2) for LPG

Fig. 6

The layout of the gas sensor mechanism of FTFs

In sensors, chemisorbed oxygen species like O⁻, O₂ ⁻ and O²⁻ appear on the surface and plays an important role in the gas sensing mechanism [24]. Usually, sensing mechanism depends on the operating temperature. Sensing mechanism of oxygen adsorbed on the surface of FTFs is given as [24, 34]

$$ {\mathrm{O}}_{2\mathrm{gas}}\leftrightarrows {\mathrm{O}}_{2\mathrm{ads}}^{-}\leftrightarrows\ 2{\mathrm{O}}_{\mathrm{ads}}^{-}\leftrightarrows\ {\mathrm{O}}_{\mathrm{ads}}^{2-} $$

(2)

The reaction mechanism for reducing gases on the surface of FTFs with oxygen species is given as [17]

$$ {\mathrm{R}}_{\left(\mathrm{gas}\right)}\leftrightarrows {\mathrm{R}}_{\left(\mathrm{ads}\right)} $$

(3)

$$ {\mathrm{R}}_{\left(\mathrm{ads}\right)}+{\mathrm{O}}_{\left(\mathrm{ads}\right)}^{-}\leftrightarrows\ {RO}_{\left(\mathrm{ads}\right)}+{e}^{-} $$

(4)

When a reducing gas reacts with chemisorbed oxygen, an electron is released back to the conduction band, resulting in changes in the resistance of a sensor.

The variation of grain size and sensitivity with nickel content for Ni-Zn-Sn FTFs is presented in Fig. 7. From this figure, it is noticed that the inverse relation between grain size and sensitivity of Ni-Zn-Sn FTFs expects the sample (x = 0.8, 0.1) for LPG. For higher concentration of Sn⁴⁺ (y = 0.2) content, the sensitivity of Ni-Zn-Sn FTFs becomes higher and grain size becomes lower as compared to lower concentration of Sn⁴⁺ content (y = 0.1).

Fig. 7

Variation of grain size and sensitivity with nickel content for Ni _x Zn_{1 + y − x} Fe_2 − 2ySn_yO₄ (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0; y = 0.1, 0.2)

Ni-Zn-Sn FTFs as LPG sensors

When Ni-Zn-Sn FTFs are exposed to LPG, they react with chemisorbed oxygen species and results in the formation of gaseous species and water vapours by removal of adsorbed oxygen due to the interaction of hydrocarbons (C_nH_2n + 2) of LPG.

The reaction of LPG with chemisorbed oxygen is given as

$$ 2{\mathrm{C}}_n{\mathrm{H}}_{2n+2}+2{\mathrm{O}}_2^{-}\ \leftrightarrows 2{\mathrm{C}}_n{\mathrm{H}}_{2n}\mathrm{O}+2{\mathrm{H}}_2\mathrm{O}+2{e}^{-} $$

(5)

This reaction shows the increased concentration of electrons in the conduction band, resulting in the increased conductance of Ni-Zn-Sn FTFs. This was reflected in increased current after exposure to the gas as expected [24, 27].

From the plot of temperature versus sensitivity presented in Fig. 5a, b, it is observed that the sensitivity and operating temperature of Ni-Zn-Sn FTFs for LPG varies in the range of 65–84 and 154–208 °C, respectively. The variation of sensitivity and optimum temperature of Ni-Zn-Sn FTFs with nickel for LPG is presented in Fig. 8. It is seen that the sensitivity of Ni-Zn-Sn FTFs decreases with increased nickel content (up to x = 0.6) and slightly increases thereafter for further increment in nickel content (for x = 0.8 and 1.0). This change in the sensitivity may be due to the DC electrical resistivity, and activation energy of Ni-Zn-Sn FTFs decreases with increased nickel content (up to x = 0.6) and slightly increases thereafter for further increment in nickel content (for x = 0.8 and 1.0); these results are presented in Fig. S1 (in Supplementary Information). This is also due to the Yafet-Kittel model for (x = 0.8 and 1.0) the sensitivity increases with an increase in nickel content [27, 28]. From this plot, it can be further noticed that the sensitivity of Ni-Zn-Sn FTFs for higher concentration of Sn⁴⁺ (y = 0.2) is higher than that for lower concentration of Sn⁴⁺ (y = 0.1). This is attributed to the decreased grain size in these FTFs with increased Sn⁴⁺ substitution.

Fig. 8

Variation of sensitivity and optimum temperature of Ni-Zn-Sn FTFs with nickel content for LPG

From this figure, it is also noticed that the optimum temperature increases with an increase in nickel content. This is due to the increasing grain size of FTFs with Ni²⁺ content. For Ni-Zn-Sn_0.2 FTFs, optimum temperature is lower than that for Ni-Zn-Sn_0.1 FTFs. It is found that there is an inverse relation between operating temperature and sensitivity in Ni-Zn-Sn FTFs till the Neel’s two-sublattice model is observed (up to x = 06). Thereafter, for the Yafet-Kittel model (x = 0.8 and 1.0), the sensitivity is seen to follow the same trend of increment as that of operating temperature with increased nickel content [28, 29].

The response and recovery characteristics of Ni-Zn-Sn FTFs (x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0; y = 0.1 and 0.2) for LPG are presented in Fig. S2 (a-b) (in Supplementary Information). From Fig. S2, it is found that the response and recovery time lies in the range of 10–28 and 20–37 s, respectively, for lower (y = 0.1) and higher (y = 0.2) concentrations of Sn⁴⁺.

The variation of response and recovery times of Ni-Zn-Sn FTFs (x = 0, 0.2, 0.4, 0.6, 0.8,1.0; y = 0.1 and 0.2) with nickel content for LPG is presented in Fig. 9. From this figure, it is noticed that the response and recovery times for lower concentration of Sn⁴⁺ (y = 0.1) increase with increasing nickel content. However, for higher concentration of Sn⁴⁺ (y = 0.2), the response and recovery times increase with an increase in nickel content (up to x = 0.6, i.e. till the Neel’s two-sublattice is dominant) [28, 29]. Thereafter, there is a decrement in the response and recovery times for further increase in nickel content (x = 0.8 and 1.0, i.e. for the predominance of Yafet-Kittel model) [28, 29] for higher concentration of Sn⁴⁺. It is also observed that the response and recovery times of Ni-Zn-Sn FTFs for higher concentration of Sn⁴⁺ (y = 0.2) are lower than that for lower concentration of Sn⁴⁺ (y = 0.1).

Fig. 9

Variation of response and recovery times of Ni-Zn-Sn FTFs with nickel content for LPG

The Ni-Sn FTF sensor shows short response (10 s) and good recovery time (20 s) for LPG.

Ni-Zn-Sn FTFs as an ethanol sensor

When Ni-Zn-Sn FTFs are exposed to ethanol, the reaction with chemisorbed oxygen species on the surface is given as [30, 31]

$$ {\mathrm{C}}_2{\mathrm{H}}_5{OH}_{\mathrm{gas}}+{\mathrm{O}}_{\mathrm{ads}}^{2-}\to {\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{O}}_{\left(\mathrm{ads}\right)}^{-}+{OH}_{\mathrm{ads}}^{-} $$

(6)

$$ {\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{O}}_{\left(\mathrm{ads}\right)}^{-}\to \left({\mathrm{C}}_2{\mathrm{H}}_5\right){\mathrm{O}}_{\mathrm{ads}}+{\mathrm{O}}_{\mathrm{ads}}^{-}+{\mathrm{e}}^{-} $$

(7)

$$ {\mathrm{C}}_2{\mathrm{H}}_5{OH}_{\mathrm{gas}}+{\mathrm{O}}_{\mathrm{ads}}^{2-}+\mathrm{hole}\to {CO}_2+{\mathrm{H}}_2\mathrm{O}+{\mathrm{V}}_{\mathrm{o}}^{\bullet \bullet } $$

(8)

In these reactions, ethanol is adsorbed by the oxygen species (${\mathrm{O}}_{\left(\mathrm{ads}\right)}^{2-}$) and produces adsorbed gas species (${\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{O}}_{\left(\mathrm{ads}\right)}^{-}$), along with a hydroxide group. Adsorbed gas species further gives gaseous species and an electron. The ethanol gas is exposed to chemisorbed oxygen species and hole to produce carbon dioxide, water molecules and doubly charged oxygen vacancy (${\mathrm{V}}_{\mathrm{o}}^{\bullet \bullet}\Big)$. During the sensing mechanism of gas with the surface of FTFs, electrons enters in the conduction band, leading to decrease the resistance of  Ni-Zn-Sn FTFs [30, 31].

The variation of sensitivity with operating temperature of Ni-Zn-Sn FTFs (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0; y = 0.1, 0.2) for ethanol is presented in Fig. 10a, b. From these figures, it is revealed that the sensitivity and optimum temperature of Ni-Zn-Sn FTFs lies in the range of 55–91 and 152–183 °C, respectively. The variation of sensitivity and optimum temperature of Ni-Zn-Sn FTFs (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0; y = 0.1, 0.2) with a nickel content for ethanol is presented in Fig. 11. From this figure, it is noticed that the sensitivity and the optimum temperature of Ni-Zn-Sn FTFs decrease with an increase in nickel content. This is due to the increase in grain size of the FTFs with an increase in Ni²⁺ content (Fig. 7). It is observed that the Zn-Sn FTF has high sensitivity (91 %) to ethanol. It means that the Zn-Sn FTFs strongly interact with ethanol as compared to other FTFs [32]. The Ni-Zn FTFs with higher concentration of Sn⁴⁺ (y = 0.2) show lower operating temperature and higher sensitivity to ethanol compared to that with a lower concentration of Sn⁴⁺ (y = 0.1). The decrement of operating temperature is due to the decrease in grain size of FTFs (Fig. 7) [24, 33]. Kapse et al. [30] reported that the higher sensitivity and lower operating temperature for Pd (1.0 wt%) doped Ni_0.6Zn_0.4Fe₂O₂ as compared to Ni_0.6Zn_0.4Fe₂O₂ for ethanol.

Fig. 10

Variation of sensitivity with operating temperature of a Ni _x Zn_{1 + y − x} Fe_2 − 2y Sn _y O₄ (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0; y = 0.1) and b Ni _x Zn_{1 + y − x} Fe_2 − 2y Sn _y O₄ (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0; y = 0.2) for ethanol

Fig. 11

Variation of sensitivity and optimum temperature of Ni-Zn-Sn FTFs with nickel content for ethanol

The variation of response and recovery characteristics of Ni-Zn-Sn FTFs for ethanol is presented in Fig. S3 (a-b) (in supplementary Information). From these figures, it is found that the response and recovery times lie in the range of 10–25 and 20–40 s, respectively. The variation of response and recovery times of Ni-Zn-Sn FTFs with a nickel content of ethanol is presented in Fig. 12. The response and recovery times of Ni-Zn-Sn FTFs increase with increasing nickel content and decreases with increasing Sn⁴⁺ content. This is due to the increment and decrement of grain size with substitution of Ni²⁺ and Sn⁴⁺ in FTFs (Fig. 7).

Fig. 12

Variation of response and recovery time of Ni _x Zn_{1 + y − x} Fe_2 − 2y Sn _y O₄ FTFs with nickel content for ethanol

Ni-Zn-Sn FTFs as a chlorine sensor

Exposure of Ni-Zn-Sn FTFs to chlorine results in the formation of chlorides. The reaction of chlorine with the surface of FTFs is given as [10, 34]

$$ {Cl}_2+2{\mathrm{O}}_{\mathrm{surf}}^{2-}\ \leftrightarrows 2{Cl}_{\mathrm{surf}}^{-}+{\mathrm{O}}_2+2{e}^{-} $$

(9)

$$ {Cl}_2+2{\mathrm{O}}_{\mathrm{o}}^{\mathrm{x}}\leftrightarrows 2{Cl}_{\mathrm{o}}^{\prime \prime }+{\mathrm{O}}_2+2{e}^{\prime } $$

(10)

The reactions (9) and (10) represent an electron donation to the conduction band of Ni-Zn-Sn FTF sensor indicating decreased resistance [10].

The variation of sensitivity with operating temperature of Ni-Zn-Sn FTFs for chlorine is as presented in Fig. 13a, b. The sensitivity and operating temperature of Ni-Zn-Sn FTFs lies in the range of 54–85 and 146–202 °C, respectively.

Fig. 13

Variation of sensitivity with operating temperature of a Ni _x Zn_{1 + y − x} Fe_2 − 2y Sn _y O₄ (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0; y = 0.1) and b Ni _x Zn_{1 + y − x} Fe_2 − 2y Sn _y O₄ (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0; y = 0.2) for chlorine

The variation of sensitivity and optimum temperature of Ni-Zn-Sn FTFs with a nickel content for chlorine is presented in Fig. 14. From this figure, it is revealed that the sensitivity of FTFs decreases, whereas optimum temperature increases with increasing nickel content. The increase in optimal temperature and decrease in sensitivity are mainly affected due to smaller surface area and greater grain size of FTFs under investigation (Fig. 7).

Fig. 14

Variations of sensitivity and optimum temperature of Ni _x Zn_{1 + y − x} Fe_2 − 2y Sn _y O₄ (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0; y = 0.1, 0.2) with nickel content for chlorine

From Fig. 14, it is also observed that the sensitivity of each FTFs is higher whereas the optimum temperature of each FTFs becomes lower for higher concentration of Sn⁴⁺ (y = 0.2) in Ni-Zn FTFs than that for lower concentration of Sn⁴⁺ (y = 0.1). It is found that the optimum temperature for Ni-Zn-Sn FTFs (x = 0, 0.2; y = 0.1, 0.2) is the same, but the sensitivity is slightly different. This can be attributed to the lattice constant of Sn⁴⁺-substituted Zn FTFs [35].

Response and recovery characteristics of Ni-Zn-Sn FTFs for chlorine are presented in Fig. S4 (a-b) (in supplementary Information). From these Fig S4, it is seen that the response and recovery times of Ni-Zn-Sn FTFs lie in the range of 10–30 and 20–35 s, respectively. The variation of response and recovery times of Ni-Zn-Sn FTFs with a nickel content for chlorine is presented in Fig. 15. From this figure, it is clear that the response and recovery times of Ni-Zn-Sn FTFs increase with increasing nickel content. The response and recovery times of Ni-Zn-Sn FTFs for higher concentration of Sn⁴⁺ (y = 0.2) are lower than that for lower concentration of Sn⁴⁺ (y = 0.1). This is due to decrement in the grain size for increased concentration of Sn⁴⁺. The Sn⁴⁺-substituted Zn FTF sensor under investigation has a short response (10 s) and good recovery time (15 s) for chlorine compared to other FTFs.

Fig. 15

Variations of response and recovery time with nickel content of Ni-Zn-Sn FTFs (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0; y = 0.1, 0.2) for chlorine

From Figs. 8, 11 and 14, it is clear that the sensitivity of all the ferrites decreases with an increase in Ni²⁺ content for test gases like LPG, ethanol and Cl₂. This is attributed to the increase in grain size of the Ni-Zn-Sn ferrites with the increase in Ni²⁺ content. From these figures, it is also observed that optimum temperature of the FTFs increases with the increase in Ni²⁺ content for LPG and chlorine except ethanol.

The compositional variation of sensitivity of Ni-Sn, Ni_0.6Zn_0.4Sn and Zn-Sn FTFs for test gases is presented in Fig. 16. From this figure, it is confirmed that the sensitivity of Zn-Sn FTF is higher for ethanol gas as compared to the other gases like LPG and ethanol. Response and recovery time of these FTFs is also well as compared to other FTFs (Fig. 12 ). Therefore, Zn-Sn FTFs is highly sensitive as compared to Ni-Sn and Ni_0.6Zn_0.4Sn FTFs.

Fig. 16

Variation of sensitivity of Ni-Zn-Sn FTFs (x = 0, 0.6, 1.0; y = 0.1, 0.2) for LPG, ethanol and chlorine

Conclusions

Nanosize Ni-Zn-Sn ferrite powders were prepared by oxalate co-precipitation method. The FTFs were deposited on a glass substrate by a screen printing technique. The XRD study confirms the formation of cubic spinel structure. The FT-IR spectra confirm the formation of ferrites. From the gas sensing characteristics of the FTFs under investigation, it can be concluded that

• 1. The sensitivity of Ni-Zn-Sn FTFs is in the order of ethanol > Cl₂ > LPG.

• 2. The highest response (91 %) is shown by Zn-Sn FTFs to ethanol.

• 3. The lowest response (61 %) is shown by Ni-Sn FTFs to ethanol.

• 4. The sensitivity of Ni-Zn-Sn FTFs decreases with increases in Ni²⁺ content for ethanol and chlorine. It is attributed to increases in grain size to increase in Ni²⁺ content.

• 5. The sensitivity of Ni-Zn-Sn FTFs for all test gases is higher for higher concentration of Sn⁴⁺ (y = 0.2) in Ni-Zn FTFs than that for lower concentration of Sn⁴⁺ (y = 0.1). The high sensitivity is due to its smaller grain size in Sn⁴⁺ (y = 0.2) compared to that of Sn⁴⁺ (y = 0.1) Ni-Zn FTFs.

• 6. The response and recovery times of Ni-Zn-Sn FTFs increase with increasing Ni²⁺ content, and response and recovery time decreases with increasing Sn⁴⁺ concentrations.

• 7. The optimum temperature of Ni-Sn-Zn FTFs increases with increases in nickel content for LPG and chlorine. The optimum temperature of Ni-Zn-Sn FTFs for all test gases was decreased with the increase in Sn⁴⁺ concentrations.