Introduction

Nowadays, advances in industrial processes and technology are changing the human lifestyle, but are also increasing air pollution and its consequences. Therefore, gas sensors have become necessary devices to monitor environmental conditions to control the air quality. Ethanol is a clear, non-toxic, and flammable liquid that is used widely in the medical, food processing, plasticizer, plastics, and cosmetics fields.1,2 Ethanol vapor is slightly lighter than air and is very toxic by inhalation or skin contact. Exposure to excessive concentrations of ethanol can severely harm the human body, as it targets the nervous system and affects brain function. Industrial explosions are another hidden danger of excessive ethanol concentrations. Therefore, the ability to efficiently detect these materials is essential. In recent years, nanostructures have attracted more attention as most of them are multifunctional materials with several applications.3,4,5,6,7,8,9,10 Nanocrystalline semiconductor metal oxides are one group of these nanostructures known as resistance-type gas sensing materials.5,11,12 These are recommended for gas detection due to their low cost, ease of synthesis, good sensitivity, and long-term stability. Recently, spinel ferrites and their composites with different structures, sizes, and morphology have been applied in biosensors, corrosion protection, drug delivery, ceramics, medical diagnostics, microwave absorbers, and transformer cores, and also in gas sensing setups to decrease working temperature and improve selectivity.11,13,14 Among the spinel group, NiFe2O4 is a soft ferrite with an inverse spinel structure that is known as a multicomponent oxide,15 which motivates researchers to employ it in chemical sensing due to multiple oxidation states.16,17

Xu et al. synthesized porous lanthanum ferrites (LaFeO3) under different pH values for ethanol gas sensing.18 Godbole et al. proposed a gas sensor based on MgFe2O4 nanoparticles which showed a rapid response toward alcohol vapors.11 In another work, Tarttelin Hernández et al. studied the gas detection efficiency of a NiFe2O4 sensor prepared by a levitation-jet synthesis (LJS) method.19

According to applied nanostructures in nanosensors, some researchers focus on ferrite-based nanocomposites. Zambare et al. proposed an effective ethanol gas sensor based on the Zn-substituted cobalt ferrite composition,20 and Mhlongo et al. improved ethanol sensing by doping of Cr into In2O3 nanofibers.21 In addition, carbon-based nanocomposites and nanostructures with a large effective surface area are considered in the fabrication of nanosensors for various gas detection applications. Nitrogen-doped carbon dots and carbon nanosheets,22 graphene-based nanosensors,23 and MgCo2O4 nanosheets24 are examples that provide sufficient sensing active sites to improve the sensor’s performance.

In the present study, building upon previous studies, we aim to synthesize nickel ferrite nanoparticles by the hydrothermal method and fabricate a stable and repeatable sensor by applying a simple method that decreases the complexity. The prepared sensor based on NiFe2O4 shows a high response value, sensitivity, and selectivity to ethanol vapor.

Experimental Methods

Synthesis of NiFe2O4 Nanoparticles

All chemicals used herein are of analytical grade. Nickel ferrite nanoparticles were synthesized by the hydrothermal technique and calcination procedure as follows:

First, 3 mmol Fe(NO3)3·9H2O and 6 mmol Ni(NO3)2·6H2O were dissolved in 60 mL deionized water and stirred for 20 min at room temperature. Next, 3 M sodium hydroxide solution was added dropwise to the homogeneous reaction solution until the pH value was equal to 11. The above mixture was transferred to a 100 mL stainless steel autoclave and heated in an oven at 160°C for 12 h. After it was cooled to room temperature, the precipitate was separated and washed several times with deionized water, and then the obtained precipitate was dried at 80°C for 12 h. Finally, the as-prepared powder was calcined at 500°C for 2 h with a heating rate of 5°C/min. The synthesis procedure is shown in Fig. 1.

Fig. 1
figure 1

The schematic diagram of the synthesis procedure.

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Characterization

The crystal structure of NiFe-NP was determined by x-ray diffraction (XRD) using the D8 Advance diffractometer (Bruker, Germany) equipped with Cu-Kα radiation (λ = 0.1542 nm). The morphology was investigated through field-emission scanning electron microscopy (FESEM, TESCAN) working at an accelerating voltage of 150 kV, and the elemental composition of the synthesized sample was determined by energy-dispersive x-ray spectroscopy (EDX). Brunauer–Emmett–Teller (BET) analysis was performed using a Belsorp-mini II (MicrotracBEL Corporation) system to estimate specific area and pore size by nitrogen adsorption–desorption isotherm analysis. Vibrating-sample magnetometry (VSM) was performed to investigate the magnetic properties of the sample at room temperature using a MDKB VSM setup with an applied field up to 15 kOe. To reveal the bandgap energy of the synthesized nanoparticles, the UV–Vis spectra were obtained by a UNICO S-2150 spectrophotometer at room temperature. The Fourier transform infrared (FTIR) spectra were recorded using a PerkinElmer FTIR (Spectrum RXl) instrument.

Sensor Fabrication and Gas Sensing

The sensor fabrication procedure is described step by step as follows:

First, a thin film of nickel was deposited on a 1.5 \(\times \hspace{0.17em}\)1.5 cm2 glass slide via the thermal evaporation method. Then, nickel ferrite nanoparticles were mixed with deionized water and homogenized using the sonication process. After that, the corresponding suspension was coated over the prepared substrate by the drop-casting method using a microsyringe. Finally, the sensor was heated over a hot plate for 10 min at 60°C to evaporate water. All sensing measurements were carried out by adjusting the sample in an 8.3 L test chamber containing two microheaters to evaporate the liquid analyte and control the sensor operating temperature. The relative humidity of the media was equal to 20%. For sensor response measurements, a given amount of the test analyte was inserted into the test chamber using a microsyringe and evaporated very rapidly. The schematic diagram of the test measurement used for testing the fabricated sensor is depicted in Fig. 2.

Fig. 2
figure 2

The schematic diagram of the test measurement setup.

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The sensor responses were calculated by using Eq. 1:

$$ {\text{Response}}(\% ) = (R_{{\text{g}}} - R_{{\text{a}}} )/R_{{\text{a}}} \times 100 $$
(1)

Here, the air is considered as the reference gas; Ra is the sensor’s electrical resistance in air and Rg represents the electrical resistance in the presence of the test gas. The electrical resistance was recorded by a digital multimeter (APPA-505). The response time is defined as the time taken by the sensor to reach 90% of the maximum response value.25

Results and Discussion

Structural Characterization

The XRD pattern (Fig. 3) identified the crystallinity, purity, and single-phase nickel ferrite that was formed in an inverse cubic spinel structure. The corresponding peaks were well matched to JCPDS card No. 74-2081 data.

Fig. 3
figure 3

XRD patterns of NiFe2O4 nanoparticles.

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The experimental lattice constant (a), crystallite size (D), and x-ray density (ρ = \(\frac{8M}{{{\text{Na}}^{3} }}\)) were determined by considering the angle (θ) and full width at half maximum (β) of the special peak (311), using Bragg’s law and the Scherrer formula \(\left( {\frac{0.89\lambda }{{\beta \cos \theta }}} \right)\).15 The calculated values of lattice parameter, crystallite size, and x-ray density for NiFe2O4 nanoparticles annealed at 500°C are 0.836 nm, 18.2 nm, and 5.33 g/cm3, respectively.

The FESEM images illustrate the morphology of the obtained sample. As shown in Fig. 4, the NiFe2O4 nanoparticles have an average diameter of about 65–70 nm. Furthermore, the EDX analysis result verified the high purity of the synthesized sample and the existence of nickel, iron, and oxygen with expected values.

Fig. 4
figure 4

(a) FESEM image, (b) EDX analysis of NiFe2O4 nanoparticles (Color figure online).

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The FTIR spectrum of NiFe2O4 nanoparticles is in the wave number range of 400–1000 cm−1. The formation of nickel ferrite nanoparticles in inverse spinel structure was further identified by the FTIR spectrum. (Fig. 5 shows the FTIR spectra in the wave number range from 400 cm−1 to 1000 cm−1). Two main peaks at 418 cm−1 and 598 cm−1 correspond to the metal-oxygen band.

Fig. 5
figure 5

FTIR analysis of synthesized NiFe2O4 nanoparticles.

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The band appearing at 418 cm−1 is assigned to octahedral metal stretching vibrations (Fe3+-O and Ni2+-O) and the strong absorption band at 598 cm−1 corresponds to the intrinsic stretching vibration of the metal at the tetrahedral site Fe ↔ O.26,27

Figure 6 illustrates the UV–Vis spectrum of NiFe2O4 nanoparticles. Here, with respect to the Tauc relation, the bandgap in the direct transition is calculated using Eq. 2 as follows:

$$ (\alpha hv)^{2} = (h\nu - E_{{\text{g}}} ) $$
(2)

where α is the absorption coefficient, h is the Planck constant, ν is the frequency of light, and Eg is the bandgap of the sample.28 The bandgap can be estimated using the (αhν)2 versus (hν) plot by extrapolating the straight-line region of the curves to the horizontal axis. The obtained optical bandgap value is equal to 2.58 eV.

Fig. 6
figure 6

The plot of (αhν)2 versus the photon energy (hν).

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Figure 7 shows N2 adsorption–desorption isotherms for annealed nickel ferrite nanoparticles. The curve confirms that the sample belongs to type V based on the IUPAC definition and the hysteresis loop related to the mesoporous feature in the nanoparticle structure. The specific area, average pore diameter, and total pore volume were calculated using the BET method as 39.75 m2g−1, 20.72 nm, and 0.21 cm3g−1, respectively.

Fig. 7
figure 7

N2 adsorption–desorption isotherms of nickel ferrite nanoparticles.

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The M-H loop obtained from VSM analysis at room temperature (300 K) is shown in Fig. 8. The magnetic hysteresis curve presents the soft ferromagnetic nature of the NiFe2O4 nanoparticles. The saturation magnetization (Ms), remanence magnetization (Mr), and coercivity (Hc) are 24.6 emu g−1, 6.2 emu g−1, and 225 Oe, respectively, as evaluated using Fig. 8.

Fig. 8
figure 8

Magnetization curve at room temperature.

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Gas Sensing Properties

In order to study the gas sensing properties, the obtained nickel ferrite nanoparticles were applied to the gas sensor structure. First, the sensor is examined at different temperatures in the range of 170–280°C to determine the OT as an important parameter in the sensing process, reaction kinetics, and temperature dependency behavior. As shown in Fig. 9, the sensor has a maximum response value (195%) at 230°C and 100 ppm ethanol concentration. The sensor presents increasing and decreasing behavior around the OT. Below the OT, the target gas molecules do not have enough energy to activate and overcome the energy barrier, but by increasing the temperature to the OT, the molecules gain adequate energy, leading to adsorption of more oxygen species and response enhancement. However, by increasing the operating temperature from OT, the adsorbed gas molecules may escape without reaction. To investigate the dynamic response of nickel ferrite nanoparticle-based sensors toward ethanol, the ethanol concentration was varied from 25 ppm to 500 ppm.

Fig. 9
figure 9

(a) The response of the sensor to 100 ppm ethanol at different temperatures, (b) the response of the sensor in different ethanol concentrations, (c) the response-recovery time of the sensor, and (d) the response of sensors at their optimum temperature to three cycles of 100 ppm ethanol and air.

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As expected, an increasing trend was recorded for the response values by increasing the ethanol vapor concentration, exposing the sensor to more ethanol molecules. The sensor’s response at 230°C to different ethanol concentrations is presented in Fig. 9. The response and recovery time to 100 ppm ethanol are 73 s and 75 s, respectively, as shown in Fig. 9.

Figure 9d shows the response values of the sensor upon exposure to 100 ppm ethanol concentration and air for three cycles. In each cycle, after recovery and return to the initial value, we injected 100 ppm ethanol again for the next time measurement. This confirms that the present sensor is repeatable and has successful operation in ethanol sensing.

To evaluate the limit of detection (LOD), the sensor was tested from 7 ppm to 18 ppm ethanol at 230°C operating temperature, which is shown in Fig. 10. The fabricated sensor had a low order of detection (7 ppm) with a 15% response. Moreover, it demonstrated a linear response in this range of ethanol concentration.

Fig. 10
figure 10

The response of the sensor to different concentrations (inset: determination the limit of detection).

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Figure 11 shows the stability of the sensor within 30 days, which is an important parameter of gas sensors. The long-term stability of the fabricated sensor was studied on the test setup when the temperature of the microheater was optimal, starting with 100 ppm ethanol until it reached a maximum and then allowed to recover and returned to the initial state. The results reveal that the sensor has long-term stability over the test period.

Fig. 11
figure 11

Response stability over a period of 30 days upon exposure to 100 ppm ethanol.

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To examine the ability of the prepared sensor to distinguish different volatile organic compounds (VOCs), the sensor was tested at 230°C and 100 ppm ethanol (C2H6O), acetone ((CH3)2CO), methanol (CH3OH), toluene (C7H8), formaldehyde (CH2O) and dichloromethane (CH2Cl2). The sensor shows the highest response to ethanol (195%), based on the results compiled in Fig. 12. The selectivity of the prepared sensor for ethanol is obvious, whereas the response shows significant differences for other analytes. Therefore, the sensor is considered an ethanol sensor with an acceptable response.

Fig. 12
figure 12

Response of the sensor to different analytes with 100 ppm concentration.

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Here, the resistance reveals an increase in the presence of ethanol, which means that the NiFe2O4 nanoparticles have p-type semiconductor properties. The gas sensing performance of the prepared sensor is investigated concerning the resistance change in the ethanol gas medium. This resistance variation is demonstrated by the surface charge model by considering the number of absorbed oxygen and adsorption sites on the surface of nickel ferrite nanoparticles which simplifies the reactions. In the air atmosphere, oxygen molecules are adsorbed onto the nanoparticle surfaces, leading to the ionization and formation of active oxygen species (O2−, O−, and O2−) by capturing electrons.29,30 As the optimum operating temperature of the fabricated sensor is 230°C, the chemical reaction between electrons and adsorbed oxygen occurs as follows:

$$ {\text{O}}_{{2({\text{gas}})}} \leftrightarrow {\text{O}}_{{2({\text{ads}})}} $$
(3)
$$ {\text{O}}_{{2({\text{ads}})}}^{ - } + e^{ - } \leftrightarrow 2{\text{O}}_{{({\text{ads}})}}^{ - } \quad (100^\circ {\text{C}} \prec T \prec 300^\circ {\text{C}}) $$
(4)

Therefore, the electrical resistance of the p-type semiconductor decreases due to an increase in the number of holes accumulated in a thick layer near the ferrite surfaces. Once the sensor is exposed to a reducing gas atmosphere such as ethanol, the oxygen reacts with it, resulting in ionization via the following equation11:

$$ {\text{C}}_{2} {\text{H}}_{5} {\text{OH}} + 6{\text{O}}_{{\left( {{\text{ads}}} \right)}}^{ - } \xrightarrow{{{\text{NiFe}}_{2} {\text{O}}_{4} {\text{surface}}}}2{\text{CO}}_{2} \uparrow + 3{\text{H}}_{2} {\text{O}} \uparrow + 6e^{ - } $$
(5)

Thus, the electrons produced are re-injected into the semiconductor, transferred to the conduction band, and recombined with holes in the NiFe2O4 nanostructure, which leads to an increase in the electrical resistance, as presented schematically in Fig. 13.

Fig. 13
figure 13

Gas sensing mechanism in the studied sensor.

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A comparison between this study and previous literature is presented in Table I. It can be found that our low-cost sensor which is made of uniform nickel ferrite nanoparticle thin film exhibits good sensitivity and stability at a moderate operating temperature.

Table I Ethanol sensing results based on different nanostructures
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Conclusions

In summary, the NiFe2O4 nanoparticle-based sensor is fabricated through a simple procedure by using a hydrothermal synthesis route. Characteristics (structural, magnetic, UV–Vis, and FTIR spectra) of the nanoparticles are discussed in detail. Based on measurements in different conditions, the sensor exhibited the highest response value (195%) to 100 ppm ethanol at 230°C. It demonstrated high sensitivity, and is capable of detecting low concentrations of ethanol (7 ppm). The response and recovery time of the sensor are 73 s and 75 s, respectively. It is worth mentioning that there is a condition in the response and recovery process because of the test setup design, whereas the fabricated sensor was tested in a real situation with the presence of air in the chamber; otherwise it could obtain much better results when the chamber was purged before the start and by using an inert gas such as argon in the chamber for the recovery routine. Although the response and recovery times may seem relatively high compared to some other investigations, this sensor is completely reliable for detecting ethanol in actual circumstances. In addition, the sensor shows a good response value and stability, which are the main characteristics of this device. The sensor was repeatable and can be completely recovered by exposing it to the air at its operating temperature.