Facile synthesis of cerium doped Co-Mn-FeO nano ferrites as highly sensitive and fast response humidity sensors at room temperature

2.1 Materials and methods

The raw chemicals such as cobalt nitrate hexahydrate [Co (NO₃)2·6H₂O], manganese-based nitrate dihydrate [Mn (NO₃)₂·9H₂O], ferrous based nitrate nano hydrate [Fe (NO₃)₃·9H₂O], cerium nitrate [Ce (NO₃)₃], glucose [C₆H₁₂O₆] and urea [CO(NH₂)₂] were purchased from Sigma Aldrich chemical company (South India).

2.2 Synthesis of cerium (Ce³⁺) doped cobalt-manganese-iron ferrites

In this work, the solution combustion technique was used to synthesis Co_0.5Mn_0.5Fe_2−xCe_xO₄ (x = 0.0, 0.050 and 0.1) ferrite-based NPs. Throughout the synthesis process we have maintained 1:1 M ratio of ferrites and fuels respectively to obtain highly pure Co_0.5Mn_0.5Fe_2−xCe_xO₄ nanoparticles powder.

During synthesis, the urea and glucose acts as reducing agents whereas oxidizing agent is metal nitrate salt. The two oxidizing and reducing agents were taken in appropriate stoichiometric ratios in 500 mL round shaped conical flask. The mixture was stirred for 2 h on a magnetic stirrer at 2000 rpm to attain homogeneous dispersion. The resulting solution of ferrite was shifted to a muffle furnace and heated at a temperature of 500 °C for 2 h. This nano ferrite solution allowed to boil at high temperature to remove the poisonous gases, the resulting dried mass is used as final product (Chethan et al., 2019). The detailed mechanism of synthesis techniques of Co_0.5Mn_0.5Fe_2−xCe_xO₄ ferrite is shown in the Fig. 1. The final product of nano ferrite was then transferred to a agate mortar and well grinded to obtain a fine ferrite nano powder. This Ce³⁺ doped cobalt-manganese-iron oxide samples were used to perform structural, morphological, and humidity sensing studies. To measure humidity sensing parameters, doped ferrite pellets of dimension 8 mm × 1.5 mm were used to test the humidity sensors applications.

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Fig. 1

Flowchart representing the synthesis of Co_0.5Mn_0.5Fe_2−xCe_xO₄ (x = 0.0, 0.050, and 0.1) ferrite.

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2.3 Materials characterization

The surface morphology of Co_0.5Mn_0.5Fe_2−xCe_xO₄ (x = 0.0, 0.050 and 0.1) ferrite was analysed by using scanning electron microscope (SEM) (Ultra Scan 60: Japan). X-ray diffraction (XR Ultra Dynamics-400, UK) in the region 2θ = 20° to 80° (Cu Kα, λ = 1.5418 Å) was used to determine the crystal structure of synthesized samples. Various chemical and functional groups present in the obtained nano ferrites was analysed through Fourier Transform Infrared Spectroscopy (FTIR) (Thermo Nicolet, Avatar 370 − India). The optical properties in terms of optical absorption for the synthesized ferrites was investigated using UV–visible spectrometry (Perkin Elmer-Canada) in the wavelength range of 200 nm to 800 nm. The experimental setup used to measure the humidity sensing measurement is illustrated in the Fig. 5(a). Pellet samples of 8 mm × 1.5 mm dimension were prepared applying 5 tons of hydraulic pressure to achieve uniform surface over the pellets surface. The synthesized pellets were sintered at 600 °C for the duration of 5 h. To establish good level of electrical contact, the silver paste is coated onto sintered pellets. After the post coated pellets were kept in between the two probes humidity sensing chamber using rubber cork and opposite side of the electrode was linked to the programmable computer with advanced digital multimeter (Model: HIOKI) to record the change in sensors resistance as a function of applied humidity.

3.1 SEM analysis

The exterior surface morphologies of the prepared Co_0.5Mn_0.5Fe_2−xCe_xO₄ (x = 0.0, 0.05 and 0.1) ferrite NPs were investigated by SEM and depicted in Fig. 2(a–c). The SEM micrographs indicate a porous structure with flaky morphologies for the ferrites (Pratibha et al., 2020a,b). The larges pores observed in the structure results due to the elimination of gases during the process of combustion. A smooth and homogeneous morphology was observed for the undoped ferrite sample (x = 0.0). The morphological features are affected by the concentrations of Ce³⁺ in the cobalt-manganese-iron oxide ferrites. Among all the samples prepared, Co_0.5Mn_0.5Fe_2−xCe_xO₄ (x = 0.1) shows highly porous morphology, such kind of porous morphology acts as active sites with improved active surface area and facilitates adsorption of water vapours at active sites. The major enhancement in the pore dimension and pore volume due to the doping of cerium (Ce³⁺) NPs in Co_0.5Mn_0.5Fe_2−xO₄ ferrite (which is proof via the BET analysis recorded in Table 1).

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Fig. 2

SEM images of Co_0.5Mn_0.5Fe_2−xCe_xO₄ (a) x = 0.0 (b) x = 0.050 (c) x = 0.1 ferrite.

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3.2 XRD analysis

The powder XRD method was used to investigate the structural features of the prepared Co_0.5Mn_0.5Fe_2−xCe_xO₄ (x = 0.0, 0.05 and 0.1) samples and the XRD patterns are depicted in Fig. 3. The XRD spectra of the ferrite sample shows prominent peaks corresponds to the crystal planes (1 0 0) (1 1 0) (1 1 1) (2 0 0) (2 1 0) (3 0 0) (3 0 1) (3 1 0) and (3 1 1) and shows mono phase cubic crystal structure (JCPDS: Card number: 21–1085). The creation of Fd3m ionic space lattice was confirmed through the XRD patterns. The ionic radius of Ce³⁺ was found to be (1.05 Å) which is well associated with the radius of Fe³⁺ (0.64 Å). The Ce³⁺ doping into the ferrite samples does not affect the crystal structure of the host ferrites. To determine the crystallite size, Scherrer’s equation (Sunilkumar et al., 2019a,b) which is in well agreement with the grain size obtained through SEM micrographs. $$D=\frac{k\lambda }{\beta \cos \theta }$$ Here, D represents the crystallite size, k is the parameter depends on the geometry of the sample and is approximately 0.9 for circular particles, where λ is the X-Rays wavelength used, where β is the intensity of FWHM, and θ is the angle of diffraction. The XRD-analysis reveals that, the crystallite size is approximately 40 nm. It was noticed that, lattice parameter and lattice volume parameters show small changes with increasing concentration of Ce³⁺ doping. The spectra obtained by XRD indicates a marginal change in the lattice parameter of Co_0.5Mn_0.5Fe_2−xCe_xO₄ ferrite, which results due to the cationic modification in the crystal structure.

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Fig. 3

Powder XRD patterns of Co_0.5Mn_0.5Fe_2−xCe_xO₄ (x = 0.0, x = 0.050 and x = 0.1) ferrite.

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It is noticed that, the mean crystallite dimensions were reduced considerable with increased concentrations of Ce³⁺ owing to lattice distortion in their normal sites. The total volume of the synthesized ferrites samples was determined by employing the equation $V=a^3$ and all determined values are recorded in the Table 2. The strain developed in the synthesized ferrites samples are calculated by using the equation $\epsilon =\frac{\beta \cos \theta }{4}$ . The hoping length pertain to magnetic properties such as L_a and L_b are calculated by using the equations $L_A=\frac{a\sqrt{3}}{4}$ and $L_B=\frac{a\sqrt{2}}{4}$ respectively. The hoping lengths are more useful parameters in the estimation of A-site and B-site magnetic ions.

3.3 FTIR analysis

The FTIR spectra of Co_0.5Mn_0.5Fe_2−xCe_xO₄ (x = 0.0, 0.05 and 0.1) ferrite is shown in the Fig. 4(d). The characteristic peaks corresponding to 716 cm⁻¹ and 528 cm⁻¹ stretching frequencies is attributed to the presence of Fe-Mn-O and Co-O bonds related to tetrahedral and octahedral structures of the ferrite. Peaks correspond stretching frequencies of 446 cm⁻¹ and 870 cm⁻¹ are attributed to the symmetrical lattice vibrations of the cerium, manganese, and cobalt NPs due to their localized sites (Sunilkumar et al., 2019a,b). The characteristic peak near to the stretching frequencies of 2958 cm⁻¹ is attributed to the O-H lattice vibration that arise because of absorbed water vapours in the ferrite. Further, the characteristic peaks correspond to frequencies 1644, 1525 and 1357 cm⁻¹ arise due to the vibrations of cobalt-manganese-iron oxide ferrites (Li et al., 2016). Hence, the FT-IR studies confirm the formation of cerium (Ce³⁺) doped Co_0.5Mn_0.5Fe_2−xCe_xO₄ ferrite.

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Fig. 4

(a–c) UV–Vis spectra and (d) FT-IR spectra for Co_0.5Mn_0.5Fe_2−xCe_xO₄ (x = 0.0, x = 0.050 and x = 0.1) ferrite.

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3.4 UV–visible spectroscopic analysis

The UV–visible spectra of Co_0.5Mn_0.5Fe_2−xCe_xO₄ (x = 0.0, 0.05 and 0.1) NPs investigated in the wavelength regime 200 nm to 800 nm is depicted in the Fig. 4(a–c). By increasing the concentration of cerium (Ce³⁺) NPs in Co_0.5Mn_0.5Fe_2−xCe_xO₄ ferrite, the peak intensity slightly increases. The major absorption peak around 625 nm indicates that, the photons are more absorbed around this wavelength which falls under red region of visible spectrum. In the UV–visible spectra, the energy transition can be due to the π-π* transitions of the ferrite sample (Manjunatha et al., 2019a,b). The band arising from 700 nm in the UV–Vis spectra can be assigned to the free carrier tail.

3.5 Humidity sensing studies

Humidity sensing mechanism

The humidity sensing measurements were performed in a laboratory made experimental setup as shown in Fig. 5(a). The humidity sensing mechanism is explained on the base of three possible mechanisms, physisorption, chemisorption and followed by capillary condensation. Initially, the water vapour gets ionized in the form of H⁺ and OH⁻ ions and get adsorbed over the surface of sensing material (Ravikiran et al., 2018). $$H_{2}O\leftrightarrow H^{+}+O{H}^{-}.$$

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Fig. 5

(A) Humidity sensing set-up used in the present investigation (B) Humidity sensing mechanism at the ferrite surface.

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During first stage of physisorption, the well distinguished OH⁻ ions gets linked to the sensing surface and chemisorption layer and H⁺ ions are liberated from the sensing material. The OH⁻ ions which are chemisorbed form a strong hydrogen bond with the nearby water molecule to result (H₃O⁺). The released H⁺ ionstart to migrate between water molecules forming the hydrogen bonds. This entire mechanism take place as per the Grotthuss chemical mechanism (Zhang et al., 2018). $$2H_{2}O\to H_3{O}^{+}+O{H}^{-}$$ $$H_3{O}^{+}\to H_{2}O+H^{+}$$

By enhancing the RH%, the physisorbed water vapour gets collected on the sensing surface creating the second step of physisorption mechanism. At the end step the adsorbed water vapours get collected in the capillary tubes which enhanced the protonation mechanism in the ferrite which results in the improved humidity sensing performance of the ferrites. Over all these outcomes which results into reduction of the resistance values and thus enhances in its humidity sensing performance of the ferrite (Sarwar Hasan et al., 2022). The detailed humidity sensing mechanism of prepared ferrites sample was illustrated in the Fig. 5(b).

3.6 Variation of resistance with relative humidity

The change of sensors resistance with a function of relative humidity (%) for Co_0.5Mn_0.5Fe_2−xCe_xO₄ (x = 0.0, 0.05 and 0.1) ferrite were illustrated in the Fig. 6(a). One can easily observe from the plots that the sensors resistance drops considerably with increasing relative humidity (RH) from 12 % RH to 97 % RH due to the more adsorption of water vapours over the surface of ferrite. This could be due to two prime reasons, first, the adsorbed moisture being comparatively lower because of which water molecules do not follow the change in electric filed direction and second, resistance lean on the geometry of the sample (Shah et al., 2011).

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Fig. 6

(A) Change in resistance (B) Change in Capacitance (C) Sensing response as a function of Relative humidity for Co_0.5Mn_0.5Fe_2−xCe_xO₄ (x = 0.0, x = 0.050 and x = 0.1) ferrite (D) Dynamic sensing response.

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Among the synthesized ferrite the highest change in sensors resistance was observed at 12 % RH for Co_0.5Mn_0.5Fe_2−xCe_xO₄ (x = 0.1) NPs concentration. The humidity sensitivity for different humidity at ambient temperature for the Co_0.5Mn_0.5Fe_2−xCe_xO₄ (x = 0.0, 0.05 and 0.1) ferrite determined by employing the below formula (Kotnala et al., 2008). $$\mathit{Humidity}sen\sin gresponse\left(\%\right)=\left[\frac{R_{\mathit{LH}}-R_H}{R_{\mathit{LH}}}\right]$$ where the resistance at lower humidity level represented by R_LH and the resistance due to variable relative humidity is R_H.

3.7 Variation of capacitance with relative humidity

The trend of capacitance with applied relative humidity for Co_0.5Mn_0.5Fe_2−xCe_xO₄ (x = 0.00, 0.5 and 0.1) ferrite recorded at a constant frequency of 100 Hz at ambient temperature is shown in Fig. 6(b). From the plots we can simply observed that the capacitance of the ferrite improved with increasing % of RH. In the beginning, up to 30 % RH the capacitance value remains the same, later at higher frequency region the capacitance value exponentially increases with higher RH was noticed due to more adsorption of water molecules which facilitates the accumulation of charge carriers inside the host ferrite. According to the adsorption theory, the adsorption of water molecules occur in two variable methods. The first step related to chemisorption mechanism and second step is physisorption mechanism. The additional layer on the sensing ferrite sample influences the capacitance change in the ferrite. It was found experimentally that the capacitance of the ferrite is directly proportional to the conductance and inversely proportional to the electric field (Syed Khasim et al., 2021). Among all the synthesized ferrites Co_0.5Mn_0.5Fe_2−xCe_xO₄ (x = 0.1) sample exhibited higher capacitance and which results into more humidity sensitivity.

3.8 Humidity sensing performance with relative humidity

The trend of humidity sensing response as a function of relative humidity (%) recorded at ambient temperature for Co_0.5Mn_0.5Fe_2−xCe_xO₄ (x = 0.0, 0.05 and 0.1) ferrite were illustrated in the Fig. 6(c). It is clearly observed from the plots that the humidity sensing response enhances with relative humidity from 12 % to 97 % (Srinivasamurthy et al., 2023). The humidity sensing response in the present work is better than that of nickel–iron-zinc oxide ferrite NPs synthesized via solution combustion technique (Apsar Pasha and Angadi, 2023). Among the prepared ferrite Co_0.5Mn_0.5Fe_2−xCe_xO₄ (x = 0.1) sample shows maximum humidity sensing responses of 98 % in comparison to other synthesized ferrite samples. The doping of cerium (Ce³⁺) NPs into the cobalt-manganese-iron oxide ferrite leads to the enhanced porosity and larger absorption of water molecules and hence improved sensing response.

3.9 Humidity response and recovery times

The humidity sensing response and recovery times were defined as, the time taken to achieve 90 % of humidity sensitivity (saturation) at the time one complete gasification process and the same time the degasification mechanism was utilized to measure the sensor recovery times when the humidity sensitivity drop to baseline. The response and recovery plots for Co_0.5Mn_0.5Fe_2−xCe_xO₄ (x = 0.1) ferrites studied at room temperature is shown in the Fig. 6(d). To measure the response and recovery times two sensing chambers were used, the first chamber which comprises of 11 % RH and other chamber comprises of 95 % RH. The sensing response for Co_0.5Mn_0.5Fe_2−xCe_xO₄ (x = 0.1) sample was found to be 36 s when the sample was shifted from 12 % RH to 97 % RH and at the same time the recovery time 63 s was measured when the tested sample moved from 97 % RH to 12 % RH. The improved response and recovery times of Co_0.5Mn_0.5Fe_2−xCe_xO₄ ferrite material can be used as an efficient material in the design of humidity sensors (Hiremath et al., 2023). The main reason for the improved response and recovery time in Co_0.5Mn_0.5Fe_2−xCe_xO₄ ferrite was believed that doping of rare earth nanoparticles like cerium into the Co_0.5Mn_0.5Fe_2−xCe_xO₄ ferrite which turn to more porous morphology which facilitate more absorption of water molecules over the surface of ferrite sample and it causes improved response and recovery times (Syed Khasim et al., 2023). The Comparative analysis of Humidity sensing performance of different ferrites-based composites synthesized via different methods with the recently published works were given in the Table 3.

3.10 Humidity hysteresis

The humidity hysteresis curves for Co_0.5Mn_0.5Fe_2−xCe_xO₄ (x = 0.0, 0.05, 0.1) ferrite were recorded in the regime of 11 % RH to 98 % RH as illustrated in the Fig. 7(a–c). The adsorption trend exhibited by slowly enhancing from the relative humidity range 11 % to 98 % and desorption curve is back tracing at the time of measurement. It was very curious to observe from the hysteresis plots, the desorption mechanism little slower in compare to the adsorption mechanism. This recommends that the exothermic and endothermic responses of the sensors are well compared to the adsorption and desorption mechanism occurs at different rates leading to the lower the impedance value in desorption mechanism in compare to the adsorption mechanism. Among the prepared ferrite Co_0.5Mn_0.5Fe_2−xCe_xO₄ (x = 0.1) sample shows lower hysteresis loss in the order of 2 % in compare to the other synthesized ferrites samples, this kind of behaviour might be due the improved morphological features and higher humidity sensitivity (Ramakrishna et al., 2023). The humidity hysteresis loss of the synthesized ferrites can be calculated by using the equation. $$Hysteresis\left(\%\right)=\left[\frac{a-b}{Y_{\max }-Y_{\min }}\right]$$

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Fig. 7

(A–C) Humidity hysteresis characteristics Co_0.5Mn_0.5Fe_2−xCe_xO₄ (x = 0.0, x = 0.050 and x = 0.1) ferrite (D) Stability study for x = 0.1 ferrite.

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In the above equation, “a” be the highest value and “b” be the lowest value of Y at the overage of the % relative humidity. All the humidity sensing parameters of the synthesized materials are shown in Table 1.

3.11 Stability analysis

In this test, we have tested Co_0.5Mn_0.5Fe_2−xCe_xO₄ (x = 0.1) ferrite at relative humidity of 30 % and 97 % for each 8 days in 90 days’ time interval. Fig. 7(d) shows the stability plots of 30 % and 97 % RH for Co_0.5Mn_0.5Fe_2−xCe_xO₄ (x = 0.1) ferrite recorded at ambient temperature. From the plots clearly shows that, in both humidity condition the synthesized ferrite sample exhibited outstanding stable performance over a long period with negligible loss of sensitivity which is nearly 2 %, this least loss in humidity sensitivity might be due to the aging induced effect was observed in most of the ferrites-based sensors. Therefore, the stability test indicated that, the prepared humidity sensing material is showing the outstanding stable performance at higher concentration of cerium doped cobalt-manganese-iron ferrite at ambient temperature (Syed Khasim et al., 2024). The lower concentration cerium NPs doped cobalt-manganese-iron ferrite was not examined the stability analysis, because these composites show lower humidity sensitivity in compared to the Co_0.5Mn_0.5Fe_2−xCe_xO₄ (x = 0.1) ferrite. However, the lower humidity sensing ferrite materials could find potential applications especially in energy storage, optoelectronic and electronic applications (Ramakrishna et al., 2024).