Structural parameters of the CuFe(2−x)EuxO4 (where x = 0.00, 0.01, 0.02, and 0.03) nanoparticles.
The CuFe(2−x)EuxO4 (where x = 0.00, 0.01, 0.02, 0.03) nanoparticles are synthesized by solution combustion method. The influence of Eu3+ on the structural, morphological, dielectrical, and humidity sensing study is recorded. The XRD pattern peaks of the as-prepared CuFe(2−x)EuxO4 (where x = 0.00, 0.01, 0.02, 0.03) nanoparticle confirm the polycrystalline spinel cubic structure with a small amount of CuO impurity phase at 38.87° and 48.96°. Surface morphology of the samples was studied by scanning electron microscope (SEM) images of the nanoparticles, and their respective average grain size was estimated using Image software. Chemical composition of all prepared samples was analyzed by EDS spectra. The dielectric parameters of AC conductivity, electric modulus, and impedance of the samples were measured over a range of frequencies from 0.1 KHz to 1 MHz at room temperature. Europium-doped copper ferrite samples showed good humidity sensing response, response and recover times, and stability over a %RH range of 11–91%. These types of samples are very useful for sensor application, battery applications, electronic applications, and automotive applications.
- humidity sensing
In recent years, the great developments in the applications of larger ionic rare earth-doped ferrites have been effectively realized in many prominent fields mainly in sensors, communication, and electronics. Copper ferrite metal oxide is a p-type semiconductor metal oxide and has been used in organic synthesis as a catalyst. Recently, many research groups have explored the importance of copper ferrite metal oxide in wide advanced applications because of their unique chemical, biocompatible, and physical properties . The CuFe2O4 ferrite has spinel cubic structure with general formula Fd3m. The CuFe2O4 ferrite belongs to a general formula AB2O4 in which A is a divalent ion site occupied by the Cu2+ ions and B is a trivalent ion site occupied by Fe3+ ions. The rare earth-doped CuFe2O4 ferrites have been used in many fields such as electronic devices, drug delivery systems, cancer therapy, and magnetic recording . Rare earth-doped ferrites are also useful in high voltage electronics due to its negligible eddy current losses, high electrical resistivity, high permeability, magneto-optical, and magnetoresistive properties [2, 3, 4]. Generally, the trivalent ions are lesser in size than a divalent ion, and hence exchange of cations among the A and B sites plays a vital role in studying the structural, morphological, dielectric, and humidity sensing behavior of spinel copper ferrites. Copper ferrite has considerable good attraction of potential applications in various devices, like cores in transformers and microwave absorption [3, 4]. Copper ferrite nanoparticles can be prepared by various techniques like sol-gel method, coprecipitation method, solution combustion method, etc. [5, 6]. Many researchers have explored the properties of spinel ferrites by doping with different larger rare earth ions like samarium, terbium, gadolinium, and cerium in it. The europium-doped copper ferrites can be used in high-frequency applications due to the rare earth-doped ferrites showing low dielectric loss, good sensing response, and low value of conductivity. Europium-doped copper ferrites are usually considered as inverse spinel ferrites.
In the present work, spinel copper ferrites were prepared by solution combustion method and then in Eu3+ ions will be incorporated to investigate its structural, morphological, dielectric, and humidity sensing behavior at room temperature.
2. Experimental details
2.1 Required oxidizers and fuels
The required oxidizers (metal nitrates), viz., copper nitrate [Cu(NO3)2·3H2O], europium nitrate [Eu(NO3)3·5H2O], and ferric nitrate [Fe(NO3)2·9H2O], and reducing agents, viz., carbamide [NH2CONH2] and glucose [C6H12O6], all were purchased from S.D. Fine Chemicals, Mumbai, India.
The CuFe(2−x)EuxO4 (where x = 0.00, 0.01, 0.02, 0.03) nanoparticles were synthesized by solution combustion method by using stoichiometry amount of copper nitrate, europium nitrate, and ferric nitrate as metal nitrates and carbamide and glucose as fuels. The weighed metal nitrates and fuels were taken in 250 ml borosil glass beaker; then all nitrates and fuels were diluted with distilled water and kept on magnetic stirrer about 45 min to obtain a homogeneous solution. This solution was kept in preheated muffle furnace at 450 ± 10°C temperature to ignite, to get a copper ferrite nanoparticle. Obtained CuFe(2−x)EuxO4 (where x = 0.00, 0.01, 0.02, 0.03) nanoparticles were taken in to mortar and then pestle it for getting fine powder of nanoparticles. The solution combustion technique flowchart for europium-doped copper ferrite as shown in Figure 1.
For as-synthesized CuFe(2−x)EuxO4 (where x = 0.00, 0.01, 0.02, 0.03) nanoparticles, X-ray diffraction characterization was carried out by using Cu-Kα radiation of wavelength 1.5404 Å and the diffractogram run with two theta (2θ) range from 10o to 80o. The XRD data were refined by using full proof software. The Rietveld refined XRD pattern peaks confirm the polycrystalline spinel cubic structure. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were carried out by using JEOL (Model JSM-840) instrument respectively to understand structural morphology and elemental analysis. The mean grain size of the particles was estimated from SEM micrographs by using ImageJ software. From SEM and EDS to understand structural morphology and elemental analysis. The AC eclectic parameters were measured over a range of frequencies from 50 Hz to 10 MHz using Wayne Kerr 6500B series impedance analyzer. For humidity sensing AC conduction studies, powder samples were pressed in the form of pellet under the hydraulic pressure of 5 tons. Further, two faces of these pellets were painted with silver paste for electrical contact. The pellet was held between two probes and then placed in a glass chamber through a rubber cork and the other end of the electrodes connected to the programmable HIOKI Digital Multimeter to record the resistance corresponding to various RH relative humidities from 11% RH to 97%. The schematic illustration of humidity sensing setup is shown in Figure 2.
3. Results and discussion
3.1 XRD analysis
Figure 3 shows the Rietveld refined XRD patterns of as-synthesized CuFe(2−x)EuxO4 (where x = 0.00, 0.01, 0.02, 0.03) nanoparticles. XRD patterns of all samples show the polycrystalline spinel cubic structure with space group Fd3m with a small amount of CuO impurity phase. The indexed XRD peaks are well matched with spinel cubic structure of JCPDS card number 74-2400 and the CuO impurity peaks observed at 38.87° and 48.96° (JCPDS 80-1268) [7, 8]. The lattice parameter is found to increase with increase in Eu3+ concentration due to difference in ionic radius. The ionic radii of Fe3+ (0.67 Å) are lesser than that of Eu3+ (0.99 Å) ions; this confirms the occupancy of europium on an octahedral site. The average crystallite size of CuFe2−xEuxO4 (where x = 0.00, 0.01, 0.02, 0.03) nanoparticles estimated from the Debye-Scherrer relation and the average particle size of all the samples were found to be in the range of 16–51 nm . The strain values are calculated by using equationand are tabulated in Table 1. Further, the hopping length of LA and LB is calculated, the hopping length increasing with increase in Eu3+ concentration. This happened because Fe3+ ions are replaced by the relative number of Eu3+ in octahedral site and the variation of hopping length of tetrahedral site (LA) and octahedral site (LB) is shown in Table 1.
|Eu3+ content||Lattice parameters (Å)||Crystallite size D in nm||Volume (Å3)||Strain Є (radian)||Space group||Average grain size||Hopping length (Å)|
|x = 0.0||8.126||25||539.29||1.38 × 10−3||Fd3m||28||3.518||2.873|
|x = 0.01||8.131||16||540.23||2.14 × 10−3||Fd3m||30||3.520||2.874|
|x = 0.02||8.143||21||542.71||1.63 × 10−3||Fd3m||40||3.526||2.879|
|x = 0.03||8.154||51||544.91||6.98 × 10−3||Fd3m||33||3.531||2.883|
3.2 SEM and EDS
The surface morphology and elemental analysis of the CuFe(2−x)EuxO4 (where x = 0.00, 0.01, 0.02, 0.03) nanoparticles were performed with scanning electron microscope and energy-dispersive X-ray spectroscope. From Figure 4(left) we can clearly see that all the particles are spherical in shape and exhibit smooth surface with an average grain size of 20–40 nm. All samples of the SEM micrographs show highly porous nature, and the appearance of the dry foamy powder is due to the evolution of the gases during the combustion process . Figure 4(right) shows the EDS spectrum of CuFe(2−x)EuxO4 (where x = 0.00, 0.01, 0.02, 0.03) nanoparticles. The CuFe(2−x)EuxO4 (where x = 0.01, 0.02, 0.03) of EDS spectrum depicts Cu, Fe, Eu, and O peaks are clearly seen with Cuo impurity peaks (Figure 4(right)). CuFe(2−x)EuxO4 (where x = 0.00) of EDS spectrum depicts Cu, Fe, and O peaks are clearly seen with Cuo impurity peak. The Eu3+ peak appeared in all samples except when x = 0, and its intensity of Eu3+ peak increase with europium concentration increases. The grain size distribution histogram of CuFe(2−x)EuxO4 (where x = 0.00, 0.01, 0.02, 0.03) nanoparticles is shown in Figure 4(left), and the average grain size of CuFe(2−x)EuxO4 (where x = 0.00, 0.01, 0.02, 0.03) nanoparticles is estimated from SEM micrographs using ImageJ software. The average grain sizes of all the particles are well matched with the crystallite size as shown in Table 1.
3.3 Dielectric studies
3.3.1 Variation of frequency with AC conductivity
Figure 5 shows the variation of frequency with AC conductivity plots, respectively, for CuFe(2−x)EuxO4 (where x = 0.00, 0.01, 0.02, 0.03) nanoparticles measured over the frequency range of 0.1 kHz to 1 MHz at room temperature. The figure clearly reveals the AC conductivity of each sample increases linearly with frequency. The exchange of electrons between A site and B site enhances the AC conductivity. The sample with a higher concentration of europium ions shows high values of AC conductivity. The substitution of europium ions on the copper ferrites obstructs the exchange of electrons between A sites and B sites, resulting in the decrease in the AC conductivity. At the lower-frequency region, the effect of grains and grain boundary is dominant, and it causes decreases in the exchange of electron between Fe2+ ions and Fe3+ ions, so small AC conductivity values have been observed. The polarization of all the samples of spinel copper ferrites is increased at high frequency. In the copper ferrite, the part of the grains is very essential at greater frequency than grain boundary’s part; it causes the conduction to be enhanced in spinel ferrites . The doping of europium ions to the Fe3+ ions from the octahedral site disrupts the conduction mechanism, and the AC conductivity decreases. The maximum value of AC conductivity is found to be x = 0.00 concentration.
3.3.2 Variation of frequency with real and imaginary part of electric modulus
Figure 6 shows the variation of frequency with real and imaginary parts of electric modulus plots respectively for CuFe(2−x)EuxO4 (where x = 0.00, 0.01, 0.02, 0.03) nanoparticles. These were studied over the frequency range of 0.1 kHz to 1 MHz at room temperature. To study the character of grain boundary and grains over the frequency range of 0.1 kHz to 1 MHZ, electric modulus analysis was taken out. For this basis, the graphs plot between real and imaginary parts of electric modulus along the y-axis and the frequency along the x-axis were taken. In copper ferrites, the real parts of the electric modulus show very low values at lower-frequency region and increase linearly with the increase in the frequency of the external applied field . In the case of the imaginary part of electric modulus (), high electric modulus at lower-frequency region is shown and again decreases with increase in the frequency of external applied field. The real and imaginary parts of electric modulus were maximum at x = 0.00 concentration.
3.3.3 Cole-Cole plots
Figure 7 shows the imaginary part of electric modulus with real part of electric modulus plots, respectively, for CuFe(2−x)EuxO4 (where x = 0.00, 0.01, 0.02, 0.03) nanoparticles at room temperature. To examine the effect of grain boundaries and grains within spinel ferrites, a plot can be drawn between real and imaginary parts of dielectric constant and dielectric loss or using the values of the real and imaginary parts of impedance, but no satisfactory results were obtained from the aforesaid formalisms. The figure plot between real and imaginary parts of dielectric modulus clearly shows the semicircles within the given range of these quantities. The Cole-Cole plots between the real part of the electric modulus and imaginary parts of the electric modulus can give good results. The role of grains and grain boundaries is very good because semicircles are observed at all the samples [9, 12]. The maximum peak of the diameter observed semicircle increases with the substitution of europium rare earth ions. The maximum intensity of peak is observed at x = 0.03 concentration.
Figure 7 shows Cole-Cole (the imaginary part of impedance with real part impedance) plots for CuFe(2−x)EuxO4 (where x = 0.00, 0.01, 0.02, 0.03) nanoparticles at room temperature.
In these impedance analysis plots, we observed only one semicircle clearly at x = 0.00 concentration, this study indicating the role of grain boundaries predominated, and the contribution from the grain was not resolved from this impedance analysis . Sivakumar et al. reported a similar result from nanocrystalline cobalt ferrites . The radii of the semicircles decreased with increasing concentration; this sign indicates a decrease in relaxation.
3.4 Humidity sensing studies
3.4.1 Variation of resistance and sensing response with relative humidity
In Figure 8, the curves are plotted between the relative humidity along the x-axis and resistance along the y-axis for CuFe(2−x)EuxO4 (where x = 0.00, 0.01, 0.02, 0.03) nanoparticles at room temperature. For all samples, the resistances are decreased from 1 × 107 Ω to 100 × 107 Ω for relative humidity varying from 11 to 97% RH. From the figure, the maximum variation in resistance is observed for CuFe(2−x)EuxO4 (where x = 0.03) sample as compared to other samples. So, the humidity sensing response of each sample was calculated by using the following equation and plotted against %RH shown in Figure 9.
The CuFe(2−x)EuxO4 (where x = 0.03) has shown the maximum sensing response; hence it is essential to know its sensing mechanism. The sensing mechanism is discussed on the basis of three sequential steps: chemisorptions, first step of physisorption, and second step of physisorption, followed by capillary condensation. In the initial stage, the water molecule adsorbed to the sensing surface and gets self-ionized to form H+ and OH− ions .
In the beginning stage, the dissociated OH− ions get attached to the sensing surface forming a chemisorption layer, and H+ ions are released. These chemisorbed two OH− ions form a hydrogen bond with the neighboring water molecule to form bulk water (H3O+). This forms the first physisorbed layer. Thus formed bulk water dissociates to form H2O and H+ ion. The released H+ ions transfer from one water molecule to another through the braking and making of bonds. This is in accordance with the Grotthuss mechanism .
As the RH increases, the physisorbed water molecules get piled up on one another forming the second step of physisorption. At the last stage, the adsorbed water molecule condenses in the capillary pores, leading to increase in the protonation. These results in the decrease in the resistance and in turn increase in its conductivity.
3.4.2 Sensing response and recovery
Nowadays, the sensing response and recovery time characteristics and stability testing are required for device fabrication of humidity sensing material. The response and recovery time were measured only for CuFe(2−x)EuxO4 (where x = 0.03) because of good sensing response as compared to other samples. In response and recovery time studies, we used two chambers, in that one chamber containing lower relative humidity of 11% RH another of higher relative humidity of 95% RH. The sensing response time of 63 s was recorded when the sample was moved from relative humidity of 11% RH to relative humidity of 97% RH, and the recovery time of 164 s was recorded when sample was moved from relative humidity of 97% RH to relative humidity of 11% RH (Figure 10). The difference between sensing response time and recovery time is small. These studies clearly show that the response and recovery time of Eu3+-doped CuFe2O4 sample is slightly better than response and recovery time of nickel copper-zinc ferrite synthesized by coprecipitation method .
Stability testing is one of the important tests for practical application in device fabrication of sensing material. For stability testing, CuFe(2−x)EuxO4 (where x = 0.03) sample pellet was tested at relative humidity of 97% RH and 55% RH for every 10 days in 2 months. Figure 11 shows the stability curves at 55% RH and 97% RH for CuFe(2−x)EuxO4 (where x = 0.03) sample at room temperature. The figure clearly indicates that both relative humidity samples show highly stable response at room temperature during that period. So, sample shows that humidity sensing material is highly stable at larger concentration of europium-doped copper ferrite at room temperature and this is the proof for good practical application. The less concentration of europium-doped copper ferrite sample was not tested for stability because at lower concentration samples show less sensing response compared to higher concentration. However low sensing response ferrites also have good potential applications such as electronic and battery applications [17, 18].
The CuFe(2−x)EuxO4 (where x = 0.00, 0.01, 0.02, 0.03) nanoparticles were synthesized by solution combustion method using mixture of fuels for the first time. XRD patterns reveal the polycrystalline spinel cubic structure having space group Fd3m with small amount of impurity phases at 38.87° and 48.96°. The SEM micrograph reveals the formation of dry foamy powders during combustion process and porous nature of the sample. The average grain size of the samples was measured from SEM micrographs. The chemical composition was analyzed by using EDS analysis. The dielectric parameters vary with frequency. In the copper ferrite, the role of the grains is very essential at greater frequency as compared to grain boundary’s role. The radii of the semicircles decreased with increasing concentration; this sign indicates a decrease in relaxation. The humidity sensing response at higher concentration is very good as compared to other concentration. The response and recovery curve time was good as compared to other ferrite samples. The sample shows high stability at higher concentration with good sensing response for sensor applications. At lower concentration, europium-doped sample shows low sensing response; however, the low sensing responsible ferrites are used in battery and electronic applications.