**3.1 XRD analysis**

**Figure 3** shows the Rietveld refined XRD patterns of as-synthesized CuFe(2x)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

**Figure 3.** *Rietveld refined XRD patterns of the CuFe(2x)EuxO4 (where x = 0.00, 0.01, 0.02, 0.03) nanoparticles.*

*Enhanced Humidity Sensing Response in Eu3+-Doped Iron-Rich CuFe2O4… DOI: http://dx.doi.org/10.5772/intechopen.90880*

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 [9]. The strain values are calculated by using equation *ɛ* ¼ *<sup>β</sup>cos<sup>θ</sup>=*<sup>4</sup> and 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**.

#### **3.2 SEM and EDS**

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 3** shows the Rietveld refined XRD patterns of as-synthesized

CuFe(2x)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

*Rietveld refined XRD patterns of the CuFe(2x)EuxO4 (where x = 0.00, 0.01, 0.02, 0.03) nanoparticles.*

**Figure 2**.

**Figure 3.**

**78**

**3. Results and discussion**

*Mineralogy - Significance and Applications*

**3.1 XRD analysis**

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 [10]. **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


**Table 1.**

*Structural parameters of the CuFe(2*�*x)EuxO4 (where x = 0.00, 0.01, 0.02, and 0.03) nanoparticles.*

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 [11]. 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.

*The variation of frequency with AC conductivity for the CuFe(2x)EuxO4 (where x = 0.00, 0.01, 0.02, and*

*Enhanced Humidity Sensing Response in Eu3+-Doped Iron-Rich CuFe2O4…*

*DOI: http://dx.doi.org/10.5772/intechopen.90880*

**Figure 5.**

**Figure 6.**

**81**

*x = 0.00, 0.01, 0.02, and 0.03) nanoparticles.*

*0.03) nanoparticles.*

*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(2x)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

*The variation of frequency with real and imaginary parts of electric modulus for the CuFe(2x)EuxO4 (where*

*(Left) SEM micrographs and (right) EDS spectra of the CuFe(2x)EuxO4 (where x = 0.00, 0.01, 0.02, and 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

*Enhanced Humidity Sensing Response in Eu3+-Doped Iron-Rich CuFe2O4… DOI: http://dx.doi.org/10.5772/intechopen.90880*

**Figure 5.** *The variation of frequency with AC conductivity for the CuFe(2x)EuxO4 (where x = 0.00, 0.01, 0.02, and 0.03) nanoparticles.*

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 [11]. 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(2x)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

#### **Figure 6.**

*The variation of frequency with real and imaginary parts of electric modulus for the CuFe(2x)EuxO4 (where x = 0.00, 0.01, 0.02, and 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

*(Left) SEM micrographs and (right) EDS spectra of the CuFe(2x)EuxO4 (where x = 0.00, 0.01, 0.02, and*

**Figure 4.**

**80**

*0.03) nanoparticles.*

*Mineralogy - Significance and Applications*

modulus along the y-axis and the frequency along the x-axis were taken. In copper ferrites, the real parts of the electric modulus *M*<sup>0</sup> show very low *M*<sup>0</sup> values at lowerfrequency region and increase linearly with the increase in the frequency of the external applied field [12]. In the case of the imaginary part of electric modulus (*M*00), 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.

decreased from 1 � <sup>10</sup><sup>7</sup> <sup>Ω</sup> to 100 � <sup>10</sup><sup>7</sup> <sup>Ω</sup> 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

*The variation of resistance with relative humidity for CuFe(2*�*x)EuxO4 (where x = 0.00, 0.01, 0.02, and 0.03)*

<sup>¼</sup> resistance of lower relative humidity � resistance at any relative humidity resistance of lower relative humidity*:*

*The variation of sensing response (%) with relative humidity for CuFe(2*�*x)EuxO4 (where x = 0.00, 0.01, 0.02,*

(1)

equation and plotted against %RH shown in **Figure 9**.

*Enhanced Humidity Sensing Response in Eu3+-Doped Iron-Rich CuFe2O4…*

*DOI: http://dx.doi.org/10.5772/intechopen.90880*

Humidity sensing response

**Figure 8.**

**Figure 9.**

**83**

*and 0.03) nanoparticles.*

*nanoparticles.*

**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 [13]. Sivakumar et al. reported a similar result from nanocrystalline cobalt ferrites [14]. 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 xaxis 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

**Figure 7.**

*Imaginary part of electric modulus with real part of electric modulus and the imaginary part of impedance with real part impedance, respectively, for CuFe(2*�*x)EuxO4 (where x = 0.00, 0.01, 0.02, and 0.03) nanoparticles.*

*Enhanced Humidity Sensing Response in Eu3+-Doped Iron-Rich CuFe2O4… DOI: http://dx.doi.org/10.5772/intechopen.90880*

#### **Figure 8.**

modulus along the y-axis and the frequency along the x-axis were taken. In copper ferrites, the real parts of the electric modulus *M*<sup>0</sup> show very low *M*<sup>0</sup> values at lowerfrequency region and increase linearly with the increase in the frequency of the external applied field [12]. In the case of the imaginary part of electric modulus (*M*00), 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

**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)

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 [13]. Sivakumar et al. reported a similar result from nanocrystalline cobalt ferrites [14]. The radii of the semicircles decreased with increasing

*3.4.1 Variation of resistance and sensing response with relative humidity*

concentration; this sign indicates a decrease in relaxation.

In these impedance analysis plots, we observed only one semicircle clearly at

In **Figure 8**, the curves are plotted between the relative humidity along the xaxis 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

*Imaginary part of electric modulus with real part of electric modulus and the imaginary part of impedance with real part impedance, respectively, for CuFe(2*�*x)EuxO4 (where x = 0.00, 0.01, 0.02, and 0.03) nanoparticles.*

electric modulus were maximum at x = 0.00 concentration.

*3.3.3 Cole-Cole plots*

*Mineralogy - Significance and Applications*

nanoparticles at room temperature.

**3.4 Humidity sensing studies**

**Figure 7.**

**82**

*The variation of resistance with relative humidity for CuFe(2*�*x)EuxO4 (where x = 0.00, 0.01, 0.02, and 0.03) nanoparticles.*

decreased from 1 � <sup>10</sup><sup>7</sup> <sup>Ω</sup> to 100 � <sup>10</sup><sup>7</sup> <sup>Ω</sup> 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**.

Humidity sensing response

#### **Figure 9.**

*The variation of sensing response (%) with relative humidity for CuFe(2*�*x)EuxO4 (where x = 0.00, 0.01, 0.02, and 0.03) nanoparticles.*

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<sup>+</sup> and OH� ions [15].

$$\rm H\_2O \Leftrightarrow H^+ + OH^- \tag{2}$$

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<sup>+</sup> ). This forms the first physisorbed layer. Thus formed bulk water dissociates to form H2O and H<sup>+</sup> 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 [16].

$$\text{CH}\_2\text{O} \rightarrow \text{H}\_3\text{O}^+ + \text{OH}^- \tag{3}$$

$$\text{H}\_3\text{O}^+ \rightarrow \text{H}\_2\text{O} + \text{H}^+ \tag{4}$$

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 [17].

#### *3.4.3 Stability*

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

**Figure 11.**

**85**

**Figure 10.**

*nanoparticles.*

*nanoparticles.*

*The humidity sensing stability characteristic for CuFe(2x)EuxO4 (where x = 0.00, 0.01, 0.02, and 0.03)*

*The sensing response and recovery characteristic for CuFe(2x)EuxO4 (where x = 0.00, 0.01, 0.02, and 0.03)*

*Enhanced Humidity Sensing Response in Eu3+-Doped Iron-Rich CuFe2O4…*

*DOI: http://dx.doi.org/10.5772/intechopen.90880*

*Enhanced Humidity Sensing Response in Eu3+-Doped Iron-Rich CuFe2O4… DOI: http://dx.doi.org/10.5772/intechopen.90880*

#### **Figure 10.**

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

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

dissociates to form H2O and H<sup>+</sup> ion. The released H+ ions transfer from one water molecule to another through the braking and making of bonds. This is in accordance

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

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

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

nickel copper-zinc ferrite synthesized by coprecipitation method [17].

H2O ⇔ H<sup>þ</sup> þ OH� (2)

2H2O ! H3O<sup>þ</sup> þ OH� (3) H3O<sup>þ</sup> ! H2O þ H<sup>þ</sup> (4)

). This forms the first physisorbed layer. Thus formed bulk water

self-ionized to form H<sup>+</sup> and OH� ions [15].

*Mineralogy - Significance and Applications*

with the Grotthuss mechanism [16].

*3.4.2 Sensing response and recovery*

bulk water (H3O<sup>+</sup>

conductivity.

*3.4.3 Stability*

**84**

*The sensing response and recovery characteristic for CuFe(2x)EuxO4 (where x = 0.00, 0.01, 0.02, and 0.03) nanoparticles.*

**Figure 11.** *The humidity sensing stability characteristic for CuFe(2x)EuxO4 (where x = 0.00, 0.01, 0.02, and 0.03) nanoparticles.*

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].
