Synthesis and characterization of Novel Gadolinium doped CoCr2O4: Focus on Structural, Electric and Humidity Sensing properties

doi:10.21203/rs.3.rs-1623271/v1

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Synthesis and characterization of Novel Gadolinium doped CoCr2O4: Focus on Structural, Electric and Humidity Sensing properties

https://doi.org/10.21203/rs.3.rs-1623271/v1

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In this paper, we presented structural, Fourier infrared spectroscopic, and dielectric analysis of cobalt chromate (Co²⁺Cr₂³⁺O₄) with 2 mole % of gadolinium (Gd³⁺) rare earth metal additions under humidity and non-humidity circumstances. The Gd³⁺ doped Co²⁺Cr₂³⁺O₄ samples are prepared by solution combustion method and sintered for 2 hours at 650°C to get single phase. Doped and undoped samples were characterise X-ray powder diffraction (XRD) analysis provides the detailed formation of the Co²⁺Cr₂³⁺O₄ phase and crystallinity. Furthermore, the average crystallite sizes are found in the range of 18 to 7 nm. The general nature of ferrite materials is revealed via FTIR analysis. The octahedral and tetrahedral stretching band in FTIR spectra were confirmed ferrite structure without impurity. We investigated the relevant conductivity of the samples, the reaction time of the capacitive sensor, and the humidity influence on the relative permittivity characteristics at a constant frequency range of f = 1 kHz. Our findings indicate that Gd³⁺ doped Co²⁺Cr₂³⁺O₄ could be exploited as an active material in humidity sensor applications.

CoCr2O4

solution combustion method

humidity sensor applications

permittivity

electrical conductivity

Ferrites having a spinel cubic structure with general formula (A)(B₂)O₄ with the space group Fd3m[8]. The tetrahedral and octahedral sites are denoted by "A" and "B," respectively. In spinal cations A²⁺ occupy tetrahedral positions, while cations B³⁺ occupy octahedral sites. Apart from these two perfect spinel configurations, they can be partially inverted. However, if the inversion is incomplete, the cation distribution may become random. In recent years, numerous methods have been developed to obtain materials with the AB₂O₄ spinel material include sol-gel, co-precipitation, ball-milling, combustion, hydrothermal heating, and microwave hydrothermal procedure. Due to its chemical stability ferrites are used in the field of sensing and electronic industries. In order to obtain the best possible performance, various additions or substitutions with metal ions are made to the standard stoichiometric structure of the spinel. CoCr₂O₄materials have better thermal and chemical stability due to this we intended to chosen rare earth(Gd³⁺) doped CoCr₂O₄ to understand humidity study.

As per the literature survey, no report is available for Gd³⁺ doped Co²⁺Cr₂³⁺O₄. Doping spinel structure with massive rare earth ions can alter the structural, vibrational, electrical, and humidity sensing properties [10-12]. The effect of rare earth doping on crystal and physical properties was investigated. Nano ferrites are materials with a high electrical resistance. As the ambient humidity increases, the resistivity of the ferrite micro particles reduces [13]. Spinel ferrite nanoparticles can be used as humidity sensors (HS) due to their high porosity, large surface/volume ratio, high resistivity, low cost, and ease of production [14]. The humidity in the air can change the resistivity of a ferrite by nearly three orders of magnitude. Relative humidity (RH) refers to the amount of water vapours in the air at a certain temperature and is commonly used to express humidity. According to a literature analysis, ferrite nanoparticles are used in the vast majority of sensors [15-17].Researchers are also synthesising Co²⁺Cr₂³⁺O₄ nanomaterials using various methods such as solution combustion, sol gel, sonochemical assisted route, flame spray pyrolysis, precipitation method, and so on [18-22].In the present investigation, we have used solution combustion method to prepare Gd³⁺ 2 mole % doped cobalt chromate with a general formula of CoCr_2-xGd_xO₄(x=0 and 0.02). In the next paragraphs, we presented the synthesis procedure, the structural, Fourier infrared spectroscopic, electrical, Fourier vibrational, and humidity behaviours of the CoCr_2-xGd_xO₄(x=0. and 0.02) in order to identify the optimal composition for sensor applications.

2.1 Materials

In the present work, we have used cobaltous nitrate, chromium nitrate, and gadolinium nitrate as oxidizers and glucose and urea as fuels to prepare Gd doped Co²⁺Cr₂³⁺O₄powder. Fig. 1 represents the molecular structure of oxidizers and fuel and the colour of those elements.

2.2 Method of preparation

A stoichiometric technique is used to weigh oxidizers and fuel. All of the weighted chemicals are combined in a 500 ml glass jar with purified water. After it has been dissolved, all of the compounds in the water jar are found to have a clear and homogeneous solution. The jar with the homogeneous solution is kept in the furnace. We must start the furnace and set the temperature to 400 degrees Celsius. Due to the high temperature maintained in the furnace, all water molecules get evaporated and it will start burning immediately. This process will take around 20 to 30 minutes to get powder. Further powder was used for further studies for XRD and FTIR studies. The powder samples were uniaxially pressed, using a hydraulic press, into a cylindrical mould, which developed a down force of 10 tonnes, yielding disk-shaped samples for electrical investigations with a diameter of 6 mm and a thickness of approximately 1 mm.

At ambient temperature, the crystalline phase of all samples was detected using a Rigaku Miniflex diffractometer (Rigaku Corporation, Tokyo, Japan) with CuK-α radiation (λ = 1.5418 Å). All X-ray diffraction (XRD) patterns were analysed using the Full Prof programme and the Rietveld refinement approach. Observed typical XRD patterns of Gd doped CoCr₂O₄ powder were equated with the International Committee on Powder Diffraction data that match well with the file (JCPDS Card No. 780711). The Debye–Scherrer relation, as discussed in the previous paper, is used to calculate the average crystallite size [23]. There are no more peaks in the pattern, indicating that the Gd³⁺ doped Co²⁺Cr₂³⁺O₄ nanoparticles have been entirely produced. The values of crystallite size, lattice parameter, and unit cell volume are obtained by Rietveld refinement, which is presented in Table 1. Further lattice parameters were found to increase with an increase in Gd³⁺ content due to the larger ions (Gd) replacing the smaller ions vis versa [24]. The concentration of Gd³⁺ increases all parameter were found increases due to larger ions replace iron ions.

Table.1

The value of crystallite size, lattice parameter, unit cell volume obtained by Rietveld refinement

Gd content

Crystallite size in

Lattice parameter

Volume

Strain (%)

Rwp

Rexp

χ²

x= 0

8.3339

578.82

1.601

1.08

1.9

1.5

0.9

x= 0.02

8.3210

576.13

1.486

1.51

2.16

2.19

1.02

FTIR spectra of the CoCr_2-xGd_xO₄ (x=0 and x=0.02) samples at room temperature are shown in Figure 3. The spectra was acquired in the range of 350 to 4000 cm^-1. The spectrum exhibits two major absorption bands below 600 cm^-1, these are the characteristics of ferrites. The frequency range of the high frequency band is 500–550 cm^-1, whereas the frequency range of the low frequency band is 550–600 cm^-1. These two bands correspond to the vibrations of metal ion-oxygen complexes at the A and B sites in the ferrite structure [8, 25].It is discovered that when Gd³⁺ substitution rises, the high frequency band expands, but the low frequency band decreases. This has an effect on the Gd³⁺- O vibration, causing the octahedral vibrational frequency to drop. This might be one of the causes of the rise in octahedral peak intensity. The tetrahedral frequency vibration rises as the concentration of Gd³⁺ ions with smaller ionic radius and atomic weight increases in the tetrahedral location. All of our samples have an octahedral frequency band split.

The transport qualities of the samples are critical for using them as a humidity sensor. Silver electrodes were plated on both sides of the disc-shaped sample for the electrical tests. The permittivity and electrical resistivity measurements were performed at room temperature (RT) with an Impedance Phase Analyzer (Wayne Kerr 6400P, 20 Hz–20 MHz). The influence of humidity on electrical properties was tested in an enclosed box at 25 degrees Celsius with a known relative humidity ranging from 0% to 97% RH. Figure 4 depicts the permittivity and electrical resistivity of CoCr_2-xGd_xO₄[x=0 and 0.02] dependent on frequency with or without the presence of humidity.

Fig 4(a) represents Variation of relative permittivity with frequency of doped and undoped samples. Further it reveals that the relative permittivity of the samples decreases with increasing in frequency and constant at higher side. This is the general behaviour of the ferrite system. In fact, we notice a massive permittivity of the order 10⁷. In reality, the permittivity's behaviour can be classified based on frequency. For low frequencies (10³ Hz), the real component of the dielectric constant increases with temperature, indicating thermal activation of charge carriers and so influencing polarisation. To understand this pattern, we use the Maxwell–Wagner double layer model, which was created for inhomogeneous structures. CoCr_2-xGd_xO₄ at lower frequencies, the value of relative permittivity (ε′) is very high. It drops significantly at low frequencies and becomes nearly frequency-independent at high frequencies [26]. This type of dielectric behaviour in ferrites has been explained by the Maxwell-Wagner and Koops phenomenological theory [27-29]. Space charge polarisation is created by the material's inhomogeneous dielectric structure, according to the Maxwell and Wagner two layer model. A dielectric medium is formed in this notion by highly conducting grains separated by narrow weakly conducting grain boundaries. Grain borders are found to be more effective at low frequencies, whilst grains themselves are proven to be more effective at higher frequencies. Ferrites have a dielectric polarisation that is comparable to the conduction process. The electronic exchange between Cr²⁺ ions, Cr³⁺ ions and Gd³⁺ ions causes local electron displacement in the direction of the applied electric field, which causes polarization in the spinel structure of CoCr_2-xGd_xO₄. Polarization reduces as frequency increases until it reaches a fixed value. The decrease in ε′ with increasing frequency is owing to the fact that, above a certain frequency, the electronic exchange between Cr²⁺, Cr³⁺, and Gd³⁺ ions does not follow the frequency of the applied AC field. Electrons must pass via the boundaries of the good and weak conducting grains. Electrons collect and generate considerable space charge polarisation due to the grain boundaries. As a result, large values of the dielectric constant exist in the low frequency region. Electrons rapidly shift their direction of travel as frequency increases, impeding electron transit inside dielectric materials and limiting charge build-up at grain boundaries. Furthermore, it was discovered that the dielectric constant falls with increasing Gd3+ content. We've also seen how the dielectric constant rises with time

Fig 4(b) Variation of electrical resistivity with frequency of CoCr_2-xGd_xO₄[x=Gd 0.00); and x=(Gd 0.02)]. It is worth noting that the electrical resistance varies according on the composition. It is discovered that the value of electrical resistivity decreases with decreasing crystallite size of samples, with the maximum value of resistivity achieved in the case of the sample with x = 0 having the highest granulation. The smaller grains result in a large no. of grain boundaries which act as the scattering center for the flow of electrons and therefore increase of resistivity (Kingry et al. 1976). Further the resistivity and dielectric constant behaves in an opposite manner. Electrical resistance reduces as frequency increases. Our findings show that the electrical resistivity of the samples is heavily impacted by the size of their crystallites.

All of the samples experience a drop as the frequency increases, which is expected. The electrical resistance on the higher side decreases with increasing frequency and eventually reaches zero.. The dispersion is greater on the low-frequency side, which might be attributed to differences in the concentrations of Cr³⁺, Cr²⁺ and Gd³⁺ ions in the various samples. The presence of a considerable amount of Cr²⁺ ions, which increase polarisation and conduction processes, may be responsible for the maximum dispersion seen in Gd³⁺ =0.02 samples. The hopping of charge carriers between Cr³⁺ and Cr²⁺ ions on octahedral sites can be used to characterise the CoCr_2-xGd_xO₄ conduction process. Furthermore, humidity raises the permittivity and conductivity of the material by filling open pores with vapours, as expected. When compared to non-doped CoCr₂O₄, which has a high electrical resistivity, the Gd³⁺ substitution adds to a decrease in electrical resistivity, putting the material's resistivity into the measurable zone, which is beneficial for sensor applications [30].

Fig 4(c) (d) represents variation of relative permittivity with relative humidity and electrical resistivity with relative humidity of CoCr_2-xGd_xO₄[x=Gd 0.00); and x=(Gd 0.02)]. Here the relative permittivity is increases with increasing RH level. The four types of powder were crushed using a hydraulic press to evaluate the effect of humidity on electrical characteristics. Porous silver electrodes were deposited on both surfaces of disk-samples with 6 mm diameters and a thickness of roughly 1 mm. Figures 4 and 5 demonstrate the electrical resistivity and relative permittivity of Gd³⁺ doped materials exposed to varied humidity conditions vs frequency at room temperature in the range of 20 Hz–20 MHz. Humidity sensitivity was evaluated in sealed boxes at a constant temperature and known relative humidity. The reaction durations were calculated by adjusting the relative humidity from 0% RH to 97.5% RH, and the recovery time was calculated by adjusting the relative humidity back to 0% RH to 97.5% RH.Electrical resistivity varies considerably with frequency for all samples, ranging by four magnitude orders from 20 Hz to 20 MHz. There was also a two-order-of-magnitude shift in resistivity with relative humidity discovered[31-33]. Humidity affects the material's electrical characteristics, causing a drop in electrical resistivity and an increase in permittivity at any frequency [34]. Furthermore, it can be demonstrated that the fuel agents dramatically alter the material's electrical characteristics, with similar results published in the literature.

The time response of CoCr_2-xGd_xO₄ samples were investigated in a closed enclosure at 25 °C constant temperature, when the sample was moved from 0%RH to 97%RH, by using an Impedance Phase Analyzer model Wayne Kerr 6400P at 1 kHz fixed frequency.

We study the effects of relative humidity dynamics on the electrical resistivity and capacity responses of porous materials [17, 35, 36]. The normalized electrical resistance and capacitance with relative humidity is depicted in fig 5(a) and 5(b)

On inspection of figure 5 (a), it is found that the resistance of the doped composition increases with increase in relative humidity and graph 5(a) shows that exponential behavior. The exponential nature of the graph reveals that in the beginning, resistance decreases rapidly, whereas for upper range of relative humidity slow decrease in resistance is observed. The value of normalized resistance is found high in Gd 0.02 content.

Fig 5(b) represents variation of electrical capacity in normalized form for CoCr_2-xGd_xO₄[(Gd 0.00); (Gd 0.02)] It was observed that for relative humidity increases the value of capacitance also increases. This increasing rate was faster at low at lower humidity level but high at higher humidity level. In general the capacitance value was also found to be increased with increasing of humidity. As RH increased, more H⁺ ions would be available in adsorbed water layers on the surface of sensor material, and, as a result, the capacitance value becomes higher. At lower humidity condition, due to discontinuity of water particles, much energy was required to transfer the protons from one hydroxyl group to its adjacent group via hopping mechanism.

In the present work Gd³⁺ doped and undoped CoCr₂O₄powderwere prepared successfully by solution combustion method using glucose and urea as fuels for the first of its kind. The impact of the o to 2 mole% rare earth (Gd³⁺) on the structural, Spectroscopic and electrical characteristics of CoCr₂O₄ studied in the present investigation. The phases, humidity conditions, cation distribution, are all highly related to electrical characteristics. Rare earth dissolution inside the crystalline spinel-type lattice of CoCr₂O₄ has a considerable effect on the electrical resistivity and relative permittivity in the frequency range 20 Hz–20 MHz and under various humidity circumstances. The value of electrical resistivity decreases with decreasing crystallite size of samples, with the maximum value of resistivity achieved in the case of the sample with x = 0 having the highest granulation. The Gd³⁺ substitution adds to a decrease in electrical resistivity, putting the material's resistivity into the measurable zone, which is beneficial for sensor applications. It was discovered that the Gd doped CoCr₂O₄ sample is the most susceptible to humidity. The representation from Figure 6 shows acceptable response times of about 150 seconds for electrical resistivity and about 200 seconds for electrical permittivity.

Acknowledgment

The Research Center for Advanced Materials Science (RCAMS)” at King Khalid University, Saudi Arabia, for funding this work under the grant number KKU/RCAMS/G012-21. Also, the authors extend their appreciation to the Deputyship for Research & Innovation, Ministry of Education, in Saudi Arabia, for funding this research work through the project number: (IFP-KKU-2020/10).

Research Data Policy and Data Availability Statements

The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request
The data that support the findings of this study are not openly available due to unpublished this work anywhere and are available from the corresponding author upon reasonable request.

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Synthesis and characterization of Novel Gadolinium doped CoCr2O4: Focus on Structural, Electric and Humidity Sensing properties

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Abstract

Figures

1. Introduction

2. Cocrgdo Synthesis Procedure

3. Xrd Analysis Of Gd Doped Cocro Powder Using Rietveld Refinement

4. Fourier Infrared Spectroscopy Analysis.

5. Electrical Properties

6. Humidity Influence On Electrical Properties

7. Humidity Sensors Characteristics

8. Conclusion

Declarations

References

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