Effects of CE doping on the structural,optical and electrical properties of spinel SrCo 2 O 4 thin films

Abstract

Cerium-doped strontium cobaltite spinel (SrCo₂O₄) thin films were synthesized using a sol–gel dip process and subsequently deposited onto glass substrates by a coating technique. The influence of Ce doping on structural, optical and electrical properties has been studied using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), UV–Vis spectrophotometry and impedance spectroscopy. The XRD analysis indicates that all samples exhibit a single-phase cubic spinel structure assigned to the Fd3 m space group, with the (311) plane emerging as the dominant orientation. IR spectra revealed the presence of vibrational modes associated to spinel SrCo₂O_4. Deposited thin films show average transmittance above 80% in the visible range. The impedance measurements show that the equivalent circuit of the diagram of Ce doped SrCo₂O₄ layers is an RpCp parallel. when the resistance Rp decreases while the capacitance Cp increases with Ce doping. Cerium incorporation alters the structural, optical, and electrical characteristics of SrCo₂O₄, leading to an increase in average crystallite size to 27.93 nm, a decrease in the optical band gap to 1.40 eV, a reduction in resistance to 33.06 Ω, and an enhancement in capacitance to 7.32 nF at a doping level of 5%.

Keywords

thin films doping sol-gel dip-Coating

Introduction

Faced with a critical energy situation in today's world, research has increased in order to develop functional materials with low size, cost and environmental impact. In this context, structured nanomaterials based on oxide semiconductors are essential for the development of devices for the production, conversion and storage of energy.^1,2 Among semiconducting metal oxides, spinel-type continues to fascinate researchers with their exceptional optical, electrical, mechanical, magnetic, elastic, and structural properties, leading to the prospect of extensive exploitation and therefore potential applications in many areas of technology, including optoelectronics devices.^3–10 Spinels constitute a large family of mixed oxides whose generic formula is AB₂O₄, with A and B being metal cations with oxidation states 2 and 3, respectively.¹¹ Among transition metal oxides, substituted products of cobalt cobaltite (Co₃O₄) with general formula MCo₂O₄ (M = Ni, Cu, Zn, Mn, Mg, Cd, etc.) are an important group of materials which have given extensive interest for electrochemical applications, especially for batteries, supercapacitors, and electrocatalysis, owing to their particular structural, optical and electrical properties.^12–17 Doping with metallic and non-metallic elements could further contribute to the design and manufacturing of materials with improved morphological and optoelectronic properties.¹⁶ Several metallic dopants have been added to the antiferromagnetic p-type transparent semiconductor Co₃O₄ for the improvement of its electrochemical stability like: Sb, Cr, Ni, Cu, Zn, Sn, Mg, Mo, Fe, Ag, etc. Indeed, the presence of two metal cations with mixed valences facilitates electron transport with relatively low activation energy.^18–26 Because cerium exhibits multiple oxidation states such as Ce³⁺ and Ce⁴⁺, it can be used as a dopant to improve the electrochemical properties of many nanostructured materials.^27,28 The choice of an appropriate preparation method has a significant impact on the properties and applications of thin films.²⁹ Number of synthesis methods is known to prepare thin films of cobalt-based spinel oxides such as: thermal decomposition,³⁰ spray pyrolysis,³¹ microwave,³² radio-frequency magnetron sputtering,³³ pulsed laser deposition³⁴ and sol-gel dip-coating.^35–37 The sol-gel dip-coating method offers benefits such as speed, simplicity in fabrication, excellent reproducibility and film uniformity.³⁸ However, the homogeneity and properties of the films obtained by this method depend on a number of factors, notably the solubility of the coating components used as well as the concentration and viscosity gradients induced by evaporation as a function of time in the solution.^39,40 In the present work, three oxides pure SrCo₂O₄ and Ce-doped SrCo₂O₄ at 3 and 5%, were deposited on pyrex glass by dip coating process at room temperature and their structural, morphological, optical and electrical properties were investigated.

Experimental details

Ce-doped spinel SrCo₂O₄ thin films were deposited on cleaned Pyrex glass substrates using the sol–gel dip-coating method. The first precursor solution was prepared by dissolving 2.91 g of cobalt nitrate (Co(NO₃)₂·6H₂O) (Sigma Aldrich, 99.8%) in 120 mL of ethanol (C₂H₆O) (Sigma Aldrich, 99.8%). Magnetic stirring for 30 min yielded a uniformly colored solution. The second precursor, a strontium nitrate solution, was obtained by dissolving 4.36 g of Sr(NO₃)₂ in 120 mL of ethanol. After blending both solutions, citric acid (Sigma Aldrich, 99.5%) was added. The mixture was then slowly heated to 80 °C and maintained under magnetic stirring for 5 h, forming a purple-red sol.

Cerium precursor solutions were prepared by dissolving 0.2 g of Ce(NO₃)₃·6H₂O (Sigma Aldrich, 99.9%) in ethanol and stirring for 2 h, producing a clear and slightly viscous solution.

The prepared sols were loaded into the dip-coating system. Depositions were performed as single-layer coatings onto well-cleaned Pyrex glass slides (25 mm × 75 mm × 1 mm) at room temperature, under 40% relative humidity, and at a withdrawal speed of 50 mm/min. Three films were prepared: one undoped SrCo₂O₄ and two Ce-doped SrCo₂O₄ films containing 3% and 5% Ce. The dip-coated layers were dried at 100 °C for 20 min and subsequently annealed at 500 °C. The complete procedure is summarized in Figure (1).

Figure 1.

The sol–gel dip-coating process for fabricating Ce-doped SrCo₂O₄ thin films.

Characterizations

The prepared thin films of SrCo₂O₄ and Ce-doped SrCo₂O₄ were characterized by carrying out structural, optical and electrical studies. The X-ray diffraction pattern of the prepared samples was collected by using a Rigaku miniflex 600 X-ray diffractometer with Cu-Kα line (λ=1.5406 Å) in a wide range of 2θ, i.e., from 0° to 80°.

The FTIR spectrum was recorded on the Shimadzu 8400 FTIR spectrophotometer from 400 cm⁻¹ to 4000 cm⁻¹. Optical properties from UV–Vis spectroscopic studies were obtained by using the Shimadzu 1650 UV–Vis spectrophotometer. Finally, the analysis by complex impedance spectroscopy was carried in the frequency range 75 kHz to 20 MHz with oscillation amplitude of 1 V using Agilent 4284A LCR-meter.

Results and discussion

Structural characterization

The phase and crystal structure information of the SrCo₂O₄ and Ce-doped SrCo₂O₄ thin films were studied using XRD as displayed in Fig. For the SrCo₂O₄ thin film, the diffraction peaks at 2θ = 18.81, 31.13, 36,61, 38.23, 44.49, 55.19, 59.19, 65.07, 68.45 and 73.55 are corresponds to (111), (220), (311), (222), (400), (422), (511), (440), (531), (620) planes, respectively. The ten diffraction peaks recorded can be well indexed with the cubic spinel structure (space group of Fd-3 m) of Co₃O₄ (JCPDS no. 42-1467).^41–42 No new diffraction peaks on the diffractograms of the doped samples confirming the formation of single-phase spinel cubic structure and indicating that Ce was substituted into the crystal lattice of strontium cobalt oxide (Figure 2).

Figure 2.

XRay diffraction pattern for SrCo2O4 and Ce-doped SrCo2O4.

In addition to the average crystallite size (D), d spacing, lattice constant (a), and unit cell volume (V_cell) of thin films were calculated from the most intense (311) plane of the samples using the following equation.^43–44

D = \frac{k λ}{β c o s θ}

(1)

Where β is full width at half maximum of intense peak, θ is the Bragg angle of the intense crystallographic plane, k is a constant (0.9) and λ = 1.5406 Å.

a = d_{h k l} \sqrt{h^{2} + k^{2} + l^{2}}

(2)

V_{c e l l} = a^{3}

(3)

The elastic deformation of thin films, which contributes to peak broadening, can be estimated by using the formula⁴⁵:

δ = 1 / D^{2}

(4)

The microstrain and dislocation density of thin films are calculated from the following equations (Figure 3).⁴⁶

ε = β c o s θ / 4

(5)

Figure 3.

Crystallite size, Microstrain and dislocation density variations for pure and Ce-doped SrCo2O4.

The calculations gave the results recorded in Table 1. The results grouped in Table 1, show that the crystallite size of the samples increases while the dislocation density and lattice strain decrease for low Ce doping, then the opposite phenomenon is obtained with the increase in the doping rate.

Table 1.

Structural parameters as crystalline size, lattice parameter, cell volume, micro strain and dislocation density of SrCo₂O₄ and Ce-doped SrCo₂O₄.

Sample	Undoped SrCo₂O₄	SrCo₂O₄ :Ce 3%	SrCo₂O₄ :Ce 5%
Β	0,0061928	0,0055822	0,0052333
Θ	18,305	18,42	18,44
cos θ	0,949398	0,94876577	0,9486554
Sinθ	0,314075	0,31598023	0,3163114
Average crystallite size D (nm)	23,58	26,18	27,93
d _hkl (Å)	2453	2438	2435
Lattice constant a (Å)	8136	8086	8076
Unit cell volume V_cell (Å)³	538,56	528,69	526,73
Dislocation density δ (lines/m²)	17,98 10¹⁴	14,59 10¹⁴	13,42 10¹⁴
Microstrain ε (line⁻² m⁻⁴)	0,0014698	0,0013240	0,0012411

Infrared spectroscopy

Infrared absorption spectra of the pure and doped SrCo₂O₄ are shown in Figure 4.Two strong peaks appeared for all the samples at 582 cm⁻¹ and 661 cm⁻¹ are respectively attributed to the M-O vibrations in the octahedral and tetrahedral sites of the AB₂O₄ spinel lattice,⁴⁷ thus confirming the formation of SrCo₂O₄ material with a spinel structure. The peak at 1618 cm⁻¹ and the broad band centered at about 3389 cm⁻¹ are respectively assigned to the bending and stretching modes of the -OH groups of the adsorbed water.⁴⁸ The FTIR spectra of the doped samples are similar to that of the undoped sample, which illustrates the similar nature of the chemical bonds. The interaction between the cerium ions incorporated with the native strontium and cobalt ions can be deduced from the change in intensity of the peaks relating to the spinel structure (ie. the peaks at 582 cm⁻¹ and 661 cm⁻¹).⁴⁹

Figure 4.

IR spectrum of pure and Ce-doped SrCo2O4.

Optical characterisation

The influence of cerium doping concentration on the optical properties of SrCo₂O₄ thin films was studied by optical measurements. Figure 5.A illustrates the transmittance spectra of both undoped and Ce-doped films at different concentration within the wavelength range of 350-900 nm. All deposited films present a high transparency, the undoped SrCo₂O₄ films displayed an optical transmittance surpassing 80%. With increasing Ce doping levels, the transmittance values progressively improved, reaching values exceeding 80%. These differences in optical transmittance over the visible range are due to changes in crystallite size and surface roughness generated by Ce incorporation.⁵⁰ A similar behavior has been observed by Boudjouan, F et al., where enhanced crystalline quality was found to reduce light scattering from the grains constituting the film, thereby increasing its transparency.⁵¹

Figure 5.

(A) Transmittance, (B) Absorbance and (C) Absorption coefficient (D) Tauc plot of spectra of pure and Ce-doped SrCo2O4.

Figure 5.B depicts the UV-Vis absorption spectra of pure SrCo₂O₄ and Ce-SrCo₂O₄ thin film. This figure indicates that the undoped SrCo₂O₄ film exhibits higher absorbance compared to the doped ones. The decrease in absorbance observed in the doped samples may be attributed to defect states arising from lattice imperfections caused by the varying Ce concentrations and may also be linked to the deterioration of crystallinity at higher doping levels.⁵² The absorbance spectra shown in Figure 5.C were also used to determine the optical band gap of the undoped SrCo₂O₄ and Ce-SrCo₂O₄ nanostructures. Tauc plots have been drawn for each nanostructure based on the Transmittance data, using the Tauc relation presented in the following equation:

(α h ν)^{m} = A^{*} (h ν - E g)

(6)

Here, α denotes the absorption coefficient, ν is the photon frequency, A is a constant, and h represents the photon energy. The exponent m takes the value 1/2 for an indirect allowed transition and 2 for a direct allowed electronic transition. The band-gap (Eg) values were obtained by extrapolating the linear region of the Tauc plot of (αhν)² versus hν, as illustrated in Figure 5.D, and the corresponding results are summarized in Table 2.

Table 2.

The optical band gap and Urbach energy for the undoped and Ce-doped SrCo₂O₄ thin films.

Ce content at. %	Bandgap (eV)		Urbach energy (meV)
Ce content at. %	E_g1 (eV)	E_g2 (eV)	E_u1 (meV)	E_u2 (meV)
0	1,48	1.76	48	697
3	1,44	1.71	70	704
5	1,40	1.70	150	753

The direct upper bandgap value of Ce-doped SrCo2O4:Ce Thin Films was observed in the range of (1.76 - 1.70 eV). According to Table 2, it is observed that the bandgap continues to decrease with the increase in Ce content; this decrease may be due to the formation of sub-bands between the energy bandgap and the merging of their sub-bands with the conduction band to form a continuous band. This result has been obtained by several researchers who have doped different materials with cerium.^53–55

Compared to absorption behavior in strongly doped crystalline semiconductors, sub-bandgap absorption can be described by the Urbach law which relates to the characteristic distribution and width of band-tail states.⁵⁶Urbach tails generally develop in materials with low crystallinity, structural disorder, or amorphous character, as these states extend into the bandgap.^57,58

The relationship between the absorption coefficient (α) and photon energy (hν) in the low-photon-energy region follows the empirical Urbach rule, expressed as⁵⁹:

α = α_{0} e x p (\frac{h ν}{E_{u}})

(7)

l n α = \ln α_{0} + (\frac{h ν}{E_{u}})

(8)

Where $α_{0}$ is the pre-exponential factor, ( $h ν$ ) the photon energy and $E_{u}$ is the band tail width or disorder energy.

Hence, the Urbach energy $E_{u}$ of the SrCo₂O₄ thin films may be estimated from the slope of the linear fit obtained by plotting ln(α) versus the incident photon energy (hv), as represented in Figure 6.

Figure 6.

Urbach energy of undoped and doped SrCo2O4 films with different percentage ratios of Ce.

The lowest Urbach energy was obtained for the undoped SrCo₂O₄ film, indicating strong crystallinity and a low density of defects. However, the Urbach energy rose with doping, implying more structural disorder, which may be related to the growth of surface roughness. It can be observed from the data in Table 1 that the $E_{u}$ values increase with the Ce doping level. This increased disorder correlates with a reduction in the optical band gap energy (Eg), in line with the well-established inverse relationship between bandgap energy and Urbach one in disordered semiconductors. Figure 7. shows this linear relationship between the band gap energy and the Urbach tail width which is consistent with previous reports and may be attributed to the reduction in grain size. In addition, the increase in Urbach energy can cause a narrowing of the band gap due to the extension of the band tails.⁶⁰

Figure 7.

The variation of optical band gap and. Urbach energy of pure and Ce dopedCo2O4 thin film.

Refractive Index and extinction coefficient

The refractive index (n) and other dispersion parameters play a crucial role in determining the electronic properties of semiconductor materials employed in optoelectronic devices.⁶¹

The refractive index (n) which fully describes how the material interacts with light, is related to the complex refractive index (n*) through the following relation⁶²:

n^{*} = n + i k

(9)

Here, n indicate the real part of the complex refractive index, representing the extent to which light is retarded through the material. It is an important optical characteristic for solar cell absorber layers, while k is the imaginary part, also referred to as the extinction coefficient, it quantifies the attenuation of light within the material due to absorption or other inelastic scattering processes, such as the Compton effect, photoelectric effect, and pair production. The following relations contribute to determining the value of n and k, respectively⁶³:

n = \frac{1 + \sqrt{R}}{1 - \sqrt{R}}

(10)

k = \frac{α . λ}{4 π}

(11)

Where R, $α$ and $λ$ are reflectance, absorption coefficient and incident wavelength, respectively.

The calculated refractive index spectra (n) and the extinction coefficient for the synthesized SrCo₂O₄ layers are shown in Figure 8. A and B. As we can see, a further increase in Ce content leads to a reduction in the refractive index, which is likely associated with defect formation and impurity incorporation in the thin films induced by Ce doping.⁶⁴ Materials with a low value of (n) are very suitable for SC absorber material and an antireflection layer.⁶⁵

Figure 8.

(A) Refractive index and (B) extinction coefficient spectra of pure and Ce-doped SrCo2O4.

Dielectric constants

The polarizability of a solid is directly proportional to its dielectric constant, and its variation with photon energy provides insight into the nature of photon–electron interactions within the films over the investigated energy range.

The materials having a higher value of dielectric constant are enriched with the intrinsic charge carrier.⁶⁶ The real and imaginary parts of complex dielectric constants mainly depend on the values of n and k, are expressed as⁶⁷:

ε_{r} = n^{2} - k^{2}

(11)

ε_{i} = 2 n k

(12)

Where $ε_{r}$ and $ε_{i}$ are the real and imaginary parts of the dielectric constant, n is the refractive index and k is the extinction coefficient respectively. The variation of $ε_{r}$ and $ε_{i}$ values of the samples with photon energy are shown in Figure 9 and listed in Table 3. It's well observed that the real and imaginary parts follow the same pattern. Since according to the previous relation, $ε_{r}$ depends on the square of n (subtraction is negligible), whereas $ε_{i}$ is related to multiplication of n and k. For this, the real parts of the dielectric constant are higher than that of the imaginary parts of the dielectric constant.⁶⁸

Figure 9.

(A) Variation of real dielectric constant (B) imaginary dielectric constant with wavelength of SrCo₂O₄ thin films.

Table 3.

The refractive index, extinction coefficient and dielectric constants values of the samples at wavelength of 750 nm.

Samples	Refractive index	Extinction coefficient	Real part of the dielectric constant	Imaginary part of the dielectric constant
Pure SrCo₂O₄	1.26	0.024	1.59	0.061
SrCo₂O_4: Ce 3%	1.14	0.009	1.31	0.020
SrCo₂O_4: Ce 5%	1.12	0.007	1.27	0.016

Complex impedance analysis

In order to explain the conduction and polarization processes of the synthesized materials, a complex impedance spectroscopy analysis was conducted in the frequency range of 75 kHz to 20 MHz at room temperature. The Nyquist plots (-Z” vs Z’) of the pure and doped SrCo₂O₄ is showed in Figure 10. As shown in Figure 5, the complex impedance spectra of the thin films at different doping ratios are all configuration as depressed semicircles. It shows that with increasing doping rate, the semicircle of the Nyquist plot becomes smaller, suggesting that the strength of the material gradually decreased. The literature has shown that the amplitude of the real and imaginary part of the impedance are strongly affected by the resistance.⁶⁹ The electrical properties of the material mainly linked to the electrical properties of the grains and the grain boundaries are described by an equivalent electrical circuit comprising two branches consisting of a parallel connection (RpCp).⁷⁰

Figure 10.

Nyquist plots of undoped and Ce-doped SrCo2O4 thin films.

The diagrams of the films correspond to semicircles whose equation below allows us to reduce the value of the capacity of the grain boundaries of the oxide layer from the Nyquist plots⁷¹:

C_{p} = \frac{1}{2 π f_{m a x} R_{p}}

(13)

Where $f_{m a x}$ is the frequency of the external applied field at the apex of the circular arc. The resistance $R_{p}$ was determined by the intersection of the Nyquist plot with the real axis.

The electrical capacitance (C_P) and resistance (R_P) of the pure and doped thin films were calculated from the impedance complex measurement and their variations are graphically represented in Figure 10. The reduction of the bandgap energy has a direct impact on the conductivity of the doped materials thus causing the decrease in resistance. On the other hand, the capacitance increases from 6.41nF to 7.32 nF with Ce doping (Figure 11).

Figure 11.

Variation in the resistance and capacitance of thin SrCo2O4 films as a function of Cerium doping rate.

We notice from Figure 6 and Table 4, this shift is also due to the introduction of cerium ions into the SrCo₂O₄ lattice, which induces a variation in particle size and consequently introduces more grain boundaries in the samples. Two conduction mechanisms are simultaneously present: conduction through the grains and conduction through the grain boundaries. The effect of the grain boundaries in the samples becomes more dominant compared to the contribution of the grains in the conduction mechanism.

Table 4.

Values of: $f_{m a x}$ , Rp, and Cp of and Ce-doped SrCo₂O₄ thin films.

Samples	$f_{m a x}$ (KHZ)	Rp(Ω)	Cp (nF)
Pure SrCo₂O₄	519.44	47.75	6.41
SrCo₂O₄ :Ce 3%	519.44	40.95	7.13
SrCo₂O₄ :Ce 5%	519.44	33.06	7.32

The results obtained suggest that cerium doping is a promising strategy for optimizing Co3O4-based electrode materials for batteries, paving the way for the development of efficient and cost-effective energy storage systems.^72,73

Conclusion

Structural, optical, and electrical properties of pure SrCo₂O₄ and Ce-doped SrCo₂O₄ thin films synthesized using a Dip-coating method with different Cerium concentrations of 0,3% and 5% are investigated. XRD patterns and IR spectra indicated that Ce ions substituted the Sr ions without altering the material spinel structure. The average nonmetric size of the crystallites increases then begins to decrease with the increase in the doping rate. The films showed highly transparency in the visible region which is favourite for the optoelectronic application in the development of transparent device, and the transmittance increases with the raising. The direct bandgap energy of revealed the SrCo₂O₄ thin flms depend strongly on the Co/Ce molar ratio. The complex impedance spectra of the thin films at different doping ratios are all configuration as depressed semicircles.

Footnotes

Acknowledgments

The authors would like to extend their thanks to Prof. El-Habib BELARBI as well as all the members of the Synthesis and Catalysis Laboratory of the Faculty of Material Sciences at the University of Tiaret for all the measurements carried out within the framework of this work.

ORCID iD

Abdelmalek Kharroubi

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

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