Influence of Post-Annealing Treatment on Some Physical Properties of Cerium Oxide Thin Films Prepared by the Sol–Gel Method

Abstract

In this study, thin films of Cerium Oxide CeO₂ were fabricated using the sol–gel technique and deposited onto a glass substrate. The annealing process was carried out at various temperatures ranging from 200 to 600 $°\mathrm{C}$ to investigate the structural, morphological, and optical properties of the films and their interrelations. X-ray diffraction (XRD) patterns revealed the crystalline nature of the prepared films, with film quality exhibiting enhancement with increasing annealing temperature. The average crystallite size, dislocation density, microstrain, and lattice constant were determined from XRD patterns. Higher annealing temperatures were found to increase the crystallite size values from 4.71 to 15.33 nm and decrease the dislocation density and microstrain of the unit cell. Scanning electron microscope (SEM) images illustrated the uniformity of the films, presenting a spheroid shape. Optical properties such as transmittance, absorbance, reflectance, the direct band gap, extinction coefficients, the refractive index, and optical conductivity were assessed using optical measurements. The direct optical band gap of the CeO₂ film was observed to decrease from 3.99 to 3.75 eV with increasing film thickness. Using the Wemple and DiDomenico (WDD) single-oscillator model, dispersion energy parameters were calculated based on the refractive index. The nonlinear optical properties of the CeO₂ thin films were evaluated using these dispersion energy parameters. The improvement of optical parameters holds significance in standardizing CeO₂ thin films for various optoelectronic applications.

1. Introduction

Nanocrystalline thin films have attracted increasing research interest over the last three decades due to their small and intermediate particle sizes, typically ranging from 1 to 100 nm in at least one dimension. These thin films are extensively used in diverse fields including solar cells, storage systems, chemical and biological sensors, and optoelectronic devices.

Ceria (CeO₂), among metallic oxides, has received considerable attention due to its wide optical energy gaps ranging from 3.4 to 3.6 eV [1,2], high transparency in the visible and near-Infrared regions, and high refractive index [3]. It exhibits high chemical resistance, resistance to photo corrosion, and an excellent capacity for oxygen storage [4]. CeO₂ is a rare-earth oxide and is classified as an n-type semiconductor with a fluorite structure (FCC). Each cerium site is surrounded by eight oxygen sites, and each oxygen site is coordinated with the four nearest cerium cations [5]. Cerium oxide is an interesting material because it can exist in both +3 and +4 oxidation states, corresponding to CeO₂ and Ce₂O₃, respectively, which are appropriate for valency change switching procedures [6]. Cerium oxide thin films are widely used in electronics, catalysis, hydrogen production, gas sensing, and energy-related fields. The structure of cerium oxide thin films can vary depending on factors such as the deposition method, deposition conditions, substrate type, and composition. By controlling epitaxial growth and these parameters, polycrystalline films with varying grain sizes ranging from a few nanometres can be prepared [7].

Many studies have reported on the physical properties of CeO₂ thin films fabricated using various physical and chemical methods, including thermal evaporation, electron beam evaporation, spray pyrolysis, laser ablation deposition, sputtering, pulsed-laser deposition, and the sol–gel method [8,9,10,11,12].

In a recent study, Anandan and Parthasarathi Bera [13] investigated the growth of CeO₂ on a Si₃N₄ substrate using magnetron sputtering. They found that the interfacial reaction between CeO₂ and Si leads to the creation of silicate, and heat treatment improves this interfacial reaction. Additionally, CeO₂ thin films were deposited onto glass substrates using electron beam evaporation. It was observed that the film exhibited a crystalline structure with a smaller crystallite size, and the optical band gap increased with an increase in film thickness [14]. Moreover, cerium dioxide films deposited by atomic layer deposition (ALD) on TiN and Si substrates at 250 °C exhibited polycrystalline cubic phases on both substrates. However, an improvement in the diffraction signal was observed along the <111> direction for the TiN substrate compared to the Si substrate due to the higher interface roughness of CeO₂/TiN [15]. Furthermore, CeO₂ was synthesized with different molar concentrations using the spray pyrolysis method at 350 $°\mathrm{C}$. The resulting thin films exhibited cubic fluorite structures, with a preferred orientation of crystals along the (111) direction. Crystallite size, dislocation density, and lattice constant were found to be affected by the molar concentration [6].

The sol–gel method is a large-scale production technique, offering several advantages such as simplicity, low cost, the ability to produce large-area films with uniform thickness, and good adherence properties [16].

The flexibility of cerium oxide properties is thus a strength and a weakness and can open a variety of new concepts for thin-film growth. Changing the preparation conditions and methods can produce films with various properties. Annealing of the films can reduce stress and improve the crystallinity of the relaxed films, which can be used for device purposes. There are only a few studies on the effect of annealing treatment on the physical properties of CeO₂ using different preparation methods [17].

In this work, our aim is to explore a simple method of obtaining good-quality CeO₂ films using the sol–gel method followed by the spin coating technique and investigate the changes in structural, morphological, and optical properties as a result of changing the post-annealing treatment.

2. Experimental Method

Cerium oxide (CeO₂) thin films were prepared using the sol–gel method onto glass substrates. Cerium (III) chloride (CeCl₃, 99.9% pure, anhydrous, Thermo Scientific Chemicals) and distilled water were used to create precursor solutions at concentrations of 0.5 M. To achieve a clear and homogeneous solution, 2.4 g of cerium chloride was dissolved in distilled water and stirred using a magnetic stirrer at 60 °C for 2 h. Subsequently, the solution was kept at room temperature for 1 week. Glass substrates, cut from standard microscope glass slides with dimensions of 2 cm × 2.5 cm, were employed for coating. To ensure better adhesion, the glass substrates were properly washed with distilled water, methanol, and acetone in an ultrasonic bath and then dried to eliminate water molecules. The films were deposited using a spin coater (KW-4A) operating at 500 revolutions per minute for 10 s and then at 3000 revolutions per minute for 30 s. Finally, the films were annealed at various temperatures (200, 300, 400, 500, and 600 °C) using a furnace with a heating rate of 5 °C/min. The chemical reaction process was described as the thermal decomposition of cerium chloride to form clusters of cerium oxide in the presence of water, as follows [18]:

4 CeCl₃ + 3 H₂O = Ce (OH)₃+ 3 HCl

(1)

Ce (OH)₃ + O₂ + 2 H₂O = 4 Ce (OH)₄

(2)

Ce (OH)₄ = CeO₂ + 2 H₂O

(3)

The resulting CeO₂ thin films exhibited desirable properties, including thinness, strong adherence to the substrates, and uniformity.

Characterization

The structural characteristics of the thin films were analyzed using a Shimadzu XRD 700 X-ray diffractometer (Tokyo, Japan) with Cuk$\alpha$ radiation (λ = 0.15406 nm) to obtain XRD patterns. The elemental composition was assessed using Energy-dispersive X-ray (EDX) spectroscopy. The surface morphology of the films and film thickness were examined using Thermo Scientific Quattro (ESEM). The samples were mounted on aluminum stubs using double-sided sticky discs of conductive carbon, then gold-coated (up to 2–3 nm thick) using the sputtering method. Additionally, the optical properties were determined using the Thermo Scientific Evo 201 spectrophotometer, and the measurements were carried out within the wavelength range of 300 to 1000 nm.

3. Results and Discussion

3.1. Structural Analysis

The structural characteristics were investigated using X-ray diffractograms. The X-ray plots of CeO₂ thin films with various annealing temperatures are depicted in Figure 1. A distinct intense broad peak was observed at an angle of 32.28$°$ along the (200) plane in all samples. The X-ray plot peaks closely match the cubic ceria of a fluorite structure, consistent with the JCPDS data 34-0394, with a strong preferred orientation along the (200) plane [19]. It is evident from the XRD spectrum that the intensity of the (200) peak increases with higher annealing temperatures, which may be attributed to the relaxation of the structure induced by the heating treatment. The size of crystallites, dislocation density, and film microstrain were assessed to determine the effect of annealing temperature on lattice parameters.

Scherrer’s equation was used to calculate the average crystallite size:

$$D=\frac{0.94 \lambda }{\beta COS\theta }$$

(4)

where D is the average crystallite size, λ is the wavelength of the X-ray, $\beta$ is the full width at half maximum, and ϴ is the Bragg diffraction angle; the constant is equal to 0.94 for spherical crystallites with cubic symmetry.

Table 1. XRD-calculated parameters at different temperatures for CeO₂ thin films.

Film	Annealing Temperature (°C)	Average Crystallite Size D (nm)	$\mathbf{Dislocation} \mathbf{Density} \mathit{\delta }$ (×1016 lines/m3)	$\mathbf{Microstrain} \mathit{\epsilon }$	$\mathbf{Lattice} \mathbf{Constant}$ $\mathsf{(}\mathbf{\AA }\mathsf{)}$
CeO2	200	4.71	4.50	0.0768	5.498
	300	13.92	0.518	0.0260	5.538
	400	14.16	0.498	0.0255	5.541
	500	14.31	0.488	0.0262	5.542
	600	15.33	0.425	0.0236	5.546

presents the computed structural parameters. The size of the crystallites was determined to range between 4.7 nm and 15.33 nm for annealing temperatures of 200–600 $°\mathrm{C}$.

An increase in annealing temperature provides higher energy in the system, leading to the creation of thin films with larger crystallite sizes and good crystallinity. As the heating temperature increased, higher energy controlled crystallite growth, causing the particles to increase in size and resulting in a sharper intensity of the diffraction peaks. A similar trend was observed in another study [20].

On the other hand, the dislocation density δ and lattice strain $\epsilon$ were calculated using the following equations:

$$\delta =\frac{1}{D^2}$$

(5)

$$\epsilon =\frac{\beta cos\theta }{4}$$

(6)

According to Table 1, the reduction in the dislocation density and microstrain of the unit cells indicates the formation of higher-quality films, which can be observed from the sharp peaks in Figure 1. This reduction accompanying annealing temperature is attributed to the relaxation promoted by the higher energy provided to the system. The decrease in dislocation density indicates the formation of high-quality films.

The lattice constant is estimated by the Miller indices (hkl) and the interplanar spacing d using Equation (7):

$$d=\frac{a}{\sqrt{h^{2 }+k^2+l^2}}$$

(7)

From Table 1, it is evident that the lattice parameter values, which are consistent with those reported for CeO₂ thin films prepared by different techniques [6], exhibit significant improvement with an increasing annealing temperature. This enhancement can be attributed to the formation of oxygen vacancies and an increase in Ce³⁺ ions.

3.2. EDX Analysis

The elemental composition of the films was investigated using EDX spectroscopy. Figure 2 depicts the EDX spectra of the CeO₂ thin film. The highest peak corresponding to cerium was recorded at 4.4 keV, while the oxygen peak was observed at 0.25 keV. Other smaller peaks originate from the glass substrate and potential impurities such as Si and H. The detected Si element was from the substrate and the H element was expected to originate from the starting material used during the preparation of the thin films.

3.3. Microstructural Analysis

Scanning electron microscope measurements were performed to examine the surface morphology of CeO₂ deposited on glass and annealed at various temperatures. Figure 3 illustrates SEM images, revealing uniform film surfaces that effectively cover the substrates. A pronounced influence of annealing temperatures on the morphology of the CeO₂ films was observed. The films displayed a homogeneous distribution of grains on the substrates with low porosity at a high annealing temperature of 600 $°\mathrm{C}$. After annealing, the grain size increased and the film surfaces became denser. At higher annealing temperatures, spheroid shapes were formed by the growth of film crystals from a nucleation site. A similar result was shown in another study for CeO₂ prepared by the chemical route method [17].

Figure 4 displays the thickness of the CeO₂ thin films measured from SEM cross-sections at different annealing temperatures. As shown in Figure 4, the thickness of the films increased with higher annealing temperatures across all samples. This phenomenon can be attributed to the increased energy gained by the molecules due to the elevated annealing temperatures, as well as the enhancement of the potential force between the substrate and the films. This results in a reduction in porosity, as mentioned previously, and an increase in film thickness. The porosity of the films is inversely related to crystallite size as well as film thickness.

3.4. Optical Analysis

The optical band transition in the films offers vital details on the film’s electronic structure and helps in the construction of optoelectronic devices. The films’ transmittance, absorbance, and reflection spectra were obtained using UV–vis spectroscopy measured in room-temperature air. The wavelength dependence of the optical transmittance spectra for the CeO₂ films annealed at different temperatures (200–600 $°\mathrm{C}$), within the wavelength range of 300–1000 nm, is depicted in Figure 5. It is clearly seen that all films are highly transparent in the visible and near-infrared regions, with transmittance values ranging between 60% and 80%, indicating uniformity and good adherence to the glass substrates. However, it is observed that the transmission of CeO₂ decreased with increasing annealing temperatures due to certain physical effects, such as crystal orientation, low porosity, and an increase in the film thickness. Similar results have been reported by Elmas et al. for CdS thin films grown on glass substrates using a spray pyrolysis technique annealed at different annealing temperatures [21]. This finding is similar to that of CdS because the same dependencies of annealing temperature and transmittance have not been found for CeO₂ thin films. The decrease in transmittance is crucial for certain applications, particularly optical devices that necessitate controlled light transmittance [22].

The absorbance of CeO₂ thin films at different annealing temperatures is shown in Figure 6. It is observed that the band absorption edge occurs in the visible region at 315 nm and the absorbance spectra sharply increase with decreasing wavelength, indicating the initiation of interband transitions at the critical edge.

Reflectance measurement is key to understanding real absorption. Figure 7 displays the experimental reflectance spectra of CeO₂ thin films at different annealing temperatures. As can be observed, reflectance is clearly dependent on the post-annealing temperature. A larger crystallite size leads to increased optical reflectance. Another study observed similar results for CeO₂ thin films deposited by magnetron sputtering [23]. The increase in optical reflectance of the thin films with increasing film thickness can be attributed to light scattering within the film layers, resulting in reflections from the film surface.

Tauc’s equation was employed to predict the optical band gap (E_g) of the CeO₂ films [16]:

$$\left(\alpha hv\right)=A{(hv-Eg)}^n$$

(8)

where $hv$ is the photon energy and A is the transition constant; the exponent n depends on the type of transition and $\alpha$ is the optical absorption coefficient calculated using the following relation:

$$\alpha =\frac{1}{t }\ln T$$

(9)

where t is the film thickness and T is the transmittance.

For direct transition, n = 1/2. Tauc plots of CeO₂ films annealed at different annealing temperatures are shown in Figure 8. By extrapolating the linear portion at $\alpha =0$, the optical band gap energy of the films can be determined. It was found that, for all deposited films, an increase in annealing temperature led to a decrease in optical band gaps from 3.99 to 3.75 eV. Nunes et al. [24] observed a decrease in the band gap of TiO₂–CeO₂ multilayer thin films with increasing temperatures. The obtained values for the band gap E_g are in good agreement with the literature [9,23,25]. The reduction in the energy band gap can be attributed to the presence of oxygen ion vacancies, which act as positively charged centers occupied by electrons. These vacancies serve as donor centres, with their energy levels lying close to the valence band. Post-heating treatment modifies the energy gap by lowering the conduction band edge and correspondingly raising the valence band edge, akin to the Moss–Burstein shift observed in semiconductors [24,26]. Additionally, the narrowing of the film’s band gap may also result from the improvement in crystallinity with increasing annealing temperatures, as evidenced by the XRD results.

The extinction coefficient ($\kappa )$ was calculated from the following equation:

$$\kappa =\frac{\alpha \lambda }{4\pi }$$

(10)

where $\lambda$ is the wavelength of incoming light. The spectra of the extinction coefficient of the CeO₂ thin films deposited at different annealing temperatures are shown in Figure 9. A continuous variation in the extinction coefficient is obtained with increasing annealing temperatures.

Studying the refractive index of films offers several advantages. High-refractive-index films have been used in optoelectronic applications. The n values are influenced by many factors, such as molecular structure, film thickness, dopants, lattice defect, porosity, and stretching [24,26]. The relationship between reflectance and the refractive index (n) can be determined using the following equation:

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

(11)

The refractive index rises in direct proportion to the increase in annealing temperature, as seen in Figure 10. Furthermore, data analysis revealed a variation in refractive indices along the film thickness, consistent with established findings indicating enhanced crystallinity with increasing film thickness. Films with higher crystallinity typically contain larger crystallites, resulting in a larger refractive index.

The optical conductivity was obtained from the relation given by the following equation [25,27]:

$$\sigma =\frac{\alpha nC}{4\pi }$$

(12)

Figure 11 displays the variations in optical conductivity for CeO₂ thin films with different annealing temperatures. As observed from the figure, there is an enhancement in optical conductivity with increasing annealing temperatures. This variation in optical conductivity makes these films good candidates for many applications, such as nanophotonic systems, optical coatings, and optical devices.

The values of the refractive index can be fitted using a single oscillator by the Wemple and DiDomenico (WDD) relationship to estimate the dispersion parameters, such as oscillator energy (E_o), dispersion energy (E_d), and the static refractive index (n_o) [28].

$${(n^2-1)}^{-1}=\frac{E_0}{E_d}-\frac{1}{E_0{E}_d} {\left(h\nu \right)}^2$$

(13)

Figure 12 illustrates the (n² − 1)⁻¹ plots against (h$\nu$)², where linear regression was used to determine the values of E₀ and E_d from the intercept $\frac{E_0}{ E_d}$ and the slope, (E₀E_d)⁻¹. The obtained values of E₀ and E_d are listed in

Table 2. Dispersion parameters for CeO₂ thin films.

Film	$\mathbf{Annealing} \mathbf{Temperature} \mathsf{\left(}\mathsf{°}\mathbf{C}\mathsf{\right)}$	E0 (eV)	Ed (eV)	no	${\mathit{\chi }}^{\left(1\right)}$ (esu)	${\mathit{\chi }}^{\left(3\right)}$ × 10−13 (esu)	n2 × 10−11 (esu)
CeO2	200	2.081	5.028	1.18	0.192	79.5	25.20
	300	2.542	4.875	1.23	0.152	12.47	3.82
	400	2.591	4.3519	1.26	0.133	4.33	1.29
	500	2.67	3.675	1.31	0.109	0.781	0.249
	600	4.388	3.304	1.52	0.059	0.07	0.017

. The oscillator energy E₀ increases with increasing annealing temperature, as suggested by the WDD model (E₀ ≈ 0.7E_g). This increment may be attributed to the decrease in the energy gap. Conversely, the dispersion energy E_d values decrease when increasing the annealing temperature from 200 to 600 $°\mathrm{C}$.

The static refractive index (n₀) can be determined at zero photon energy by the WDD model as follows:

$$n_0=(1+\frac{E_o}{E_0} )$$

(14)

The estimated values of n₀ are listed in Table 2, revealing an enhancement with increasing annealing temperature, attributed to the densification of the annealed films. Films with high refractive indices have potential applications in various optoelectronic devices.

3.5. Nonlinear Optical Properties of the CeO₂ Thin Films

For a better understanding of the optical properties, the values of linear optical susceptibility ${\chi }^{\left(1\right)}$ and third-order nonlinear optical susceptibility ${\chi }^{\left(3\right)}$ can be determined using the dispersion energy parameter by the following relations [29]:

$${\chi }^{\left(1\right)}=\frac{E_{d/E_0}}{4\pi }$$

(15)

$${\chi }^{\left(3\right)}=6.82\times {10}^{-15}{\left(\frac{E_d}{E_0}\right)}^4$$

(16)

The calculated values of ${\chi }^{\left(1\right)}$ and ${\chi }^{\left(3\right)}$ are listed in Table 2 for CeO₂ thin films. It is evident that the value of χ⁽³⁾ changes with increasing annealing temperatures, which can be attributed to alterations in the material structure and homogenization of the films due to the increasing rates of chemical reactions during the annealing process [30].

The nonlinear refractive index n₂ can be determined from the following relation [31]:

$$n_2=\frac{12\pi {\chi }^{\left(3\right)}}{n_0}$$

(17)

The values of n₂ are recorded in Table 2. The results indicate that the nonlinear refractive index of CeO₂ thin films decreased with increasing annealing temperatures due to the reduction in defect gap states and changes in polarization of the films during the annealing treatment. Therefore, the nonlinearity parameters decreased during the annealing process.

4. Conclusions

In summary, CeO₂ thin films were successfully fabricated via the sol–gel technique and deposited onto glass substrates, followed by annealing at different temperatures (200, 300, 400, 500, and 600 $°\mathrm{C}$). The effects of annealing temperature on the structure, morphology, and optical properties of the CeO₂ films were thoroughly investigated. XRD analyses confirmed a noticeable improvement in the crystallinity of the deposited films with increasing annealing temperatures, revealing a cubic ceria fluorite structure. The main XRD diffraction peak located at 2ϴ = 32.28° corresponds to the (200) plane phase. Annealing temperature escalation led to an increase in crystallite size (from 4.71 to 15.32 nm) and film thickness (from 18 to 67.78 mm), along with a reduction in dislocation density and microstrain of the unit cells due to the relaxation promoted by higher energy. EDX analysis confirmed the presence of cerium particles on the surface, while SEM surface morphology revealed a homogeneous distribution of spheroid crystals with low porosity at higher annealing temperatures. UV-visible transmission spectra showed high transparency of all films in the visible and near-infrared regions, with the band absorption edge observed at 315 nm. However, film reflectance increased with increasing film thickness. Variations in the annealing temperature resulted in modifications to the electronic structure of the films, leading to a decrease in the energy gap. The extinction coefficient, refractive index, and optical conductivity of the films were found to increase with increasing annealing temperatures. Oscillator energy (E₀), dispersion energy (E_d), and the static refractive index were calculated using the WDD model. Nonlinear optical parameters, such as linear optical susceptibility ${\chi }^{\left(1\right)}$, third-order nonlinear optical susceptibility ${\chi }^{\left(3\right)}$, and the nonlinear refractive index n₂, exhibited a decline during the annealing process due to variations in polarization and changes in the film structure. The changes in the physical properties of CeO₂ thin films are dependent on the post-annealing treatment. Thus, the annealing effects on the physical properties of CeO₂ thin films will be useful for the formation of highly crystalline and high optical conductivity thin films for optoelectrical devices, thin-film sensors, and solar cells.