Research Article | Open Access
Y. J. Acosta-Silva, M. Toledano-Ayala, G. Torres-Delgado, I. Torres-Pacheco, A. Méndez-López, R. Castanedo-Pérez, O. Zelaya-Ángel, "Nanostructured CeO2 Thin Films Prepared by the Sol-Gel Dip-Coating Method with Anomalous Behavior of Crystallite Size and Bandgap", Journal of Nanomaterials, vol. 2019, Article ID 5413134, 8 pages, 2019. https://doi.org/10.1155/2019/5413134
Nanostructured CeO2 Thin Films Prepared by the Sol-Gel Dip-Coating Method with Anomalous Behavior of Crystallite Size and Bandgap
Amorphous CeO2 thin films were deposited by a dip-coating method on Corning glass substrates and annealed for one hour at the temperatures () of 250, 450, and 550°C in air for crystallization. The precursor solution was prepared by dissolving cerium acetate in methanol, lactic acid, glycerol, and trimethylamine at 55°C. X-ray diffraction (XRD) patterns showed the cubic structure of CeO2. From XRD data and employing the Scherrer formula, the crystallite size (CS) was calculated to be within the to interval. SEM micrographs revealed cracks of the films annealed at 250 and 450°C, even though for 550°C, the film shows a homogeneous morphology free of cracks. CS increases (from 4.0 to 10 nm) and thickness decreases (from 217 to 182 nm) when increases. The UV-vis spectra exhibited an average transmittance of 80% in the 300 to 2000 nm wavelength range. Also, from XRD, it was observed that the lattice shrinks and from transmittance that the bandgap energy increases with . The Raman spectra exhibit 461 cm-1 assigned to F2g mode of the fluorite cubic structure, where F2g hardens when increases as an effect of the shrinkage of the lattice.
Cerium dioxide is one of the most abundant rare earth that have attracted attention due to its interesting properties that include chemical stability, high values for the refractive index, optical transparency in the visible region, and hardness [1, 2]. This oxide has a wide versatility for its multiple properties adequate for many industrial applications, which can be an appropriate substitute of ZnO and TiO2 [3, 4]. The CeO2 is stable even in the substoichiometric form (CeO2-, ) and thus has been easily produced by several deposition techniques . The CeO2 thin films have been used in different applications such as solar energy conversion , gas sensors , catalysis , and corrosion-resistant and protective coatings . The CeO2 thin films can be prepared by several methods such as pulsed laser deposition (PLD) , magnetron sputtering , spray pyrolysis [12, 13], and chemical vapor deposition (CVD) . Another critical topic apart from the deposit techniques is the first-principles calculations of different materials. Goyenola et al.  have made recent studies. However, most of these preparation methods require a large amount of energy, complex devices, and vacuum during film deposition. The sol-gel dip-coating technique is simple and economical; the possibility to process at low temperature, large surface area coatings, and substrates with complex shapes can be used. Also, films with crystalline quality, chemical stability, and high optical transmission can be obtained [16, 17].
On the other hand, the CeO2 films are usually synthesized by the sol-gel method using cerium chloride or cerium nitrate as the precursor and citric acid as the stabilizer. Wang et al.  reported the synthesis of CeO2 films by the sol-gel method using cerium nitrate as the precursor and citric acid as the chelating agent, and they also studied the effect of annealing temperature on oxygen vacancies. Verma et al.  prepared CeO2 films with cerium chloride heptahydrate (0.22 M) and citric acid with different molar ratios; the addition of citric acid to the precursor leads to homogeneity and a reduced ion storage capacity in the films. Škofic et al.  also reported the synthesis of CeO2 films using cerium chloride heptahydrate in a mixture of citric acid and ethanol, where the films were heat-treated in an air or argon atmosphere. The structural, electrochemical, and optical properties of these films depended on the preparation conditions.
In the present work, thin films of CeO2 were synthesized by the sol-gel dip-coating method; the precursor solution was prepared using cerium acetate as the precursor and lactic acid as the stabilizer. The films were analyzed by X-ray diffraction (XRD), Raman spectroscopy, UV and visible spectroscopy (UV-vis), and scanning electron microscopy (SEM). In the characterization, the effect of the annealing temperature on the structural, optical, and morphology properties of CeO2 thin films was studied, where it was found that stress induced by oxygen vacancies (VOs) prevails on the quantum confinement effect. To know the way of how VOs affect the lattice by considering the quantum confinement and strain requires first-principles studies at nanometric dimensions, similar to the work carried out by Goyenola et al. in fullerene-like CS . In this sense, it is worth mentioning that Kossoy and coworkers have associated elastic anomalies to stress due to VOs in CeO2 . These anomalies can produce desirable or undesirable effects in the material since affecting the band structure and, hence, the optical properties . Besides, the application of ferromagnetic semiconductor and oxides as the material base for spintronics has stimulated the search of these types of materials. Recent works report that strain induces ferromagnetism in semiconductors and oxides , such is the case of ferromagnetism induced in Nd-doped CeO2 nanoparticles due to VOs provoked by the doping .
The CeO2 thin films were prepared by the sol-gel dip-coating method. The precursor solution was prepared by dissolving 1 mol of cerium acetate (Ce(CH3CO2)3·xH2O) in 100 mol of methanol; after that, 1.5 mol of lactic acid, 0.2 mol of glycerol, and 1 mol of trimethylamine were added to the solution at 55°C and kept under stirring conditions for 1 hour. Finally, the precursor solution was cooled at room temperature. Thin films were deposited by the dip-coating method on glass substrates, and withdrawal speed of 10.0 cm/min was used; the surrounding relative humidity during the film removal was lower than 30%. The dip-coating process was applied eight times; after each coat, the film was dried at 250°C for 5 min; when the cycle was completed, the films were dried at 250°C for 1 hour in air and afterward sintered at different temperatures () (250, 450, and 550°C) for 1 h. The phase of the CeO2 thin films was determined using a Rigaku D/max-2100 diffractometer (Cu kα radiation, 1.5406 Å) in the range of 20-90° for an incidence angle of 0.5°. The crystallite size was calculated employing the Scherrer equation. Structural properties were also studied by Raman spectroscopy. Raman spectra were measured at room temperature in a wavelength range between 300 and 700 nm using a Labram-Dilor Raman spectrometer, with a He-Ne laser as the exciting source. The morphology and thickness of the films were studied by scanning electron microscopy using an XL 30ESEM Philips microscope at 50000x. Optical transmittance was measured with a Cary 5000 UV-vis-NIR spectrophotometer in the 200-1000 nm range. The surface roughness of the films was measured by using an atomic force microscope (AFM), Park Scientific Inst. System.
3. Results and Discussion
3.1. Structural Properties
3.1.1. X-Ray Diffraction (XRD)
X-ray diffraction (XRD) patterns of the films for different annealing temperatures are displayed in Figure 1. The XRD pattern for the film annealed at 550°C presents peaks at 28.54°, 33.07°, 47.48°, 56.34°, 59.09°, 69.41°, 76.70°, 79.07°, and 88.42° corresponding to the planes (111), (200), (220), (311), (222), (400), (331), (420), and (422), respectively, which are in good agreement with JCPDS card No. 43-1002 of the fluorite cubic structure. For the diffraction patterns of the films annealed at 250 and 450°C, the absence of some diffraction peaks suggests a decreasing in crystallinity. The left inset of Figure 1 exhibits the shift toward larger 2θ of the (111) diffraction values when increases. This behavior, in turn, indicates that the lattice shrinks, as observed in the right inset where the (111) and (002) interplanar spacings (ISs) are plotted as a function of , and how both ISs decrease when rises. The decreasing of the interplanar spacings is originated by strain provoked by the creation of VOs.
3.1.2. Crystallite Size and Thickness
Figure 2 illustrates the variation of the thickness, measured from SEM images, with (left axis). The decreasing of the thickness when increases suggests that, since the nonannealed films were pieces of the same sample, the CeO2 lattice shrinks probably due to the VO formation, in such a way that more temperature equals larger desorption of oxygen. The crystallite size (CS) of the CeO2 films was calculated from the (111) and (002) diffraction peaks using Scherrer’s equation (), where the shape factor is 0.9, , is the full width at half maximum (FWHM) of the (111) peak, and is Bragg’s angle . Figure 2 (right axis) displays the CS dependence with the annealing temperature; observe that two different values of CS were obtained from the two peaks; the asterisks indicate mean values. Larger CS generally corresponds to better crystallinity; thus, CeO2 films annealed at 550°C got the best crystalline quality. The average crystallite size of the CeO2 films was found to be in the range to , which agrees with films synthetized by the sol-gel method . Strain alters the real value of CS ; Figure 2 shows that CS values calculated depend on the crystalline direction. The reason could be that, as can be observed in XRD patterns of Figure 1, the films show a certain grade of preferred orientation along the (111) direction, which is where CS is measured. Consequently, due to the free surface of the films, the strain is larger than that on the (002) that have more projection to the substrate (see inset of Figure 2) which opposes the strain. This effect is also observed in the IS of the right inset of Figure 1.
3.2. Morphological Properties
3.2.1. Scanning Electron Microscopy (SEM)
The morphology of the films was also determined by scanning electron microscopy. The surface morphology of the CeO2 films annealed at 250°C, 450°C, and 550°C is shown in Figures 3(a)–3(c). The SEM image of the CeO2 annealed at 250°C shows an extensive cracking. This is probably due to shrinkage of the film upon drying . Other reasons for cracks might be the internal stress ; also, the withdrawing speed is too high . Similar results were reported by Carvalho et al. , Mihalache and Pasuk , and Suresh et al. . The SEM image of the CeO2 film annealed at 550°C presents a film with homogeneous morphology and free of cracks.
AFM images also indicate that the changes of surface morphology and roughness of CeO2 thin films were highly dependent on . SEM images of cross-sections of the films also allow to measure the thickness. Figure 4 shows the thickness of the CeO2 films annealed at 450°C (Figure 4(a)) and 550°C (Figure 4(b)). The micrographs clearly show a decreasing in thickness with the increasing of . Similar results were reported by Suresh et al. .
3.2.2. Atomic Force Microscopy (AFM)
The surface morphology of the CeO2 thin films was also investigated by atomic force microscopy (AFM). Figure 5 shows AFM 3D images of the CeO2 thin film morphology sintered at 250, 450, and 550°C, respectively. In Figure 5(a), the AFM image of the CeO2 thin film sintered at 250°C shows that the surface is smooth and uniform. When the sintering temperature is increased at 450 (Figure 5(b)) and 550°C, on the surface of films, sintered small grain agglomerates appear (Figure 5(c)) together and randomly distributed. The measured Root Mean Square (RMS) surface roughness of the CeO2 thin films sintered at 250, 450, and 550°C was 2.5, 3.5, and 6.0 nm, respectively.
3.3. Optical Properties
Figure 6 shows the transmission spectra of the CeO2 films annealed at different temperatures. All the sintered films have good transparency in the 400-2000 nm region. The transmittance reaches ~75% for the films annealed at 450 and 550°C. The energy bands of CeO2 can be split into two: the valence band derived from O2p states and the conduction band from the Ce5d states. In between lay the empty Ce4f states with a strong atomic-like character, leading to two bandgaps. Some works have been reported that explore the band structure, though not leading to precise values for these bandgaps, as pointed out by Castleton et al. . However, the most common values reported are bandgap taken as 2.6–3.4 eV; 5.5– 6.5 eV range is chosen for O2p → Ce5d [30, 31].
UV-Vis spectra show a strong absorption band below 400 nm, that is, the absorption edge, due to the charge-transfer transition from O2- (2p) to Ce4+ (4f) orbitals in CeO2 .
The values of both the direct () and indirect () optical bandgap energies were obtained from the linear fitting of the plot (αhν)2 and (αhν)1/2 vs. the photon energy (hν), respectively (see Figure 7), where it can be observed how the linear fits for both and shift to higher energies with increasing . The increment of bandgap when rises has been reported by Zarkov and coworkers . Figure 8 exhibits and versus . The figure shows that both values described, approximately, parallel line shapes, i.e., have the same energy separation in the 250–550°C interval of . Since , as expected for CeO2, the bandgap is indirect. However, for the nanometric CeO2 dimensions, generally, the bandgap increases when the particle size decreases, due to quantum confinement effect. The Bohr radius of CeO2 is of the order of 7–8 nm ; therefore, in our case, the quantum effects should strongly influence the results.
The decreasing of both bandgaps when the size of crystal size decreases could be related to the shrinkage of the lattice (see Figure 1), which introduces strong stress in the structure in such a way that its effect dominates the quantum confinement.
3.3.2. Raman Spectroscopy
The formation of a cubic structure in the CeO2 films was confirmed by Raman spectroscopy. Figure 9 shows the Raman spectra of the CeO2 films annealed at different temperatures. The Raman spectra of the samples exhibit a Raman mode at 461 cm-1 which has F2g symmetry assigned to the cubic structure of the fluorite and can be viewed as a first-order symmetric stretching mode of the Ce-O8 vibrational units . This mode should be very sensitive to any disorder in the oxygen sublattice resulting from thermal, doping, or crystal size , as can be observed on how the FWHM of the F2g mode increases as the CS decreases in Figure 9. Hattori et al.  reported an increase of this peak with increasing heat treatment temperature from 400 to 1000°C. The bandwidth of this peak decreases with the increase of the annealing temperature due to the increase in particle size and therefore an increase of the order of the interior of the lattice structure. Similar results were also observed by Wang et al.  and Kosacki et al. . The broadband at around 580 cm-1 is due to disorder in the oxygen sublattice (mainly VOs) . In Figure 9(a), the deconvolution of this band in three ones can be seen; the mode at 595 cm-1 is caused by VOs [40, 41]. The band located at 560 cm-1 is due to the reduction of the lattice, since the desorption of oxygen originates the Ce3+ formation, producing the Ce3+-VO complex [40, 42]. The third at ~620 cm-1 is related to the SiO2 from the glass substrate . With regard to the band at 780 cm-1, several authors have associated it with the presence of the 18O2+ [40, 44].
Figure 9(b) displays in an augmented scale the frequency of the F2g mode for the different ’s. This mode shifts to higher energies when rises. Figure 9(c) exhibits the frequency of F2g as a function of . The mode hardens with due to the strain provoked by the oxygen vacancies, whose density increases at the same time that increases. This result is consistent with those obtained by XRD, where larger promotes a larger lattice contraction (see Figure 1), that is, increasing the stress on the atomic framework .
CeO2 thin films have been synthesized successfully by the sol-gel dip-coating method. X-ray diffraction (XRD) results showed that all samples crystallized in the cubic fluorite structure and crystallinity increases with the increase of the annealing temperature. Crystallite size was found to be in the range to , which increases as augments. On the contrary, the interplanar (111) and (002) decrease when rises. This behavior was explained by the strain in the films provoked by oxygen vacancies. The Raman peak at 463 cm-1 indicates the F2g active mode of the cerium cubic structure. A broad mode at 590 cm-1 was deconvoluted in three ones: at 560 cm-1 due to the Ce3+-VO complex, at 595 cm-1 associated to VOs, and at 620 cm-1 assigned to the substrate. Based on SEM measurements, the films annealed at 250°C and 450°C show a densification process. The optical properties were also affected by because an increase leads to an increase of both indirect and direct bandgap energy values.
Data are available on request due to privacy/ethical restrictions. The data that support the findings of this study are available on request from the corresponding author (A. Méndez-López). The data are not publicly available due to restrictions, e.g., their containing information that could compromise the privacy of research participants.
Conflicts of Interest
The authors declared that they have no conflicts of interest to this work.
We appreciate the support of the “Programa para el Desarrollo Profesional Docente (PRODEP)” with the project “Concreto mejorado mediante el deposito de nanopartículas de ZrO2” and “Dirección de Planeación–Universidad Autónoma de Querétaro.” In addition, thanks are due to M. Sci. C. Zuñiga Romero, M. Sci. A. Tavira, M. Sci. A. Garcia Sotelo, M. Sci. A.B. Soto, Dr. R. Fragoso, Dr. J. Marquez Marin, Dr. A. Guillen, Eng. J.E. Urbina Alvarez, Eng. L.D. Guerrero-García, Eng. R.E. Aguilera-Perez, and Dr. M. Becerril for their technical assistance.
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