CeriumDioxide Thin Films Using Spin Coating

Cerium dioxide (CeO2) thin �lms with varying Ce concentrations (0.1 to 0.9M, metal basis) were deposited on soda-lime-silica glass substrates using spin coating. It was found that all �lms exhibited the cubic �uorite structure a�er annealing at 500C for 5 h. e laser Raman microspectroscopy and GAXRD analyses revealed that increasing concentrations of Ce resulted in an increase in the degree of crystallinity. FI� and FESEM images con�rmed the laser Raman and GAXRD analyses results owing to the predicted increase in �lm thickness with increasing Ce concentration. However, porosity and shrinkage (drying) cracking of the �lms also increased signi�cantly with increasing Ce concentrations. UV-VIS spectrophotometry data showed that the transmission of the �lms decreased with increasing Ce concentrations due to the increasing crack formation. Furthermore, a red shi� was observed with increasing Ce concentrations, which resulted in a decrease in the optical indirect band gap.


Introduction
During the last few decades, metal oxide semiconductors have become important materials, with numerous publications focusing on different types of these materials namely, In 2 O 3 , TiO 2 , SnO 2 , and CeO 2 .Recently, there has been growing interest in the use of CeO 2 [1][2][3] due to its promising characteristics, including: (i) it is an n-type semiconductor with a band gap of 3.2 eV [4,5], (ii) it is highly transparent in the visible region (400-800 nm) [4,5], and (iii) it is inexpensive.ese advantages enhance the potential for CeO 2 to be used widely in a range of applications, such as oxygen storage [6], smart windows [7], electrochemical displays [8], UV �lters [9], and catalysts [10].For the preceding applications, thin �lm CeO 2 is used most commonly owing to its �exibility of use, cost considerations, and ease of preparation.
CeO 2 thin �lms can be prepared by several techniques, including spray pyrolysis [11], pulsed laser deposition [12], sputtering [13], and spin coating [14].e latter is one of the most advantageous techniques owing to its versatility, effectiveness, and practicality.Furthermore, the operation can be done in ambient conditions and thus a vacuum system is not required.e aim of this work was to prepare CeO 2 thin �lms on soda-lime-silica glass substrates using spin coating and to investigate the mineralogy, morphology, and optical properties of these �lms.

Methodology
Solution precursors were prepared using cerium chloride heptahydrate (analytical grade, 99.9%, Sigma Aldrich) dissolved in methanol (Reagent Plus ≥ 99 wt%, Sigma-Aldrich) with magnetic stirring.e concentrations of Ce used were 0.10, 0.30, 0.50, 0.70, and 0.90 M (metals basis).To each of these solutions 5 mL of citric acid were added (0.2 M, analytical grade, 99.0 trace metal basis, Sigma Aldrich), followed by stirring at 500 rpm for 5 minutes without heating.Spin coating (Laurell WS-65052) was done by rapidly depositing ∼0.2 mL (ten sequential drops) of solution onto a microscope slide spun at 2000 rpm in air.e �lms were dried by spinning for an additional 15 s.Subsequently, all the �lms were annealed at 500 ∘ C for 5 h in air in a muffle furnace (heating rate 300 ∘ C/h, natural cooling).
e mineralogies of the �lms were determined by glancing angle X-ray diffraction (GAXRD, Philips X'pert Materials Research Diffraction, CuK, 45 kV, 40 mA, step size 0.02 ∘ 2, speed 6 ∘ /min 2) and laser Raman microspectroscopy (He-Cd UV laser excitation source, wavelength 514 nm, Renishaw inVia).e �lm thicknesses were determined using singlebeam focused ion beam (FIB) milling (FEI XP200).Fieldemission scanning electron microscopy (FESEM, Hitachi S4500; Cr-coated, secondary electron emission mode, 5 kV accelerating voltage) was used to investigate the morphologies of the �lms.e transmissions in the ultraviolet-visible (UV-VIS) range were determined using a dual-beam spectrophotometer (Perkin Elmer Lambda 35) and the optical indirect band gap was calculated from these data using the method of Tauc and Menth [15] as shown by (1).

Results and Discussion
Figure 1 shows the laser Raman spectra of the �lms.ese data clearly indicate that the peak intensity of CeO 2 increased signi�cantly with increasing Ce concentration.e GAXRD patterns of the �lms showed the same trend as the laser Raman microspectra, as seen from Figure 2. e increase in intensity of the laser Raman spectra and GAXRD patterns with increasing Ce concentration is the result of increasing amounts of material being deposited which consequently increased both the thickness of the �lms and the degree of crystallinity.Furthermore, the GAXRD patterns also con�rm that all �lms exhibited the cubic �uorite structure [2] aer annealing at 500 ∘ C. e FIB images, shown in Figure 3, show that the thickness of the �lms increased with increasing Ce concentrations, which con�rms the results observed from the laser Raman and GAXRD analyses.It is also seen that, with increasing thickness of the �lms, the extent of porosity also increased.e average thicknesses of the �lms are listed in Table 1.
Figure 4 shows FESEM images showing the surface morphologies of the �lms.It can be seen that, with increasing Ce concentrations, the number and sizes of shrinkage cracks and resultant pores increased, similar to what was observed in the FIB images of the cross-sections.It is unknown if the cracks derive from shrinkage during drying or annealing.e increasing amount of shrinkage is consistent with both the increasing amounts of removed water (from the heptahydrate) and the increasing degree of crystallinity (since crystallisation is always accompanied by shrinkage).Figure 5 shows the UV-VIS spectra of the �lms and it is seen that the transmission of the �lms decreased with increasing Ce concentrations in the �lms.Moreover, the absorption edge shied towards longer wavelengths (red shi).e increase in the thickness and light scattering from pores/cracks are responsible for the observed decrease of the transmission spectra [16][17][18].
Since, CeO 2 is known to be a transparent conductive oxide, a polycrystalline CeO 2 thin �lm is transparent to visible light (400-800 nm).However, Figure 5 shows that the transmission in visible region slightly decreased.e possible explanation for this observance is that with increasing Ce concentration, there was a drastic increase in the porosity and crack formation in the �lms (as shown in Figures 3 and 4).ese imperfections enhance the light scattered by the �lms and thereby decreases the light transmitted through the �lms.Additionally, in the ultraviolet region (>400 nm), a signi�cant red shi is observed with increasing Ce concentrations.is red shi is associated with the decrease of the indirect band gap of the �lms.
e optical indirect band gaps of the �lms were calculated using the UV-VIS data; the details are described elsewhere [15].e data, shown in Table 1, demonstrated that the optical indirect band gaps decreased signi�cantly with increasing Ce concentrations and this is attributed to the increasing crystallinity of the �lms.

Summary and Conclusions
CeO 2 thin �lms were deposited on soda-lime-silica glass substrates by spin coating using methanol solutions of varying Ce concentrations (0.1 to 0.9 M). e major conclusions of the present work are as follows.
(i) All the �lms exhibited the cubic �uorite structure phase aer annealing at 500 ∘ C for 5 h.(ii) e laser Raman microspectroscopy and GAXRD analyses showed that with increasing Ce concentration, the thicknesses of the �lms increased as did their degree of crystallinity.(iii) e FI� images con�rmed the increasing �lm thicknesses and the FESEM images showed increasing porosity and cracking with increasing Ce concentration in the �lms.(iv) UV-VIS spectra showed that the transmittance of the �lms decreased with increasing Ce concentration and hence the observation of a red shi, which decreased the optical indirect band gap.
e present work shows that crystalline CeO 2 �lms as thin as ∼100 nm can be produced by spin coating and by annealing at 500 ∘ C. e thicknesses of the �lms can be controlled through modi�cation of the Ce concentration.Further work is required in reducing the cracking of the �lms by controlling the rates of drying and/or heating (during annealing).While this is relatively easy in the latter case, the former would likely require imposition of a water-vapour-saturated atmosphere in the spin coater chamber, which would put the electronics of the unit at risk.Alternatively, longer chain alcohol solvents could be used but these would cause greater annealing shrinkages.Ultimate success is likely to require the appropriate combination of cerium salt, solvent, �lm thickness, drying rate, annealing rate, and annealing temperature.

F 1 :F 2 :
Laser Raman microspectra of CeO 2 �lms prepared using solutions of varying Ce concentrations.GAXRD patterns of CeO 2 �lms prepared using solutions of varying Ce concentrations.

F 3 :F 4 :
FIB images of the cross-sectional areas of the CeO 2 �lms produced using solutions of varying Ce concentrations.FESEM images of the surface morphologies of CeO 2 �lms prepared using solutions of varying Ce concentrations.

T 1 :F 5 :
icknesses and optical indirect band gaps of the different �lmsUV-VIS spectra of CeO 2 �lms prepared using solutions of varying Ce concentrations.