Preparation and Characterization of Thin Films by Sol-Gel Method

Abstract

The sol-gel method has been widely used to prepare several materials, such as glass fibers, catalysts, electrochemical devices, or thin films. Sol-gel is considered an economical and straightforward method compared to physical vapor deposition (PVD) or chemical vapor deposition (CVD), which are more complex and need more facilities. At the same time, almost the same quality has been evaluated for sol-gel thin films. Furthermore, chemical tailoring of raw materials to produce new functional compositions is more feasible than conventional methods such as PVD. Thin films utilizing sol-gel were prepared by dip coating, spin coating, electrochemical coating, and spray coating methods, where these methods can be used for various substrate types. Prepared thin films may be utilized in several areas of application, such as semiconductors, catalysts, or photocatalysts.

1. Introduction

Sol-gel processing began by Ebelman and Graham, who studied silica gels as early as the mid-1800s. Hydrolysis of tetraethyl orthosilicate (TEOS), Si(OC₂H₅)₄, under acidic conditions, yielded SiO₂; these early investigators observed that it was in the form of a “glass-like material” [1].

Sol-gel method can be implemented in several fields of scientific and engineering fields, such as the ceramic industry, nuclear industry, and the electronic industry or the development of new materials for catalysis, membranes, chemical sensors, photochromic applications, fibers, optical gain media, and solid-state electrochemical devices [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]. Thin film and powder catalysts have been widely produced using the sol-gel method. Several variants and manipulations have been implemented into the process to synthesize pure thin films or powders in large homogeneous concentrations and under stoichiometry control [14, 15, 16]. Process simplicity, optical complex shapes, uniform oxide complexes, special shapes of fibers and aerogels, synthesis of amorphous minerals, synthesis of porous material with high content of organic and polymeric compounds, and synthesis of highly optical transparent or low initial investment are the most advantages of the sol-gel method [17, 18, 19].

The term sol is attributed to a stable suspension containing colloidal nanoparticles (with diameters of 1–100 nm). The sol may contain amorphous or crystalline particles with dense, porous, or polymeric substructures [20, 21].

A rigid network with pores of sub-micrometer dimensions and polymeric chains whose average length is greater than a micrometer has been considered a gel [22]. A diversity of combinations of substances embraced as the “gel” can be classified into four categories: (1) structures of well-ordered lamellar; (2) networks of covalent polymer that is completely disordered; (3) formation of predominantly disordered through physical aggregation of polymer networks, (4) disordered particular structures [23, 24, 25].

The gel is attributed to a porous three-dimensional continuous solid network surrounding and supporting a continuous liquid phase. Synthesis of oxide materials and gelation is because of covalent bond formation between the sol particles [26, 27]. Gel formation can be reversed due to the presence of van der Waals interactions or hydrogen bonds. The gel structure network is dependent on the size and shape of the sol particles to a large extent [28].

2. Preparation of thin film by sol-gel method

Sol-gel monoliths have been prepared by three approaches: method 1, colloidal powders solution; method 2, alkoxide or nitrate precursors hydrolysis and polycondensation followed by hypercritical drying of gels; method 3, aging and drying of alkoxide precursors under ambient atmospheres after hydrolysis and polycondensation [29, 30].

The hydrolysis and condensation reactions (sol-gel process) would be influenced by several parameters, such as the metal alkoxide activity, solution pH, ratio of the water/alkoxide, temperature, solvent nature, and additive used. The addition of catalysts controls the rate, hydrolysis extent, and condensation reactions. By varying processing parameters, materials with different microstructures and surface chemistry can be obtained [31]. The fabrication of ceramic materials in various forms would be available by further processing the “sol.” By casting the sol into the mold, a wet gel will form. Dense ceramic or glass particles will form, followed by drying and heat treatment of the gel [32]. Meanwhile, highly porous and low-density aerogel material is synthesized utilizing supercritical conditions to remove the liquid in a wet gel. Many natural systems like opals, agates, and particles are evidence of silicate hydrolysis and condensation to form polysilicate gel. Ebelman prepared the first metal alkoxide from SiCl₄ and alcohol and found the first “precursor” for glassy materials can be Si-(OC₂H₅)₄ and gelled compound on exposure to the atmosphere [33].

$$\text{Step}1:\left(\text{Hydrolysis}\right):\mathrm{Si}\left(\text{OR}\right){}_4+\mathrm{H}{}_2\mathrm{O}\leftrightarrow \text{SiOH}\left(\text{OR}\right){}_3+\mathrm{R}-\mathrm{OH}$$E1

$$\text{Step}2:\left(\text{Water condensation}\right):\text{SiOH}\left(\text{OR}\right){}_3+\text{SiOH}\left(\text{OR}\right){}_3\leftrightarrow \left[\mathrm{Si}\right]{}_2\mathrm{O}\left(\text{OR}\right){}_6+\mathrm{H}{}_2\mathrm{O}$$

$$\left(\text{Alcohol condensation}\right):\text{SiOH}\left(\text{OR}\right){}_3+\mathrm{Si}\left(\text{OR}\right){}_3\leftrightarrow \left[\mathrm{Si}\right]{}_2\mathrm{O}\left(\text{OR}\right){}_6+\mathrm{R}-\mathrm{OH})┤$$E2

2.1 Electrochemical aided deposition (EAD)

Dip coating, spin coating, and spraying methods are usually used to prepare sol-gel films. Only flat surfaces can be coated by dip coating and spin coating, while the rheology of the precursor and fine-tuning must be considered for the spray coating method. Sol-gel thin film preparation implementing the electrochemical deposition technique has been considered in the last two decades. Woo et al. applied this technique for preparing silane films as binders [34]. Electrodeposition of silane-based films and proposed its mechanism later studied by Shacham et al. [35]. Preparation of coatings on conductive materials based on the sol-gel principle utilizing EAD is a recently developed method, and thicker and rougher sol-gel films can be prepared by this method. Application of negative potential and in-situ catalysis of sol-gel chemistry (hydrolysis and condensation reactions) are the basic principles of EAD. Reduction of oxygen or some specific ions supporting the electrolyte or hydrolysis, which leads to an increase in pH value near the electrode, would accelerate the immediate condensation of the solute on the electrode surface. Equation (3) expressed the chemical reaction in this process [36],

$${\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}+4\mathrm{e}\to 4\mathrm{O}{\mathrm{H}}^{-\left(3\right)}$$E3

During the electrodeposition process, the pH value of the bulk solution does not change, and no new materials are introduced; the silane component does not lose or gain electrons at the electrode surface. Electrodeposited films have several unique advantages compared to conventional self-assembled films [37]:

1. The base catalyst provides an additional driving force for the film, resulting in thicker and rougher film formation near the cathode surface.

2. Films with greater porosity and better intra-cohesion may be produced due to gelation, where solvent evaporation occurs during electrodeposition.

3. The derived OH-ions can catalyze the chemical bonding process between the sol-gel film and the substrate.

Coatings on complex non-planar geometries and controlling the thickness and composition of the nanocomposite are the significant advantages of depositing utilizing EAD (Figure 1).

Figure 1.

Electrochemical deposition of a sol-gel film [36].

2.2 Dip coating method

Sol-gel dip coating consists of depositing a solid film by withdrawing a substrate from a sol: gravitational draining and solvent evaporation, followed by further condensation reactions. Sol-gel dip coating, compared to methods such as chemical vapor deposition (CVD), evaporation, or sputtering, does not require complex equipment and is potentially less expensive than conventional thin film-forming processes [38]. The ability to tailor the microstructure of the deposited film is the most crucial advantage of sol-gel over conventional coating methods.

The substrate from the liquid bath is drawn vertically at a speed of U_o. By entraining the moving substrate, the liquid in a fluid mechanical boundary layer may separate above the liquid bath surface, and the outer layer returns to the bath. The fluid film terminates at a well-defined drying line by solvent evaporating and draining. The process is steady with respect to the liquid bath surface when the receding drying line velocity equals U_o.

The concomitant draining, evaporation, and hydrolysis consolidation step represents the sol-gel transition. An integrated gel film will be left due to a withdrawn drying line moving downwards with colorful parallel interference lines. By implementing volatile solvents, in comparison to the bulk sol-gel process, the complete transition will be done in a short time. The drying and keeping of the water content are almost constantly enhanced due to the evaporation and the resulting cooling. In addition, over the surface of the wet film, a downward laminar flow of vapors forms. Inhomogeneities deposition in the film properties may occur by any turbulence or variation in the atmosphere (Figure 2).

Figure 2.

Schematic of sol-gel dip coating method including both capillarity and draining regimes [39].

A fluid mechanical equilibrium between the entrained film and the receding coating liquid is the film formation process. The equilibrium state will be affected by forces such as viscous drag and the gravity force and other forces like the surface tension, the inertial force, or the disjoining pressure.

The Landau-Levich equation describing the final liquid film thickness h for pure liquids considers this fundamental theoretical approach [40],

$$h=c·\frac{{\left(\eta ,U\right)}^{2/3}}{{\gamma }^{\frac{1}{6}}{\left(\rho ,g\right)}^{\frac{1}{2}}}$$E4

where c, η, U, y, and ρ are constant, liquid viscosity, withdrawal speed, the surface tension of the liquid against air, and liquid density, respectively.

Modified dip coating techniques can be used for more complex geometries, although this method is particularly suited for coating flat or rod-shaped rigid substrates [41]. A semi-continuous dip coating process for endless flexible substrates like webs or filaments, while dip coating is a typical batch process. Double-sided coating of flat substrates, especially in the production of optical filters, is one of the advantages of the dip coating method. For oxide coatings prepared from the metal salt solution, a single-layer film thickness can be deposited ranging from only a few nanometers to approximately 200 nm. With inorganic-organic hybrid materials, due to the lower shrinkage and the higher flexibility of the network film, several microns are accessible, and colloidal systems can be implemented to produce thicker films up to 1 μm.

The high-volume industrial production of ordinary optical filters using the dip coating technique. Three-layer antireflection coatings for technical glasses (e.g., displays and lighting) form the largest market segment. Dip coating methods have been implemented by several industrial brands such as the TiOr-based solar-control glass Calorex^® (Irox^®) and the antireflection coatings Amiran®, ConturanQ, and Mirogard®Several and well-known products emerged from these activities [41].

2.3 Spin coating method

The spin coating can rapidly deposit thin layers onto relatively flat substrates. The target surface is dispensed by coating solution; the spinning action causes the solution to spread out and leave behind the coated surface of the substrate by the chosen material very uniformly that is held by some rotatable fixture (often using a vacuum to clamp the substrate in place).

The viscous drag force precisely balances the rotational accelerations within the solution. Emslie, Bonner, and Peck (EBP) [I] first described this flow condition. Simultaneously, solvent evaporation out of the top surface of the solution is also considered by Meyerhofer [42]. Spin coating runs into two stages: viscous flow controlling and evaporation controlling. Prediction of the final coating thickness, h_f, in terms of several key solution parameters, is according to [42]:

$$h_f=x{\left(\frac{e}{2\left(1,-,x\right)K}\right)}^{\frac{1}{3}}$$E5

where e, K, and x are the solution’s evaporation and flow constants and effective solids content, respectively. The evaporation and flow constants are defined as:

$$e=C\sqrt{\omega }$$E6

$$K=\frac{\rho {\omega }^2}{3\eta }$$E7

where ω, ρ, η, and C denote the rotation rate, solution’s density, viscosity, and a proportionality constant. In which C depends on airflow flow regime (laminar or turbulent) and solvent molecules diffusivity in air. Sol-gel film preparation by spin coating deposition typically results in a coating thickness below 1 μm (Figure 3).

Figure 3.

Procedure for spin coating method [43].

2.4 Spray coating method

Spray coating is suitable for non-flat samples, and the thickness can be very well controlled and surface modification over large areas. Only a few studies have studied sol-gel spray coating.

Most of the previously reported research has considered the sol-gel dip coating method low-cost and accessible. However, it is not easy to control the homogeneity and thickness of the coating over the entire sample length in the case of dip coating. Furthermore, spray coating is suitable for non-flat samples, and the thickness can be very well controlled, a better alternative as it allows surface modification over large areas. A three-step process is a typical example of an organically modified silica hydrophobic coating prepared by spraying: (1) preparation of hydrophobic alcosol, (2) alcosol spraying on 100°C glass substrates, and (3) trimethylchlorosilane surface modification created by Mahadik et al. [44]. Due to low cost and lack of specialized equipment, traditional spray coating is still the most often reported method. At the same time, there are other procedures, including spray coating with plasma, thermal spray, and powder [45, 46, 47].

3. Characterization techniques

3.1 Thin film X-ray diffraction

Thin films have been considered the basic materials for modern electronic devices of metallic conductors, semiconductors, and insulators. These films should possess specific mechanical, electrical, magnetic, or optical properties for optimal device performance, which are affected strongly by the film’s microstructural nature, such as crystalline or amorphous state, crystallographic orientation, crystallite size, strains, and stresses [48, 49]. Therefore, for the design and improvement of electronic devices, it is very important to characterize microstructural thin films.

The microstructure of thin films cannot be easily characterized by methods developed for bulk materials due to their small dimensions perpendicular to the surface. X-ray diffraction (XRD) is especially suitable for thin films since it is nondestructive, noncontact, and highly quantitative among the analytical methods.

By implementing the XRD test, several thin film features can be evaluated. Figure 4 indicates XRD patterns of highly crystalline and amorphous films prepared by the sol-gel dip coating method [50], where results indicate the formation of an amorphous TiO₂:SiO₂ thin film (Figure 4a) and a highly crystalline TiO₂:SiO₂ (Figure 4b) thin film on a glass substrate. XRD results revealed that TiO₂ crystals may not be grown by an increase in SiO₂ content of more than 20 mol percent, while highly crystalline TiO₂ has formed on the glass substrate by a lower content of SiO₂ (about 15 mol%). Furthermore, by addition of SiO₂ crystalline size and crystallinity of TiO₂ declined, where Scherrer or Modified Scherrer equations have been used to evaluate the crystalline size according to XRD pattern and full width at half maximum (FWHM, β) calculation [51]:

Figure 4.

XRD pattern for prepared sol-gel dip coating thin film (a) amorphous TiO₂:SiO₂ and (b) highly crystalline TiO₂:SiO₂.

$$D=\frac{\mathit{K\lambda }}{\beta cos\left(\theta \right)}$$E8

Or modified Scherrer equation,

$$ln\beta =ln\frac{\mathit{K\lambda }}{L}+ln\frac{1}{cos\theta }$$E9

In addition, the crystalline phase of the synthesized thin film can be indicated by considering XRD results. Figure 5 illustrate the XRD pattern of TiO₂ nanocrystalline thin film prepared by different heat treatment. As shown, with an increase in temperature, the crystalline phase has been transformed from Anatase to Brookite phases. TiO₂ can be prepared in three phases by different heat treatment methods, where there is the Anatase phase at a heat treatment temperature below 500°C. In contrast, the Rutile phase will form around 500–750°C, and the Brookite phase formation temperature is approximately 750–900°C.

Figure 5.

Phase transmission of TiO₂ from Anatase to Brookite Phase [52].

3.2 Energy dispersive x-ray spectroscopy (EDX)

Investigation of the chemical species present in a material can be evaluated by energy-dispersive X-ray spectroscopy as a powerful technique. Analysis of the energy and intensity distribution of the X-ray signal produced by the interaction of an electron beam with a specimen has been considered the basis of the EDX method. Directed an electron beam toward the sample has been analyzed in a setup that EDX is an optional tool installed on electron microscopes. The electrons can interact with the nucleus or a specific atom’s electrons by certain kinetic energy. The detection of the emitted radiation due to the loss of any amount of energy of electrons between zero and the initial energy will lead to a continuous electromagnetic spectrum that constitutes the background of the collected spectrum [53].

Ionization phenomena usually involve K shell electrons when the primary electrons interact with the atom’s electrons. The electrons from L or M shells can occupy the vacancy left by the K shell electron because an atom in the excited state tends to reach the minimum energy. X-ray radiation emission is designated as K_α, and K_β results from a transition between L and K or M and K shells. The emission of K_α and K_β X-rays identifies the elements in the analyzed sample because of the energy difference between L, M, and K levels that are well-defined for each element. In principle, a line in the continuous electromagnetic spectrum should be given by the detection of this transition level [54].

Figure 6 shows EDX results for TiO₂:SiO₂ thin film coated on the glass substrate, where indicated the presence of Ti, Si, and O in the film. Also, the EDX test revealed the presence of Na, Mg, and Ca that can be related to the glass substrate due to the passing of the X-ray over the prepared thin film and the detection of glass substrate elements. EDX test can be used for mapping and evaluating the synthesized film’s elements distribution.

Figure 6.

EDX results for prepared TiO₂:SiO₂ thin film by dip coating sol-gel method [50].

3.3 X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) is widely implemented in surface science techniques—an X-ray based on the photoelectric effect photon absorbed by a core or valence electron. The electron will be emitted by the larger incident photon energy than the binding energy [55]. A hemispherical electron energy analyzer spectrometer collected the emitted electrons by an electrostatic lens and performed energy analysis. The binding energy E_k of the electrons can be determined from the known photon energy hν and the measured kinetic energy E_B [56],

$$E_{\mathrm{k}}=\mathit{h\nu }-E_{\mathrm{B}}-\varphi$$E10

with ϕ as the spectrometer work function. In an XPS experiment, typical incident photon energies range from several 10s to well over 1000 eV, where the same order of kinetic energies will be considered.

The X-ray penetration depth into the sample is of the order of hundreds of nanometers or more, while the kinetic energy range used in conventional XPS studies, atoms in the sample photoelectrons elastic and inelastic interactions limited to the probe depth to a maximum of a few nanometers.

A variety of features is shown in the XPS survey spectrum in Figure 7a, where results indicate photoemission lines of Ga (Ga 2p1/2, Ga 2p3/2, Ga3s, Ga3p, and Ga 3d), Fe (Fe 2p1/2, Fe 2p3/2, and Fe 3p), and oxygen (O 1 s). The spin-orbit components of Ga 2p3/2 and 2p1/2 have been indicated by splitting 26.9 eV, along with a loss feature at around 1136 eV Ga2p (Figure 7b). In addition, Ga is in a +3 oxidation state confirmed by Ga 2p3/2 peak position at 1118 eV, which is characteristic of native gallium oxide [57].

Figure 7.

GFO thin films XPS analysis, (a) different photoemissions features; (b) Ga2p core-level; (c) O1s core-level; (d) Fe2p core-level [57].

Lattice-bound oxygen is assigned by the O 1 s spectrum (Figure 7c) centered at 530.01 eV, while surface hydroxyl and carbonylated groups are attributed to the other contributions. The component with the most intense in 2p3/2 is at 710.8 eV, with a 13.5 eV splitting from the 2p1/2 component consistent with a Fe+3 oxidation state.

3.4 Atomic force microscopic method

An amazing technique with unprecedented resolution and accuracy is atomic force microscopy (AFM), which allows us to see and measure surface structure. The arrangement of individual atoms in a sample or the structure of individual molecules can be studied by atomic force microscope imaging. The hopping of individual atoms from a surface has been measured by scanning in an ultrahigh vacuum at cryogenic temperatures. However, even seeing biological reactions occur in real-time can be carried out in physiological buffers at 37°C. AFM does not need to be carried out under these extreme conditions [58, 59]. The crystallographic structure of materials can be used to have images containing only 5 nm of microscopic images to show about 50 atoms, or 100 μm or larger images. Almost any sample, such as the surface of a ceramic material, highly flexible polymers, human cells, a dispersion of metallic nanoparticles, or individual molecules of DNA, can be imaged by AFM, but it is challenging [60]. Furthermore, AFM has various “spectroscopic” modes that measure other properties of the sample at the nanometer scale. A map of the height or topography of the surface building up as scanning a probe of the AFM goes along the sample surface.

Figure 8 illustrates the AFM image of a synthesized 15-layer TiO₂:SiO₂ thin film prepared by the sol-gel method. The roughness of the prepared TiO₂:SiO₂ thin film has been presented in Figure 5, while this roughness belongs to the crystalline formation of TiO₂ in the presence of amorphous SiO₂. In this work, TiO₂:SiO₂ gel was coated on a wall of an annular photoreactor to remove oily contamination from wastewater. However, according to AFM results, TiO₂:SiO₂ thin film roughness has been measured as 16 nm, where the film’s roughness is necessary to have proper contact between the reactant and catalyst [61].

Figure 8.

AFM image for TiO₂:SiO₂ thin film prepared by sol-gel method [61].

3.5 Scanning electron microscope

The scanning electron microscope (SEM) has studied surface details of several compositions, such as metals, rock minerals, polymers, corrosion deposits, filters, ceramic membranes, foils, fractured/rough surfaces, alloys, and biological samples. Conductive or non-conductive material, either in solid or powder form, can be examined in an as-received or prepared condition by SEM technique [62, 63]. A field emission gun is installed on the SEM to investigate surface features only 1 nm apart. SEM allows large areas of a sample to remain in focus at one time, yielding 3D characteristics due to its extraordinary ability to depict large depths of field.

The performance of an electron source is expressed by two essential parameters: current density and brightness. Beam current density J_b is given as [64]:

$$J_{\mathrm{b}}=\frac{\text{Beam current}}{\text{Area}}=\frac{i_{\mathrm{b}}}{\pi }$$E11

where i_b and d are the current and diameter of the beam, respectively, the brightness (β) of the electron source would be considered as a function of the total number of electrons (current, I) emitted from a unit area (A) of the source and the solid angle (Ω) of emission subtended by those electrons. This relationship can be expressed as follows [65]:

$$\beta =\frac{\text{Current}}{\text{area}\times \text{solidangle}}=\frac{I}{A\mathrm{\Omega }}=\frac{j_{\mathrm{c}}}{\pi a^2}$$E12

where j_c and α are the current density (expressed in A/cm²) and convergence angle of the beam in radians, respectively.

The SEM method has been used for analyzing thin films, where surface morphology and layer thickness can be evaluated by this method. TiO₂:SiO₂ SEM micrographs are illustrated in the Figure 9, where the surface morphology of 10 layers of TiO₂ and TiO₂:SiO₂ films can be observed in Figure 9 [61]. As indicated, films have been prepared by fractured morphology utilizing the sol-gel method. Crack formation occurs through stress and different thermal coefficients of expansion of the overlayer due to contraction and substrate during the drying and annealing processes of the films. Micro-cracks will result in better diffusion and may contribute to a higher surface area while reducing the film’s durability. According to SEM images, the crystalline size of TiO₂ doped by SiO₂ is 32 nm [61].

Figure 9.

SEM images for TiO₂:SiO₂ multilayer thin film [61].

3.6 Transmitting electron microscope

Ernst Ruska and Max Knoll, in 1931, invented the transmission electron microscope. The electron microscope resolution has improved steadily from around 100 nm in the early models to 0.1 nm and is even better nowadays [66]. High-resolution transmission electron microscopy (HRTEM) refers to phase-contrast imaging resolving single atoms or atomic clusters. A highly coherent source, a well-aligned system, a good detector, and a suitable sample are essential to achieve HRTEM [67].

Samples of electron microscopy typically have gratings behavior. Most electrons travel through the sample unchanged, and none may be absorbed. Wave interference forming A TEM image.

HRTEM image is formed in the image plane when two or more selected Bragg reflected beams interact (interfere) by a suitably large objective aperture to form an image. HRTEM imaging is a type of phase-contrast imaging [68]. As a result of the contrast that occurs from the distinction in the phase of the beams as a result of their interaction with the sample, it can be used to resolve the crystalline lattice, columns of atoms, or sub-Angstrom imaging of the lattice and even single atoms can be prepared in the case of the most modern aberration-corrected transmission electron microscopes. Second-phase or amorphous layers and atomic resolution, crystalline defects, and structure across boundaries can be observed by HRTEM imaging. Furthermore, in the case of interfaces in multilayer thin films, the topography information that it is aligned correctly in the direction of the electron beam on the interface is also provided [69]. Analysis of HRTEM images is complicated and can often require simulation to directly interpret the contrast present in the image, although it is relatively easy to obtain.

Sol-gel spin coating technique has been implemented to prepare highly stoichiometric AgInSe₂ thin films on a p-type Si(111) substrate. Different temperatures were used to anneal these films. HRTEM images of the synthesized and annealed thin films have been shown in Figure 10. The HRTEM image order of the lattice spacing was indicated as 0.3 nm. Films were indexed to a pure polycrystalline chalcopyrite AgInSe₂ structure, as seen in the selected area electron diffraction patterns of the AgInSe₂ [70].

Figure 10.

TEM images for AgInSe₂ thin film prepared by sol-gel method [70].

4. Conclusion

In this chapter, sol-gel method and coating techniques such as dip coating, spin coating, or electrochemical deposition have been investigated. Sol-gel coating methods have been considered economical and more feasible than methods such as physical vapor deposition

(PVD) or CVD, in which sol-gel has been widely utilized industrially. In addition, characterization methods for thin films have been discussed. X-ray-based methods (XRD, EDX, or XPS) were used to investigate the crystallinity and composition of the prepared thin film. Also, AFM, SEM, and TEM, which have been used to study the surface and structure of the films, were discussed in detail.