What is the sol-gel method？

The sol-gel method, or dip-coating method, is an older technique for preparing oxide thin films from solutions. In 1846, the French chemist J.J. Ebelmen discovered that mixing silicon tetrachloride (SiCl₄) with ethanol led to hydrolysis in moist air, forming a gel. In the 1930s, W. Geffcken demonstrated that hydrolysis and gelation of metal alkoxides could produce oxide-thin films.

In 1975, B.E. Yoldas and M. Yamane successfully created thin thin films of porous, transparent aluminum oxide (Al₂O₃). Since the 1980s, the sol-gel method has been used to fabricate composite oxide thin films on glass substrates, broadening its applications in advanced material science and coating technologies.

Principle of Film Formation Using the Sol-Gel Method

The sol-gel process involves several chemical steps to produce oxide films. First, raw materials are dispersed in a solvent and undergo hydrolysis, forming reactive monomers. These monomers polymerize to form a sol, which is a liquid colloidal system where the dispersed particles are 1–1000 nm in size. Over time, the sol transitions into a gel, a solid-like colloidal system with a three-dimensional network structure. The gel is then dried and heat-treated to produce nanoparticles and the desired oxide material. The final material solidifies on the glass substrate surface as an oxide film. A simplified sol-gel dip-coating process is shown in Figure 4-6.

In this method, the glass substrate is dipped into a solution containing highly reactive chemical precursors. After immersion for a specific duration, the solution adhering to the substrate undergoes hydrolysis and condensation reactions, forming a transparent and stable sol system on the glass. As the sol ages, the colloidal particles slowly polymerize, creating a gel network filled with immobilized solvents. Drying and sintering the gel result in the formation of molecular or nanostructured materials, which eventually form a uniform oxide film on the substrate.

For example, the chemical reactions to form aluminum oxide (Al₂O₃) films are as follows:
Al(OR)₃ + H₂O → Al(OR)₂OH + ROH
Al(OR)₂OH + H₂O → Al(OR)(OH)₂ + ROH
Al(OR)(OH)₂ + H₂O → Al(OH)₃ + ROH
Upon heating, 2Al(OH)₃ → Al₂O₃ + 3H₂O.
This process involves stepwise hydrolysis of aluminum alkoxide, diffusion of ROH from the film, and condensation during heating, resulting in Al₂O₃.

Figure 4-6 Gel dip coating process

Advantages and Applications of the Sol-Gel Method

The sol-gel method allows the production of a wide variety of oxide films with excellent optical and mechanical properties. Table 4-2 summarizes the properties of various oxide films prepared using this method. For instance, Al₂O₃ films are either amorphous or crystalline, depending on the precursor and processing conditions, and exhibit excellent adhesion when mixed with other oxides. Similarly, TiO₂ and SiO₂ films are widely used due to their transparency and mechanical durability.

In addition to oxides, this method can produce other types of films. During the dip-coating process, two types of forces act within the film: cohesive forces parallel to the substrate surface and adhesive forces perpendicular to it. As the film thickness increases, cohesive forces can exceed adhesive forces, introducing tensile stress that affects film quality. Therefore, controlling hydrolysis conditions is essential to enhance adhesion and ensure film integrity.

Chemical bonds between the film and the glass substrate further strengthen adhesion. For instance, hydrolysis of alkoxide compounds can form silicates or titanates that bond covalently with the substrate, ensuring durable and stable films. This characteristic makes the sol-gel method a versatile and efficient technique for producing advanced coatings with diverse applications.

Table 4-2 Properties of non-absorbing and absorbing metal oxide films prepared by gel immersion plating

Raw Material	Film Material	Color in Transmission	Structure	Remarks
Al-sec-butoxide	Al₂O₃	Colorless	Amorphous	Forms mixture with other oxides
Al(NO₃)₃·9H₂O	Al₂O₃	Colorless	Crystalline	Forms mixture with other oxides
Y(NO₃)	Y₂O₃	Colorless	-	-
La(NO₃)	La₂O₃	Colorless	-	-
Ce(NO₃)₃·6H₂O	CeO₂	Colorless	Crystalline	Forms mixture with other oxides
Nd(NO₃)₃	Nd₂O₃	Colorless	-	Absorption band at 500–600 nm reduces transmittance
In(NO₃)	In₂O₃	Colorless	Crystalline	Semiconductor
Ti(OR)	TiO₂	Colorless	Crystalline	Forms mixture with other oxides
Si(OR)₄	SiO₂	Colorless	Amorphous	Forms mixture with other oxides
TiCl₄	TiO₂	Colorless	Crystalline	Forms mixture with other oxides
ZrOCl₂	ZrO₂	Colorless	Crystalline	-
HfOCl₂·8H₂O	HfO₂	Colorless	Crystalline	Contains trace Cl in layers
ThCl	ThO₂	Colorless	Crystalline	-
Th(NO₃)	ThO₂	Colorless	-	-
SnCl	SnO₂	Colorless	Crystalline	Semiconductor
Pb(OOCCH₃)₂	PbO	Colorless	Amorphous	Diffuses into glass at 500°C
Ta₂O₅	Ta₂O₅	Colorless	-	-
Sb₂O₃	Sb₂O₃	Colorless	-	-
Cu(NO₃)₂·3H₂O	CuO	Brown	-	-
VOC	VO	Pale green to yellow	-	Optical properties depend on preparation conditions
CrO(NO₃)₉H₂O	CrO	Yellow	-	-
CrOCl	-	Orange	Crystalline	-
Fe(NO₃)₃·9H₂O	Fe₂O₃	Yellow to red	-	-
Co(NO₃)₂·6H₂O	CoO	Brown	-	-
Ni(NO₃)₂·6H₂O	NiO	Gray	-	-
RuCl₃·H₂O	RuO	Gray	-	Semiconductor
RhCl₃	RhO	Gray to brown	-	-
UO₂(OOCCH₃)₂	UO₂	Yellow	-	-

Dip-Coating Solution Preparation for Sol-Gel Process

In industrial production, dip-coating solutions are primarily prepared using oxides from Group III to Group VI elements of the periodic table, such as Al, In, Sn, Ti, Zr, Ta, Cr, and Sb, or mixtures of these oxides. To produce uniform optical coatings with desired properties, the solutions must possess specific physical and chemical characteristics. The following requirements must be met for effective dip-coating solutions:

Solubility and Stability of Precursors

The starting compounds must exhibit sufficient solubility, ensuring minimal crystallization during solvent evaporation. The compounds should remain in a gel-like or polymerized state, react with the solvent, or leave behind an amorphous residue after evaporation. This ensures uniformity in the resulting film.

Wettability

To ensure proper substrate wetting, the contact angle between the solution and the substrate should be minimized. Surfactants can be added to improve wetting properties. In cases of insufficient substrate cleanliness, reduced hygroscopicity may lead to surface roughness or coating non-uniformity. A solution with low polymerization may result in surface scratches deeper than 10 μm.

Durability of the Solution

The solution must maintain stability under consistent process conditions. Gel-like or polymerized solutions often have poor durability, which can be enhanced using stabilizers.

Drying and Heating

To achieve uniform oxide films with repeatable properties, careful drying and heating are essential. Cracks or haze should be avoided during solidification to ensure strong adhesion between the film and substrate.

Viscosity

The solution must have an appropriate viscosity to produce uniform films. A viscosity range of (1.5–2) × 10⁻³ Pa·s is ideal. Excessive viscosity may result in uneven coating thickness.

Examples of Dip-Coating Solution Preparation

The following examples illustrate common preparation methods, but may need to be adjusted based on specific film requirements and reagent purity.

SiO₂ Film

The hydrolysis reaction for preparing SiO₂ from tetraethyl orthosilicate (TEOS) is as follows:
Si(OC₂H₅)₄ + 4H₂O → H₄SiO₄ + 4C₂H₅OH
H₄SiO₄ → SiO₂ + 2H₂O
With TEOS having a molar mass of 208 g/mol and water 72 g/mol, the water-to-TEOS ratio required for normal hydrolysis is calculated as 0.346 (g water per g TEOS). The water amount should be adjusted based on ambient humidity: if relative humidity exceeds 65%, reduce water by 0.5%; if below 50%, increase by 0.5%. Excess water can cause film irregularities, while insufficient water slows film formation and weakens adhesion.

Three-Layer Anti-Reflective Film

For three-layer films, the TEOS concentration is diluted with 98% ethanol for the first two layers and 99% ethanol for the third layer. Hydrochloric acid (HCl) is added at 0.3 mL per 100 mL of solution for the first two layers and 5 mL for the third layer.

TiO₂ Films

For titanium ethoxide [Ti(OC₂H₅)₄], the HCl addition depends on concentration:

For 3–9% Ti(OC₂H₅)₄, add 0.15–0.2 mL HCl per 100 mL.
For 10–14% Ti(OC₂H₅)₄, add 0.3–0.4 mL HCl per 100 mL.

If the solution turns milky white, HCl is added dropwise until it becomes clear.

Concentration-Based HCl Addition

The amount of HCl added varies with the concentration of TEOS:

For 3–5% TEOS, add 0.05 mL HCl per 100 mL.
For 6–8% TEOS, add 0.1 mL HCl per 100 mL.
For 9–19% TEOS, add 0.15 mL HCl per 100 mL.
For 20–30% TEOS, add 0.2 mL HCl per 100 mL.

Multicomponent Oxide Films

A dip-coating solution for 4SiO₂·Al₂O₃·6K₂O·0.4Na₂O film is prepared as follows:

Dissolve 2.06 g of boric acid in 30 mL of ethanol.
Dissolve silicon ethoxide in 60 mL of ethanol.
Add 2.169 g of potassium and sodium methoxide (3.6 mL of a 30% methanol solution) to the latter. Stir until any precipitate dissolves.
Add 12.32 g of aluminum sec-butoxide and the boric acid solution. Continue stirring for 1 hour until the solution becomes clear.

Film Preparation Methods in the Sol-Gel Dip-Coating Process

The sol-gel dip-coating method includes two primary techniques: vertical dip-coating and spin-coating.

Vertical Dip-Coating Method

This method has two variations: the lifting process and the lowering process.

Lifting Process: The substrate (glass) is dipped into the solution and then gradually lifted out to achieve coating.
Lowering Process: The substrate remains stationary while the liquid level of the dip-coating solution is reduced.

Due to its operational ease, the lifting process is widely used, especially for large glass panels. A schematic of the lifting process is shown in Figure 4-7. The production steps for oxide films using the vertical dip-coating method are as follows:

The glass substrate is mounted on a dip-coating frame.
The prepared dip-coating solution is placed in a tank, and the frame is lowered into the solution.
The substrate is immersed in the solution, ensuring a uniform coating.
The frame is lifted at an optimal speed of 10–20 cm/min.
The coated substrate is dried in air, undergoing hydrolysis and dehydration to form a thin film.
The substrate is transferred to a heating furnace, where it is heated at 400–500°C for 30 minutes at a ramp rate of 7–10°C/min.
During heating, the adhered solution undergoes condensation, forming a solid oxide layer.
The substrate is cooled to room temperature, unloaded, inspected, and packaged.

Figure 4-7 Schematic diagram of gel immersion plating lifting process membrane preparation

The film thickness is influenced by parameters such as withdrawal speed, sol viscosity, and surface tension. The relationship is given by Equation 4-8:

The film thickness is influenced by parameters such as withdrawal speed, sol viscosity, and surface tension. The relationship is given by the equation: δ = η · v / γ, where δ is the film thickness, η is the viscosity, v is the withdrawal speed, γ is the surface tension, and g is the gravitational acceleration.

Limitations: Vertical dip-coating is less effective for large flat glass surfaces due to difficulties in maintaining solution stability and for coating only one side of the substrate.

Figure 4-8 Process of preparing oxide film by gel immersion plating lifting process

Spin-Coating Method

The spin-coating method, also known as the centrifugal method, is performed using a spin coater.

The glass substrate is secured on the coater.
A dropper dispenses the sol-gel solution onto the rotating substrate.
Centrifugal force distributes the solution uniformly over the surface.

This method is ideal for small substrates, with typical spin speeds around 1200 rpm. After coating, the substrate is dried and aged at 60°C for 15 minutes to form a gel film. Additional layers can be applied as needed by repeating the process. A schematic of the spin-coating process is shown in Figure 4-9.

Advantages: Spin-coating is highly effective for large-area coatings and can achieve uniform films even on slightly uneven surfaces.

Figure 4-9 Schematic diagram of gel immersion spin process membrane preparation

Advantages of the Sol-Gel Dip-Coating Method

Simple and inexpensive coating equipment without the need for costly vacuum systems.
Simultaneous coating of both sides of the glass substrate for enhanced durability or reduced layer count.
The chemically bonded thin film ensures strong adhesion to the glass surface.
Easy application of multilayer films to meet specific requirements.
Effective for coating the inner surfaces of glass products, such as tubes, where vacuum-based methods are challenging.
Low baking temperatures (380–500°C) prevent deformation of the glass.

Limitations of the Sol-Gel Dip-Coating Method

Maintaining long-term solution stability during production is challenging.
Product performance is consistent within a relatively wide range, making precision difficult.
Not suitable for small or intricate substrates with many edges or corners.

The sol-gel dip-coating method is a versatile and cost-effective process, though its applicability may be limited for specific product geometries or precision requirements.

Contact Us

Your feedback fuels our growth, and your questions drive our solutions.

We value your feedback, inquiries, and suggestions. Please feel free to get in touch with us

General inquiries

Please contact us via sales@bo-glass.com, and we will reply to you as soon as possible.

Interested to work with us

Drop your resume at info@bo-glass.com
and we will get back to you shortly.

We uses the contact information you provide to us to contact you about our relevent content, products, and services.

1