Silica (mainly composed of silicon dioxide, SiO₂) itself does not possess significant photocatalytic performance. However, as an ideal photocatalytic material carrier, its photocatalytic efficiency can be significantly improved by combining it with photocatalysts such as TiO₂ and CdS, showing great application potential, especially in environmental remediation and energy conversion.

I. Characteristics and Photocatalytic Basis of Silica
Basic Characteristics of Silica
Chemical Composition: Silica is a synthetically produced amorphous silica powder, with the chemical formula SiO₂·nH₂O, where nH₂O exists as surface hydroxyl groups.

Physical Properties: It possesses an extremely high specific surface area (500-1000 m²/g), a porous structure, and abundant surface functional groups (such as silanol groups -Si-OH), making it an ideal catalyst carrier.

Chemical Stability: It is heat-resistant, non-flammable, odorless, soluble in caustic alkalis and hydrofluoric acid, but insoluble in water and common acids.

Photocatalytic Mechanism
Basic Principle: Photocatalysis is a process in which semiconductor materials generate electron-hole pairs under light irradiation, thereby initiating a redox reaction.

The Role of Silica: Silica itself is not a semiconductor material and does not possess photocatalytic activity. However, its high specific surface area and surface functional groups can effectively adsorb pollutant molecules and act as a carrier to immobilize the photocatalyst, improving its dispersibility and stability.

Synergistic Effect: When silica is combined with the photocatalyst, a heterogeneous interface can be formed, promoting the separation of photogenerated electrons and holes, reducing the recombination rate, and thus improving photocatalytic efficiency.

II. Preparation Methods of Silica Composite Photocatalytic Materials
Sol-Gel Method
Most Commonly Used Method: By selecting suitable precursors (such as tetrabutyl titanate, sodium silicate) and additives, the hydrolysis and condensation processes are controlled to form a homogeneous silica-photocatalyst composite material.

Advantages: The pore size, specific surface area, and crystal structure of the material can be precisely controlled, improving the catalytic effect.

Hydrothermal/Solvothermal Method
High-Temperature and High-Pressure Synthesis: Under hydrothermal or solvothermal conditions, the crystallization and growth of photocatalysts such as TiO₂ and CdS on the surface of silica are promoted.

Application Example: The CdS@g-C₃N₄ composite material synthesized by the solvothermal method achieved a degradation efficiency of over 90% for methylene blue (MB) within 30 minutes.

Precipitation Method
TiCl₄ Hydrolysis Precipitation Method: Using silica as a carrier, supported nano-TiO₂ photocatalysts were prepared by controlling the final pH value (5-5.5) of the TiCl₄ hydrolysis reaction. The degradation rate of Rhodamine B reached 96.87%.

Optimized Parameters: The optimal TiO₂ loading was 30%, and the TiO₂ exhibited the anatase form after calcination at 600℃, resulting in the best photocatalytic activity.

Sol-Gel-Calcination Method
Preparation of Composite Antibacterial Agents: Titanium dioxide is loaded onto a precipitated silica support using the sol-gel method to obtain a composite powder material possessing both photocatalytic and antibacterial properties.

Innovation: The precipitated silica-titanium dioxide composite photocatalytic antibacterial agent prepared by this method exhibits significant antibacterial properties under both UV and UV light irradiation, broadening the light response range.

III. Application Areas of Precipitated Silica Composite Photocatalytic Materials
Environmental Pollution Control
Water Pollution Treatment: Precipitated silica-TiO₂ composite materials can effectively degrade organic pollutants (such as Rhodamine B and methylene blue) and heavy metal ions in water.

Air Pollution Control: It can promote the degradation of harmful gases (such as nitrogen dioxide) in the air, improving air quality.
Innovative Application: The Kunming University of Science and Technology team utilized a "structure-electric field" synergistic control strategy to prepare copper-based composite microspheres with an "egg yolk-double shell" configuration, achieving a solar-driven degradation efficiency for tetracycline antibiotics that is dozens of times higher than that of traditional materials.

Renewable Energy Development
Water Splitting for Hydrogen Production: Silica-based composite photocatalysts enhance the activity and stability of photocatalysts, promoting hydrogen production through water splitting.

CO₂ Reduction: This can be used to reduce carbon dioxide into valuable chemicals, enabling the recycling of carbon resources.

Photocatalytic Organic Synthesis

Selective Reactions: Silica-based composite photocatalysts enable selective oxidation, reduction, and hydroxylation of organic matter, exhibiting high catalytic activity and selectivity.

Green Chemistry: Provides an environmentally friendly alternative to traditional organic synthesis, reducing the generation of harmful byproducts.

Antibacterial and Water Disinfection

Novel Disinfection Technology: A research team at Sun Yat-sen University has developed a self-floating photocatalytic thin film that can efficiently accumulate and continuously sterilize under low-light conditions, significantly improving photocatalytic disinfection efficiency.

Mechanism Innovation: By precisely controlling the molecular structure of the photocatalyst, the lifetime of oxygen-centered organic free radicals (OCORs) has been successfully extended to the minute level, overcoming the limitation of the short lifetime of traditional reactive oxygen species (ROS).

IV. Advantages and Challenges of Silica Composite Photocatalytic Materials
Advantages & Characteristics:
High specific surface area: Provides abundant surface active sites, which is beneficial for the adsorption and reaction of reactants

Good chemical stability: Remains stable under various environmental conditions and can be reused.

Synergistic effect: Silica and photocatalyst form a heterogeneous interface, promoting the separation of photogenerated electrons and holes and improving photocatalytic efficiency.

Multifunctionality: The composition and structure of the composite material can be controlled to achieve efficient degradation of different pollutants.

Challenges:
Complex preparation process: Requires precise control of reaction conditions to achieve a highly uniform composite structure.

Large-scale production challenges: Achieving large-scale production of the composite material remains a challenge to be overcome.

Improving photocatalytic efficiency: Although significant progress has been made, there is still room for improvement in visible light utilization and quantum efficiency.