I. Basic Concepts of Photoreactions
1.1 Ultraviolet Light Sources
Ultraviolet (UV) light serves as the primary light source for photocatalytic reactions, with a typical wavelength range of 250–400 nm. Commonly used UV light sources are listed as follows:
• Low-pressure mercury lamps: Emit UV light at 254 nm and 185 nm, with the power of a single lamp generally not exceeding 100 W. They are categorized into hot cathode and cold cathode types. Hot cathode low-pressure mercury lamps were the first to be developed and are the most widely produced and applied; cold cathode lamps feature a longer service life. Low-pressure mercury lamps are the prevailing light sources for laboratory and small-scale photocatalytic research.
• Medium-pressure mercury lamps: Deliver a broad emission spectrum covering 254–365 nm and longer wavelengths with high power output, suitable for large-scale treatment processes.
• High-pressure mercury lamps: Primarily emit 365 nm UV light falling within the long-wave UV-A region. The 365 nm wavelength lies near the absorption edge of TiO₂, making this lamp an extensively adopted light source for TiO₂ photocatalytic experiments.
• Black light lamps: Mainly radiate 365 nm UV light with flexible power options, widely utilized in photocatalysis research.
1.2 Direct Degradation by Ultraviolet Light
UV light alone can directly degrade organic pollutants, yet its mechanism and efficiency differ fundamentally from photocatalysis:
• Conventional UV degradation (254 nm UV): Only cleaves specific chemical bonds (e.g., C–X bonds) via homolytic cleavage. This method merely degrades organics with particular structures at limited efficiency, and is primarily applied for disinfection and sterilization.
• Vacuum UV degradation (VUV, 185 nm): Shorter wavelengths carry higher energy, capable of homolytically breaking most chemical bonds and even decomposing water molecules in the medium. Nevertheless, VUV exhibits extremely shallow penetration depth in water: the irradiated annular zone in a loop reactor is merely approximately 70 micrometers thick. Dissolved oxygen within this thin layer is rapidly consumed, and insufficient oxygen supply may conversely trigger polymerization reactions. Hence, VUV degradation is currently limited to research on water purification containing relatively low-concentration organic matter.
Direct UV Degradation vs. Photocatalytic Degradation
• Direct degradation: Low efficiency, high selectivity, narrow applicable range
• Photocatalytic degradation: Complete non-selective mineralization via generation of hydroxyl radicals (·OH), high efficiency, broad applicable spectrum
II. Fundamental Principles of Photocatalytic Reactions
2.1 Classification of Photocatalytic Reactions
Photocatalytic reactions are divided into two categories based on the existing state of catalysts:
1. Homogeneous photocatalysis: Catalysts exist in dissolved form within the reaction system.
2. Heterogeneous photocatalysis: Catalysts remain solid in the reaction system.
In the field of environmental water treatment, heterogeneous photocatalysis using solid TiO₂ catalysts dominates the industry.
2.2 Development History
The development of TiO₂ photocatalysis technology can be summarized into three landmark milestones:
1. 1972: Japanese scientists Fujishima and Honda published a paper in Nature, realizing water splitting for hydrogen production using TiO₂ as a photocatalyst for the first time. This pioneering work laid the foundation for photocatalysis research, known as the "Fujishima–Honda effect".
2. 1977: Frank and Bard first applied TiO₂ photocatalysis to wastewater treatment and successfully oxidized cyanide ions (CN⁻) in wastewater, marking the transition of photocatalysis technology from fundamental research to practical applications.
3. 1990s to present: The introduction of nanotechnology and escalating environmental pollution issues have driven rapid advancement of TiO₂ photocatalysis in pollution remediation. Governments worldwide have invested substantial research resources, yielding abundant research achievements.
2.3 Semiconductor Energy Band Structure and Photocatalytic Mechanism
The theoretical basis of semiconductor photocatalysis originates from the energy band structure of semiconductors. The energy bands of semiconductor materials consist of the Valence Band (VB) and Conduction Band (CB), with an energy gap between them defined as the Band Gap (E₉).
When a semiconductor is irradiated by light with energy greater than or equal to its band gap energy, electrons (e⁻) in the valence band are excited and jump to the conduction band, leaving holes (h⁺) in the valence band. This process generates photogenerated electron–hole pairs, the starting point of all photocatalytic reactions.

Photogenerated charge carriers inside the semiconductor have three possible fates:
1. Bulk recombination: Electrons and holes recombine inside the semiconductor, releasing energy as heat (undesirable).
2. Surface recombination: Recombination occurs on the semiconductor surface (undesirable).
3. Migration to the surface to participate in chemical reactions (the target outcome).
Only carriers migrating to the surface can drive catalytic reactions.
Quantum Yield and Apparent Quantum Yield are two critical metrics for evaluating photocatalytic efficiency.
• For TiO₂ (E₉ = 3.2 eV): Maximum absorbable incident wavelength = 387.5 nm
• For CdS (E₉ = 2.5 eV): Maximum absorbable incident wavelength = 496 nm
2.4 Surface Chemical Reactions
Charge carriers migrating to the TiO₂ surface separately participate in oxidation and reduction pathways:
Oxidation Pathway (Hole Reactions)
• 
(hydroxyl radical generation)
• 
(water oxidation)
• 
(direct organic oxidation)
Reduction Pathway (Electron Reactions)
• 
(superoxide anion generation)
• 
(Fenton-like reaction)
Multiple reactive oxygen species (ROS), including ·OH, ·OOH and ·O₂⁻, are ultimately produced via surface reactions. These ROS work synergistically to gradually oxidize and mineralize organic pollutants into carbon dioxide and water.
2.5 Common Semiconductor Photocatalysts
Typical semiconductor photocatalysts include TiO₂, CdS, ZnO, WO₃, Fe₂O₃, SnO₂, SrTiO₃, etc. They are classified below based on band gap width, catalytic activity and chemical stability:
表格



Semiconductor
Band Gap (eV)
Maximum Absorbable Wavelength (nm)
Catalytic Activity
Stability
TiO₂
3.2
387.5
High
Stable (no photocorrosion)
CdS
2.5
496
High
Unstable (metal ion leaching)
ZnO
3.2
387.5
High
Unstable (metal ion leaching)
WO₃
2.8
443
Moderate
Relatively stable
Fe₂O₃
2.2
564
Moderate
Relatively stable
SrTiO₃
3.2
387.5
Moderate
Stable
Comprehensively considering catalytic activity, chemical stability and biosafety, TiO₂ stands as the dominant photocatalyst in the field: it resists photocorrosion, tolerates wide pH ranges, is non-toxic to organisms, and has abundant raw material reserves. Its wide band gap of 3.2 eV endows it with strong redox capacity. Although CdS and ZnO deliver high catalytic activity and narrower band gaps capable of absorbing visible light, they suffer severe photocorrosion under illumination accompanied by metal ion dissolution, limiting their practical industrial applications.
2.6 Three Crystal Phases of TiO₂
TiO₂ naturally exists in three crystal forms: anatase, rutile and brookite, differing in the connection mode of TiO₆ octahedra. Each octahedron in anatase shares edges with four adjacent octahedra, while octahedra in rutile and brookite share edges with only two neighboring units.
In terms of photocatalytic activity, anatase TiO₂ exhibits the highest performance. Rutile is the thermodynamically stable phase; anatase irreversibly transforms into rutile under high-temperature calcination. Brookite has poor structural stability and is rarely applied in photocatalysis. Commercial P25 TiO₂ is a mixed crystal composed of approximately 75% anatase and 25% rutile. Heterojunctions formed at the phase interface facilitate the separation of photogenerated electron–hole pairs.
III. Influencing Factors of Photocatalytic Reactions
Figure 1: Six Core Factors Governing Photocatalytic Degradation Efficiency
3.1 Catalyst Particle Size
Catalyst particle size exerts a remarkable impact on photocatalytic efficiency through two primary mechanisms:
1. Adsorption performance: Reduced particle size increases specific surface area and the number of surface adsorption sites, promoting adsorption of organic pollutants and dissolved oxygen to accelerate reaction rates.
2. Quantum yield optimization: Smaller particle sizes drastically shorten the migration time of photogenerated carriers from the bulk phase to the surface. For instance, reducing particle diameter from 1 μm to 10 nm cuts carrier migration time from 100 nanoseconds to 10 picoseconds, lowering the probability of bulk recombination.
However, significant quantum size effects emerge when particles shrink to an extremely small scale, widening the semiconductor band gap (blue shift). While this enhances redox potential, the maximum absorbable wavelength shortens, potentially preventing efficient catalyst excitation by the commonly used 365 nm UV light. Therefore, particle size must be selected through comprehensive trade-off analysis.
3.2 Light Source and Light Intensity
The correlation between degradation rate and light intensity falls into three distinct regimes:
1. Low light intensity region:

,

Reaction rate is proportional to light intensity, with constant quantum efficiency.
2. Medium light intensity region:

,

Reaction rate rises with the square root of light intensity, accompanied by declining quantum efficiency.
3. High light intensity region:

,

Degradation rate plateaus, and quantum efficiency drops sharply.
Taking chloroform treatment as an example, degradation rates stop improving when light intensity exceeds

. Excessively high light intensity leads to excessive carrier concentration and drastically accelerated recombination rates. Practical systems are recommended to operate within low-to-medium light intensity ranges for optimal quantum efficiency.
3.3 Organic Pollutant Concentration
The relationship between organic concentration and photocatalytic degradation rate follows the Langmuir–Hinshelwood model. This model assumes organic molecules first adsorb onto the catalyst surface in compliance with Langmuir isotherms, followed by a first-order reaction between adsorbed pollutants and surface reactive species.
• Low concentration (

): Rate equation simplifies to

, corresponding to apparent first-order kinetics. Increasing pollutant concentration accelerates degradation.
• High concentration (

): Catalyst surface reaches adsorption saturation; rate equation simplifies to

, corresponding to zero-order kinetics. Further concentration elevation fails to boost degradation speed and only extends treatment duration.
3.4 Solution pH Value
pH affects photocatalysis via multiple pathways:
1. Modulation of semiconductor band potential: Band potential varies with pH following the formula
. Rising pH shifts the conduction band to more negative potentials and the valence band to more positive potentials, strengthening the oxidizing capacity of semiconductor catalysts.
2. Alteration of organic speciation: Organics exist as neutral molecules or charged ions at different pH levels, which exhibit vastly different adsorption affinity toward TiO₂ surfaces and directly affect reaction rates.
3. Interactive effect with light intensity: pH demonstrates an extremely pronounced influence under low light intensity (

). Experiments show that under

, chloroform degradation rate at pH 8 is over ten times higher than that at pH 3.8.
3.5 Extrinsic Electron Acceptors
Extrinsic electron acceptors (e.g., O₂, H₂O₂, persulfate, periodate, etc.) effectively trap photogenerated electrons, suppress electron–hole recombination and improve reaction efficiency. Oxygen serves as the most fundamental electron acceptor, maintaining electrical neutrality of the system and generating additional ·OH via subsequent reactions.
H₂O₂ exerts dual effects:
• Positive effect: Decomposes into ·OH under UV irradiation or generates ·OH by capturing photogenerated electrons to promote degradation.
• Negative effect: Acts as a radical scavenger; high dosages consume ·OH and reduce degradation efficiency.
Precise optimization of H₂O₂ dosing concentration is therefore mandatory.
3.6 Inorganic Ions
Aqueous inorganic ions interfere with photocatalytic efficiency through two mechanisms:
1. Competitive adsorption: Sulfate, fluoride, chloride, phosphate and other anions compete with organic pollutants for TiO₂ surface adsorption sites, reducing effective contact between contaminants and catalysts.
2. Radical scavenging: Carbonate ions (

) are potent ·OH scavengers with a reaction rate constant of

. Bicarbonate (

) exhibits negligible reactivity with ·OH (

). The distribution of carbonate species depends strongly on pH: elevated pH increases the proportion of

and amplifies radical scavenging effects.
3.7 Reaction Temperature
The correlation between photocatalytic reaction rate and temperature generally conforms to the Arrhenius equation, yet temperature impacts differ across target pollutants. For phenol degradation, reaction rates slightly rise with increasing temperature; for chloroform degradation, especially under high light intensity, elevated temperature suppresses degradation due to accelerated recombination of ·OH radicals. Overall, photocatalysis exhibits weak temperature dependence and operates efficiently at ambient temperature.
Potential Advantages and Main Barriers of Photocatalysis
Five Core Advantages
1. Energy-saving: Utilizes inexhaustible solar radiation as the light source
2. Powerful mineralization capacity: Decomposes most organic pollutants and immobilizes heavy metals
3. High safety: Catalysts feature excellent stability, anti-photocorrosion performance and non-toxicity
4. Mild reaction conditions: No strict requirements on pH or temperature
5. Flexible scalability: Applicable to small and large-scale treatment systems with adjustable processing loads
Two Major Technical Barriers
1. Insufficient intrinsic activity of photocatalysts
2. Limited light penetration distance inside photoreactors
Chapter Summary
TiO₂ photocatalysis generates photogenerated electron–hole pairs under light excitation, which produce reactive oxygen species such as ·OH on catalyst surfaces to mineralize organic pollutants. TiO₂ stands as the preferred photocatalyst thanks to its high catalytic activity, superior chemical stability and biosafety. Catalyst particle size, light source intensity, organic pollutant concentration, solution pH, extrinsic electron acceptors and inorganic ions jointly determine overall photocatalytic efficiency. How to further enhance catalyst activity and light utilization efficiency will be covered in the next chapter focusing on catalyst modification technologies and photoreactor engineering.
