TiO₂ Particle Size and Performance: Opacity, UV Blocking, and Photocatalysis Explained
Particle Size and Optical Behavior
Particle size controls whether TiO₂ functions as a white pigment, transparent UV filter, or photocatalyst. Pigment-grade TiO₂ at 200–300 nm maximizes Mie scattering and hiding power (>30 m²/g). Below 100 nm, particles scatter visible light poorly — coatings appear transparent while still absorbing UV. Specifying particle size without defining the target function leads to mismatched product selection.
Anatase vs Rutile: Crystal Phase Sets the Performance Ceiling
Rutile's refractive index (2.70–2.73) exceeds anatase (2.52–2.56), giving rutile 10–15% better light scattering at equivalent diameters. For UV blocking, nano rutile (15–50 nm) outperforms anatase at the same size. Anatase dominates photocatalysis — its conduction band at ~−0.2 V vs NHE enables more efficient hydroxyl radical generation. Rutile is correct for whiteness and durability; anatase is obligatory for self-cleaning and photodegradation.
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Photocatalysis vs UV Screening: The Unavoidable Size Trade-off
Photocatalytic anatase (5–25 nm, BET 100–300 m²/g) generates ROS efficiently but is transparent — unusable as pigment. Nano rutile (15–50 nm, BET 50–150 m²/g) balances UV extinction with cosmetic transparency for mineral sunscreens. Pigment rutile (200–300 nm, BET 5–15 m²/g) delivers maximum hiding power with negligible photocatalytic activity. These three functions are not interchangeable.
BET Surface Area: The Functional Proxy That Matters Most
BET is the most reliable single specification for predicting TiO₂ behavior. Pigment grades: 5–15 m²/g; UV-filter: 50–150 m²/g; photocatalytic: 100–300 m²/g. Higher BET means more reactive surface but also higher oil absorption and agglomeration tendency. Surface-coated particles include coating mass — subtract 10–30% to estimate bare-core active surface.
Commercial Grade Specifications at a Glance
| Grade | Crystal Phase | Particle Size | BET (m²/g) | Refractive Index | Primary Applications |
|---|---|---|---|---|---|
| Pigment | Rutile | 200–300 nm | 5–15 | 2.70–2.73 | Architectural coatings, plastics, paper |
| UV Filter / Sunscreen | Rutile | 15–50 nm | 50–150 | 2.70–2.73 | Mineral sunscreen, clear UV coatings, cosmetics |
| Photocatalytic | Anatase | 5–25 nm | 100–300 | 2.52–2.56 | Self-cleaning surfaces, air purification, water treatment |
| Mixed Phase (P25-type) | Anatase/Rutile (~80:20) | 10–30 nm | 50–120 | 2.55–2.65 | Dye degradation, research-grade photocatalysis |
FAQ
+What particle size gives maximum opacity in white coatings?
Rutile at 200–300 nm delivers peak Mie scattering. Below 150 nm hiding power drops rapidly; above 400 nm scattering efficiency also declines.
+Why is anatase preferred for photocatalytic applications?
Anatase's conduction band at ~−0.2 V vs NHE enables stronger reductive chemistry and more efficient hydroxyl radical generation. Its nano grades (5–25 nm, BET 100–300 m²/g) provide the high surface area needed for rapid pollutant degradation.
+Can nano TiO₂ below 50 nm replace standard pigment grade?
No — nano TiO₂ is transparent in visible light and provides no hiding power. Anatase nano grades also generate surface radicals that degrade organic binders, causing chalking and coating failure.
+What BET should I specify for a mineral sunscreen active?
Nano rutile with BET 50–100 m²/g and D50 15–35 nm. Surface treatment (alumina + silica or organosilane) is mandatory to suppress photocatalytic reactivity.
+At what temperature does anatase convert to rutile?
Transformation begins at 600–700°C and completes by ~900°C. Grain growth eliminates photocatalytic activity permanently. Confirm supplier phase stability data at your process temperature.
+How do I interpret BET on surface-coated grades?
Coatings add mass without proportional surface area, so coated BET is typically 10–30% lower than bare core. For photocatalytic specs, request both coated and uncoated BET values.