Titanium dioxide nanoparticles are used in some sunscreen products. Explain why manufacturers choose to use titanium dioxide in the form of nanoparticles rather than as a bulk powder.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Titanium dioxide is a white solid that absorbs ultraviolet (UV) radiation. In bulk form it leaves a visible white residue on skin. When used as nanoparticles, the sunscreen appears transparent on the skin.
Model answer (4 marks)
Titanium dioxide nanoparticles are chosen because they have a much larger surface area to volume ratio than bulk powder, so for the same mass they present more active surface to absorb UV radiation.
The larger surface area also makes the particles more effective at blocking UV per unit mass, allowing a lower concentration to be used.
Nanoparticles can exhibit different physical properties to the same material in bulk; here the optical properties change.
Because the particles are so small they do not scatter visible light, so the sunscreen looks transparent on the skin, unlike the white bulk powder.
The larger surface area also makes the particles more effective at blocking UV per unit mass, allowing a lower concentration to be used.
Nanoparticles can exhibit different physical properties to the same material in bulk; here the optical properties change.
Because the particles are so small they do not scatter visible light, so the sunscreen looks transparent on the skin, unlike the white bulk powder.
Examiner tips
- Use the term "surface area to volume ratio" and link it to UV absorption. Mention the optical property change (no visible scattering).
- Show the logical chain: larger SA/V → more UV absorption → less product needed, and small size → transparent appearance.
Common mistakes
- Confusing bulk with nanoparticle properties; forgetting to mention transparency. Using vague terms like "better" without explaining why. Not linking surface area to UV absorption explicitly.
Mark scheme (4 marks)
- Nanoparticles have a much larger surface area to volume ratio than the bulk material
- The greater surface area to volume ratio means nanoparticles are more effective at absorbing UV radiation (per unit mass)
- Nanoparticles can exhibit different properties to the same material in bulk
- Nanoparticles are too small to scatter visible light so the sunscreen appears transparent/colourless on skin (unlike bulk powder which appears white)
Key terms in this question
Related
- All AQA GCSE Chemistry (8462) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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