Question 9

Question 1

q9a)

Below you see a sketch of a side view of a film, which is composed of stacked capsule‐ type spheres with shells of gold (yellow dots) and titania (black dots) nanoparticles.

Figure: Side view on film built up of close packed multilayered gold‐titania spheres (yellow: gold nanoparticles, black: titania nanoparticles; black line: substrate)

Which synthesis method would you use to prepare the gold and the titania nanoparticles.

Answer 1

Gold NP: citrate route (get negatively charged nanoparticles)
- NaAuCl4 in water → AuCl4- + Citrate → Au reduced to Au(0) and this nucleates and forms gold nanoparticles. Citrate on surface
→ citrate as reducing and stabilizing agent: forms a citrate bilayer, surface is negatively charged (strong pH dependance): hydrophilic

TiO2: aqueous sol-gel with Trizma as stabilizing agent (positively charged)

Question 2

q9b)

Write down a chemical mechanism for the synthesis of the titania nanoparticles based on the answer in a).

Answer 2

Non-aqeous would also be possible (use benzyl alcohol) but different possible mechanisms

Would also be possible with Ti(OEt)4

Question 3

q9c)

How would you assemble the gold and the titania nanoparticles into the spherical capsule‐type structures?

Describe the most important experimental steps.

Answer 3

1. latex ball in center (negatively charged with polymer surfactant on surface), made by droplet flow
2. Put in TiO2 nanoparticle solution, wash
3. put in gold nanoparticle solution, wash
4. repeat
5. in the end: calcination to remove latex sphere & for sintering

Question 4

q9d)

How would process the gold‐titania spheres into an ordered, close‐packed film?

Answer 4

• Dip coating with evaporation-induced self-assembly
• spin coating
  to remove charges: use NaCl → charge screening

Question 5

q9e)

Considering your answer in d), which experimental parameter you think is crucial in deciding, whether you get an ordered or disordered structure?

Answer 5

For dip coating/spin coating:
- Evaporation time (too short → no time for self-assembly)
- solvent, pressure, atmosphere & temperature influence evaporation time
- concentration of spheres

Question 6

q9f)

Could you think of an application of such a material?

Answer 6

Carbon capturing:
Can be used for the reduction of CO2 by enhancing the separation between e- & h (no recombination)
→ photocatalysis