�Solar Steam Generation

Potable water

• 1.1 billion people worldwide lack access to water
• 2.7 billion find water scarce for at least one month of the year
•  By 2025, two-thirds of the world’s population may face water shortages

A simple steam generation setup with nanoparticles

• While steam generation can be demonstrated with just a lens and a beaker these setups can be easily improved in simple ways
• Collection of steam
• Ice bath

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What is causing the steam if the overall water temperature is <100° C?

• Local heating around nanoparticles
• Works with any metal nanoparticle
• We used Cu nanoparticles because they are cheap
• Leftover Au nanoparticles from last year

Ancient Roman nanotech?

• The Lycurgus Cup from 4^th century CE Roman Empire
• When lit from the front it looks green, from the back red
• The glass is full of gold and silver nanoparticles (50 nm)
• The color when lit from the back is from enhanced scattering of red light in the glass

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Plasmonic nanoparticles

Surface plasmons are free oscillations of electrons on metal surfaces

• Electromagnetic wave + charge density oscillations propagate along surface
• EM field decays exponentially into metal and dielectric

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Metal nanoparticles behave like resonant oscillators

⇐⇒

⇐⇒

• Mass m ⇐⇒ Inductance (resistance to current change)
• Spring constant ⇐⇒ 1/Capacitance (storage of electric energy)

𝜔 =

𝑘 1

^{𝑚 =} 𝐿𝐿

The width of the resonance depends on energy losses to heating and radiation

The plasmon resonance depends on the material…

… and on particle shape

J. J. Mock et al., J. Chem Phys. 116, 7655 (2002)

Size is much less important, since we are in the quasi-static limit

λ ~ 600 nm

𝜆 ≫ 𝑑, so the particle sees a nearly uniform electric field

⇒ EM behavior is independentt of size

Plasmonic resonances

• Localized Surface Plasmons

Resonances (LSPRs) occur in metallic

nanostructures

• Strong effect on optical scattering
• LSPRs can concentrate incident E-M intensities.
• The intensity enhancement (𝑔) can

reach 10³ – 10⁷ times.

J. P Kottman, et al., J. Microscopy

202 (1), 60 (2001).

J. J. Mock et al., J. Chem Phys. 116, 7655 (2002)

Hot spot enhancement�= large local density of states (LDOS)�

• At the hot spots, there are more quantum mechanical states available for photons
• This is due to coupling to electromagnetic modes carried by electrons in the metal
• As a result, photons can both enter and leave hot spots with high efficiency.
• Some processes are therefore enhanced as 𝑔²

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Summary

• From local hot spots caused by SPR around each nanoparticle
• Can be used to purify water
• Cheap if Cu nanoparticles are used