Painting Cheap Foam With a Nickel Dye Turns Seawater Into Drinking Water With Sunlight
The cheapest way to desalinate water is to let the sun do it — if you have a material that soaks up the whole solar spectrum and turns it straight to heat. Three papers from one lab built that material, molecule by molecule.
Nearly every solar-desalination scheme comes down to one component: a photothermal layer floating at the air–water interface that captures sunlight, heats only the top millimetre of water, and evaporates it — leaving the salt behind. The catch is the material. Carbon blacks are broadband but hard to tune; most small-molecule dyes absorb only a narrow slice of the spectrum and convert little of it to heat. Getting broad absorption, high solar-to-heat conversion, and salt resistance in one cheap, paintable material has been the bottleneck. Between 2023 and 2026, Jen-Shyang Ni’s group at National Kaohsiung University of Science and Technology closed that gap with a family of organonickel bis(dithiolene) complexes — and tuned them three different ways.
The Obstacle
A good interfacial photothermal material needs to do four things at once: absorb across the visible and near-infrared (where much of the sun’s energy sits), convert that light to heat rather than re-emitting it, sit on a cheap porous support that wicks water, and reject salt so it doesn’t clog and stall. Narrow-band dyes fail the first test; brightly fluorescent molecules waste energy as light instead of heat; and many high-performing evaporators are expensive to make. The design problem is molecular: build a small molecule whose absorbed photons decay non-radiatively into vibrations — heat — across a wide band.
The Innovation
The shared building block is a square-planar nickel bis(dithiolene) complex with an intervalence charge-transfer (IVCT) transition that pushes absorption deep into the NIR-II window. Its absorbed energy relaxes through molecular motion — non-radiative decay — which is exactly what you want for a heater. The three papers are a deliberate progression in how to tune it:
- Pick the metal (proof of concept). Neutral d8 nickel, palladium and platinum bis(dithiolene) complexes (NiPN/PdPN/PtPN) were compared; the nickel complex gave the highest photothermal conversion and the best evaporator.
- Tune the ring (broadband + salt resistance). Swapping phenyl for thienyl substituents (NiTh vs NiPh) extended electron delocalisation; NiTh’s J-aggregation on the evaporator surface broadened absorption and conferred salt resistance.
- Tune the tail (packing + hydrophobicity). Alkyl-chain engineering (NiThR, R = C4→C16) controlled molecular packing and surface hydrophobicity on paper fibres; the long n-hexadecyl chain (NiTh16) gave the best-behaved evaporator.
Key Results
The arc runs metal → ring → tail: pick nickel, then broaden its absorption with thienyl rings, then engineer the alkyl tail into a salt-resistant, dye-cleaning paper evaporator — with the photophysics confirming, throughout, that light becomes heat.
Nickel Wins, and the Photothermal Efficiency Is the Reason
Of the three metals, the nickel complex delivered the highest photothermal conversion efficiency and, on filter paper, the best evaporator — a deionised-water evaporation rate of 1.406 kg·m-2·h-1 at a solar-to-vapour efficiency of 95.64% under one sun, with only a ~3% drop for seawater. Its dim emission against strong absorption is the working principle: light in, heat out.
Thienyl Rings and J-Aggregation Push Efficiency Past 100%
The thienyl complex NiTh reached a photothermal conversion efficiency of 48.6% (vs 29.2% for phenyl NiPh) and, on foam, an evaporation rate of 1.99 ± 0.10 kg·m-2·h-1 (blank foam: 0.48). Because the 3D evaporator also harvests light on its sides, measured solar-to-vapour efficiencies exceeded 100% — up to 122–147% — and NiTh’s surface J-aggregation made it salt-resistant, sustaining seawater evaporation at 1.80 ± 0.05 kg·m-2·h-1 over 15 cycles.
A Long Alkyl Tail Makes a Salt-Resistant Paper Evaporator
Increasing the chain length lifted photothermal conversion efficiency from 6.9% (C4) to 30.1% (C16). The NiTh16@filter-paper device evaporated at 1.72 ± 0.09 kg·m-2·h-1 (98.48% efficiency) under one sun, ran seawater at 1.67 kg·m-2·h-1 with no fall-off across cycles, and survived a direct-salt-loading test without precipitation.
…and Cleans Dye Wastewater to Clear Condensate
Beyond salt, the same NiTh16 evaporator purified organic-dye wastewater — methyl orange, rhodamine B and methylene blue — condensing clean water from strongly coloured feeds. One paintable material thus handles both desalination and dye-water cleanup, the two dominant water-purification duties, from sunlight alone.
Photophysics confirm the heat pathway. Across all three papers the complexes show strong, broadband UV-vis-NIR absorption but only faint emission — the signature that absorbed photons relax non-radiatively into heat. That absorption/emission characterisation, measured on the MRID (532 nm), is the evidence that ties the molecular design to the photothermal performance.
Why ProTrusTech Is Featuring This Research
A photothermal claim is only as good as the proof that light becomes heat rather than fluorescence. The steady-state photoluminescence and absorption of these organonickel complexes — dim emission against strong broadband absorption — were characterised on the MRID (532 nm), giving the direct optical evidence behind the whole series. It is a clean example of the MRID’s photoluminescence capability supporting materials design well outside conventional semiconductor PL.
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Why This Matters
Passive solar interfacial evaporation is the lowest-energy desalination route there is, and its ceiling is set by the photothermal material. This series shows that molecular engineering — not just nanostructuring carbon — can deliver cheap, paintable, broadband, salt-resistant heaters, and that the same evaporators double as dye-wastewater purifiers. It is a materials-chemistry answer to a water-security problem, and it sits alongside our other water-treatment features under Environmental & Water Treatment.
What’s Next
- Scaling the paint-on-paper/foam devices to module size while keeping salt resistance.
- Long-duration field trials on real seawater and industrial effluent.
- Extending the metal/ring/chain design rules to deeper NIR-II absorbers and to recover, not just evaporate, dissolved species.
Developing photothermal materials, solar evaporators, or NIR absorbers — and need photoluminescence and absorption characterisation to prove the energy pathway? Talk to our team.
Key Takeaways
- Organonickel bis(dithiolene) complexes are cheap, paintable, broadband NIR-II photothermal materials for solar interfacial evaporation.
- Metal choice: nickel beats Pd/Pt — 1.406 kg·m-2·h-1, 95.64% efficiency.
- Thienyl + J-aggregation: broadband absorption, salt resistance, efficiencies >100% (up to ~122–147%).
- Alkyl-chain tuning: NiTh16 paper evaporator — 1.72 kg·m-2·h-1, salt-resistant, and it cleans dye wastewater.
- MRID photoluminescence/absorption confirmed the dim-emission / strong-absorption signature that makes these complexes heaters, not emitters.
