An ultrathin MXene film absorbs invisible solar wavelengths to generate heat while staying transparent, enabling passive anti-fogging and de-icing on glass surfaces.
(Nanowerk Spotlight) Sunlight carries enough energy to melt a thin layer of ice in minutes. If a window, windshield, or eyeglass lens could quietly harvest that energy and warm itself, fogging and frost would never obscure the view. Yet capturing solar heat on a glass surface without making the glass opaque has remained an unsolved problem. The physics works against itself: a material absorbs photons to generate heat, but absorbing visible photons makes the material dark.
Every transparent photothermal coating developed to date has been forced into a compromise, either absorbing too little light to be useful or sacrificing the clarity that makes glass worth having. Nanoscale fillers such as carbon particles, metal nanoparticles, and semiconductors embedded in polymer hosts absorb only in narrow wavelength bands. Loading enough of them to broaden the absorption range clouds the material. Thin-film techniques like spray coating and sputtering concentrate the absorbers at the surface, but they produce uneven layers thicker than 100 nm that degrade transparency, heating performance, or both.
A research team at Beihang University has reported a route around this trade-off. Their study, published in Advanced Science (“Transparent Photothermal Slippery Surface Based on Monolayer Self-Assembled MXene Film for Anti-Fogging and De-Icing”), describes a film of MXene nanosheets just 2.5 nm thick that achieves 82.5% visible-light transparency while raising its surface temperature by roughly 25 °C under standard solar illumination. The solution rests on a simple but underexploited fact: less than half the energy in sunlight is actually visible.
Design of the self-assembled Mxene film with bridging structure. (a) Schematic illustration of the liquid–liquid interfacial self-assembly of 2D MXene nanosheets. (b) Dynamic illustration of the self-assembly process of MXene nanosheets driven by the Marangoni effect. Subpanel (i) shows the dynamic behavior of the MXene suspension prior to contacting the water phase; subpanel (ii) depicts the Marangoni flow and nanosheet self-assembly upon the instant the isopropanol-based MXene dispersion meets the water phase. (c) Scanning electron microscopy (SEM) image of exfoliated MXene nanosheet. (d) Optical microscopy image of a monolayer MXene self-assembled film, where the light-colored MXene sheets are uniformly oriented on the brown substrate. (e) Atomic force microscopy (AFM) image of the monolayer MXene self-assembled film. The inset image is the height profile of the monolayer MXene (2.5 nm) in a self-assembled film. (f) Height profile statistics of the monolayer MXene self-assembled film in AFM image (e). (Image: Reproduced from DOI:10.1002/advs.202522420, CC BY)
Only about 45.5% of solar energy reaching Earth’s surface falls within the visible range of 400 to 700 nm. Nearly 50% arrives as near-infrared radiation, and about 4.6% as ultraviolet. MXenes, a family of two-dimensional materials derived from layered titanium carbide (Ti₃C₂Tₓ), possess a high density of free electrons and abundant surface chemical groups that make them strong absorbers across a wide spectral range.
By tuning the film to capture ultraviolet (300 to 400 nm) and near-infrared (700 to 2000 nm) energy while letting visible wavelengths pass, the team captures the majority of solar power without blocking the view. Spectral measurements confirmed that roughly 62% of the total absorbed energy originates outside the visible band.
Fabricating such a precisely structured film required an unconventional method. The researchers dispersed single-layer MXene nanosheets in isopropanol and sprayed the mixture onto a hexane-water boundary. The surface tension difference between the two liquids generates Marangoni flow, a tangential force that rapidly spreads the nanosheets outward and lays them flat in an ordered monolayer just 2.5 nm thick. A single such layer transmits 94.3% of light at 550 nm. Tens of droplets from a 2 mg mL⁻¹ dispersion cover an area of 10 cm².
About 80% of the nanosheets settle into well-oriented single layers, but a small fraction forms stacked overlaps roughly 4.2 nm thick. These overlaps act as electrical bridges between neighboring sheets. Once enough bridges form, the film crosses what physicists call a percolation threshold: the point at which isolated conductive patches merge into one continuous network, much as scattered puddles on pavement eventually connect into a single flowing sheet.
Below this threshold, individual nanosheets absorb light only at narrow, specific wavelengths. Above it, the connected network dissipates energy broadly, behaving less like a collection of tiny antennas and more like a thin, slightly leaky metallic sheet in which frequent electron collisions convert absorbed light across a wide spectral range into heat.
With the physics of the monolayer established, the practical question becomes how many layers are needed. Stacking three monolayers produced a temperature rise of 25.1 °C (±2.9 °C) under one-sun illumination (100 mW cm⁻²) while maintaining 82.5% transmittance and a haze below 4%. At reduced intensities of 0.5 and 0.8 sun, the three-layer stack still reached gains of roughly 15 °C and 20 °C. This photothermal efficiency exceeds that of comparable transparent coatings based on gold, copper, or carbon films produced by conventional spray methods.
To translate heating into practical ice and fog resistance, the team added a slippery top layer: a thin coating of silicone oil-infused PDMS gel, a soft polymer saturated with lubricant, roughly 8 µm thick. This smooth surface allows liquids to glide off under gravity within 20 seconds. Unlike textured superhydrophobic coatings that scatter light and create haze, the lubricant layer preserves both transparency and heating efficiency. Despite PDMS having a low thermal conductivity (0.15 W m⁻¹ K⁻¹), the layer is so thin that heat generated by the MXene transfers to the outer surface with negligible loss.
The complete composite, designated TPSS, was tested at −20 °C under controlled conditions. Under one-sun illumination, the surface warmed to about 7.9 °C within five minutes, preventing deposited ice-cold water from freezing. Pre-frozen ice melted and slid away within roughly 10 minutes. At 90% relative humidity and −20 °C, the coating maintained a completely frost-free surface. Slippery surfaces lacking the MXene layer accumulated frost rapidly, while photothermal surfaces without the slippery coating could not shed melted ice. Neither component alone delivered full anti-icing and anti-fogging function.
The PDMS encapsulation also shields the MXene from environmental degradation. Bare MXene films oxidize when exposed to moisture, forming insulating titanium dioxide that disrupts the conductive network. X-ray diffraction and photoelectron spectroscopy confirmed that encapsulated MXene showed negligible oxidation after 144 hours at humidities from 10% to 90%, while unprotected films developed substantial titanium dioxide. Simulated rain and sand abrasion left TPSS heating performance stable after 10 cycles; bare MXene films failed within three to five.
The nanometer-scale thickness of the MXene layer also allows it to conform to curved and flexible substrates. Applied to flexible PET (polyethylene terephthalate) film, the coating bent and deformed without damage. In outdoor tests conducted in Beijing at 2.1 °C, a coated eyeglass lens remained clear while the wearer exhaled warm, humid breath through a face mask; the uncoated lens on the same pair fogged immediately. A building model fitted with coated transparent PMMA (polymethyl methacrylate) panels shed a pre-frozen ice layer autonomously under simulated sunlight.
The entire process relies on self-organization at a liquid interface, avoiding vacuum chambers, high temperatures, and expensive deposition equipment. Material consumption sits below 0.1 mg cm⁻². Requiring zero energy input during operation and showing documented resilience against rain and abrasion, the coating offers a credible path toward passive, sunlight-driven anti-icing and anti-fogging protection for architectural glass, vehicle windshields, eyewear, and optical sensors.
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