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Nanoparticulate selective emitters for effective radiative cooling in urban heat islands

This study demonstrates that nanoparticulate selective emitters (Nano-SEs) significantly outperform broadband alternatives in mitigating urban heat islands by achieving lower surface temperatures under the restricted sky view factors typical of dense cityscapes, thereby offering a scalable solution for enhancing urban thermal resilience.

Original authors: Ioannis Papakonstantinou, Jiayu Song, Ahmad Alduweesh, Abdulkareem Alasli, Zhijie Xu, Albert Adibekyan, Christian Monte, Haoying Zhao, Fei Guo, Ivan Parkin

Published 2026-09-09

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Ioannis Papakonstantinou, Jiayu Song, Ahmad Alduweesh, Abdulkareem Alasli, Zhijie Xu, Albert Adibekyan, Christian Monte, Haoying Zhao, Fei Guo, Ivan Parkin

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Cities are often significantly warmer than the countryside surrounding them, a phenomenon known as the urban heat island effect. This extra heat comes from the sun warming up buildings and roads, which then radiate that stored energy back into the air, creating a feedback loop that makes cities harder to cool and more expensive to live in. One promising way to fight this without using electricity is radiative cooling. This process works by allowing a surface to radiate its heat directly out into the cold vacuum of space. The atmosphere acts like a window for this heat, letting infrared radiation pass through a specific band of wavelengths while blocking it at others. If a material can emit heat strongly through this atmospheric window and reflect sunlight, it can theoretically cool itself down even when the air around it is hot.

For years, scientists have debated the best way to build these cooling materials. Some designs are "broadband," meaning they emit heat across a wide range of infrared wavelengths. Others are "selective," designed to emit heat only through that specific atmospheric window while reflecting heat at all other wavelengths. In open, rural areas where the sky is unobstructed, previous research suggested that the more complex selective designs offered only a tiny advantage over the simpler broadband ones. This led many to believe that the extra effort to make selective emitters was not worth it. However, this conclusion assumed a clear view of the sky, a condition that rarely exists in the dense, canyon-like streets of a modern city.

A team of researchers from University College London and other institutions has now shown that in the real world of city rooftops, selective emitters are far superior. They found that when a cooling surface is surrounded by hot buildings, the simple broadband materials absorb the unwanted heat radiating from those walls, causing them to warm up. Selective emitters, by contrast, reflect that stray heat from the buildings while still letting their own heat escape to space. To prove this, the researchers developed a new type of material called a nanoparticulate selective emitter. Instead of using complex, rigid layers that are difficult to manufacture, they created tiny particles of silicon oxynitride. These particles were engineered with a porous, sponge-like structure to ensure they could be fully converted during manufacturing, giving them the perfect ability to emit heat only through the atmospheric window.

The researchers tested these materials in two very different settings. First, they placed them on a rooftop in London with a clear view of the sky. As expected, the selective emitters performed almost the same as the broadband ones, cooling the surface by a few degrees below the ambient air temperature. This confirmed that in open spaces, the extra complexity of selective emitters does not provide a major benefit. The story changed completely when they simulated a dense urban environment. They set up a test where the cooling samples were surrounded by heated walls, mimicking the hot facades of tall buildings that block the view of the sky. In this scenario, the broadband materials failed to cool effectively, often becoming warmer than the surrounding air because they absorbed the heat from the walls. The selective emitters, however, continued to cool, staying significantly cooler than the broadband versions.

The advantage of the selective design grew as the surrounding heat increased. When the researchers simulated walls heated to temperatures between 40 and 80 degrees Celsius, the selective emitters stayed up to 5 degrees Celsius cooler than the broadband ones. In even more extreme simulations, where the heat source reached temperatures of 500 degrees Celsius, the gap widened further. This happens because the heat radiating from very hot objects shifts to shorter wavelengths that fall outside the atmospheric window; the selective material naturally rejects this heat, while the broadband material absorbs it. The team also created two versions of their material: one that reflects sunlight for use on opaque roofs, and another that is transparent to sunlight for use on windows or greenhouses. Both versions maintained their ability to reject heat from hot surroundings while radiating their own heat away.

Using computer models based on the actual layout of London, the team projected what would happen if entire city blocks were covered with these materials. In low-rise areas, both types of materials worked well. But in the high-rise districts, where the sky view is heavily blocked by skyscrapers, the broadband materials struggled to keep roofs cool. The selective emitters, however, maintained lower temperatures across the entire district, effectively countering the heat trapped by the buildings. This work suggests that while simple cooling materials might be enough for open spaces, the unique challenges of dense cities require the precision of selective emitters. By rejecting the heat radiating from neighboring buildings and only releasing heat into the cold universe, these new nanoparticle-based materials offer a scalable and practical way to make future cities more livable and energy-efficient.

Technical Summary: Nanoparticulate Selective Emitters for Effective Radiative Cooling in Urban Heat Islands

Problem Statement
The urban heat island (UHI) effect, driven by heat-absorbing infrastructure and waste heat, significantly elevates urban temperatures, threatening public health and energy sustainability. Passive radiative cooling, which dissipates heat to the cold universe (3 K) through the 8–13 μm atmospheric transparency window, offers a zero-energy cooling solution. However, current consensus suggests that selective emitters (which target only the 8–13 μm window) offer only marginal advantages over broadband emitters (which emit across the entire infrared spectrum) under unobstructed sky conditions. Furthermore, existing selective emitter technologies face significant material constraints, including poor scalability, rigid substrate requirements, and trade-offs between high atmospheric window emissivity ( $\varepsilon_{8-13\mu m}$ ) and spectral selectivity ( $\eta$ ).

The authors argue that the perceived marginality of selective emitters is specific to rural, unobstructed environments. In dense urban landscapes, where buildings obstruct the sky view (low Sky View Factor, SVF) and radiate heat from hot surfaces (40–80 °C) onto cooling surfaces, the performance dynamics shift. Broadband emitters absorb this parasitic urban thermal radiation, negating their cooling capacity, whereas selective emitters reflect it. Despite this theoretical advantage, practical deployment has been hindered by the lack of scalable, high-performance materials that can be tailored for diverse urban applications (e.g., opaque roofs vs. transparent facades).

Methodology
The study employs a multi-faceted approach combining theoretical modeling, material synthesis, and experimental validation:

Theoretical Modeling: The authors modeled radiative heat flux ( $Q_{urban}$ ) incident on horizontal rooftops from surrounding vertical walls at varying temperatures ( $T_w$ ) and SVFs. They compared the theoretical equilibrium temperatures of ideal broadband emitters (BE) and ideal selective emitters (SE) under these conditions, including parasitic heat transfer coefficients ( $h$ ).
Material Synthesis (Nano-SEs): To overcome material limitations, the team developed Nanoparticulate Selective Emitters (Nano-SEs) composed of silicon oxynitride ( $SiO_xN_y$ $S i O_{x} N_{y}$ ).
- Morphological Engineering: To solve the diffusion-limited nitrogen incorporation inherent in thermal nitridation of silica ( $SiO_2$ ), they utilized porous and hollow nanospheres and silica aerogels. These morphologies provide high specific surface areas, facilitating ammonia gas diffusion and enabling controlled stoichiometry.
- Integration: The Nano-SEs were integrated into polymer matrices to create two distinct configurations:
  - Solar-Reflective: Nano-SEs dispersed in Polydimethylsiloxane (PDMS) on a silver-coated substrate for opaque surfaces.
  - Solar-Transparent: Nano-SEs dispersed in Low-Density Polyethylene (LDPE) on Indium Tin Oxide (ITO) glass for applications requiring visual access (e.g., windows, greenhouses).
Characterization: Optical properties were measured using FTIR (hemispherical and angular emissivity up to 100 μm), UV-Vis-NIR spectrometry (solar reflectance/transmittance), and XPS/SEM/STEM for structural and elemental analysis.
Experimental Validation:
- Clear-Sky Tests: Outdoor testing in London (SVF = 1) to compare Nano-SEs against broadband controls.
- Urban Simulation: Controlled experiments mimicking urban rooftops surrounded by heated walls ( $T_w$ = 40–100 °C, extended to 500 °C) with variable SVFs (0.13–1).
- City-Scale Modeling: Simulation of a 1-km diameter zone in London using LiDAR data to map SVF and predict rooftop temperatures for districts equipped with either broadband or selective emitters.

Key Results

Material Performance: The optimized Nano-SEs ( $SiO_{1.2}N_{0.7}$ ) achieved a weight-averaged atmospheric window emissivity ( $\varepsilon_{8-13\mu m}$ ) of 94.1% and a spectral selectivity ( $\eta$ ) of 2.46. This outperforms state-of-the-art disordered particle-polymer coatings and single/multi-layer dielectric designs. The solar-reflective variant achieved 95.3% solar reflectance, while the solar-transparent variant maintained 87.8% solar transmittance.
Clear-Sky Performance: Under unobstructed sky conditions (SVF = 1), the experimental results confirmed that selective emitters offer negligible temperature advantages over broadband emitters (ΔT ≈ 0 °C), validating the current literature's stance for rural environments.
Urban Performance: In simulated urban environments with limited SVF and hot surroundings ( $T_w$ = 40–80 °C), the selective emitters demonstrated a significant cooling advantage, achieving surface temperatures 0.5 to 5 °C lower than broadband counterparts. As $T_w$ increased to 100 °C and SVF decreased to 0.17, the advantage persisted.
High-Temperature Sensitivity: The cooling advantage of selective emitters increased with the heat source temperature ( $T_w$ ). At $T_w$ = 200–500 °C (relevant to industrial exhaust), the blackbody spectrum shifts to shorter wavelengths, increasing the fraction of radiation outside the 8–13 μm window. Selective emitters effectively reject this radiation, while broadband emitters absorb it, widening the temperature gap.
City-Scale Impact: Modeling of London's high-rise districts showed that while broadband emitters fail to provide sub-ambient cooling in areas with low SVF (<0.4), selective emitters maintain sub-ambient temperatures, effectively mitigating the UHI effect in dense urban cores.

Significance and Claims
The paper claims to resolve the debate regarding the efficacy of selective emitters by demonstrating that their superiority is context-dependent. The authors assert that while broadband emitters are sufficient for rural settings, selective emitters are essential for dense urban environments where radiative cross-talk from hot buildings is significant.

The primary contribution is the development of Nano-SEs, a scalable, nanoparticulate platform that overcomes the traditional trade-offs between spectral selectivity and material scalability. By engineering the morphology of $SiO_xN_y$ nanoparticles, the authors achieved high $\varepsilon_{8-13\mu m}$ and $\eta$ without the complex deposition processes or substrate limitations of previous multilayer designs. Furthermore, the platform's versatility allows for tailored solar responses (reflective or transparent), enabling deployment across diverse urban surfaces from rooftops to glass facades.

The work concludes that integrating such scalable selective emitters into urban infrastructure offers a viable, passive strategy to enhance thermal resilience and mitigate the intensifying urban heat island effect, particularly as cities grow denser and vertical.

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