What is an infrared absorbing nanoparticle?

Sunlight contains more than visible light. Around 50% of the energy in sunlight reaches us as infrared radiation, most of it in the near-infrared range between 700 and 2500 nm. This is the part of the spectrum responsible for heat build-up in buildings, vehicles and optical systems. Infrared absorbing nanoparticles are materials designed to intercept this radiation before it causes unwanted heating, without blocking the visible light that passes through a window or coating.

How does infrared absorption work in nanoparticles?

When infrared radiation hits a material, it can be reflected, transmitted or absorbed. Infrared absorbing nanoparticles are designed to absorb radiation in a specific wavelength range, typically the near-infrared range around 800 – 1400 nm, and convert it into heat that dissipates within the material rather than passing through it.

The absorption coefficient of a nanoparticle determines how strongly it absorbs radiation at a given wavelength. A high absorption coefficient means the material absorbs a large fraction of the incoming radiation even in very thin layers. This is why nanoparticles are particularly useful for infrared absorbing coatings: a thin film containing nanoparticles with a high absorption coefficient can block significant amounts of near-infrared radiation while keeping the coating visually transparent.

The optical properties of the nanoparticle, including its size, shape and crystal structure, determine exactly which wavelengths it absorbs. This is why different nanoparticle materials absorb at different points in the infrared spectrum, and why choosing the right material for the right wavelength range matters.

Which materials are used as infrared absorbing nanoparticles?

Several different materials are used as infrared absorbing nanoparticles, each with different absorption ranges and properties.

Lanthanum hexaboride (LaB₆)

LaB₆ nanoparticles absorb strongly in the 800 – 1400 nm range, placing them in the near-infrared (NIR) part of the spectrum. They combine infrared absorption with extreme hardness, up to 22 GPa, which makes them useful for coatings that need to block infrared radiation while also resisting wear. LaB₆ nanoparticles work as an infrared shield at very low doses, between 10 and 100 mg per square metre, which keeps coatings thin and transparent.

Cesium tungsten oxide (CWO)

Cesium tungsten oxide nanoparticles absorb primarily in the near-infrared range above 900 nm. They are used in solar control glass and window films where strong near-infrared shielding is needed with minimal effect on visible light transmittance. Cesium is a much more expensive raw material than lanthanum, which makes LaB₆ a more cost-effective alternative for many infrared absorbing applications.

Indium tin oxide (ITO)

ITO nanoparticles absorb in the near-infrared range and are used in transparent conductive coatings. They are common in electrochromic glass and display applications, though their higher cost limits use in bulk coating applications.

Carbon black and other pigments

Carbon black absorbs broadly across the visible and infrared spectrum. It is used as an infrared absorbing pigment in applications where colour is not a concern, such as black paint for industrial heat absorption. Its broad absorption profile also blocks visible light, which limits its use in transparent coatings.

What is the difference between infrared absorption and infrared reflection?

Infrared absorbing nanoparticles and infrared reflecting materials both block near-infrared radiation, but they do so differently. An infrared absorbing material takes in the radiation and converts it to heat within the coating or material itself. An infrared reflecting material bounces the radiation back before it enters the material.

For window coatings and architectural glazing, absorption is often preferred because it avoids reflecting heat back at people standing outside a building. For roofing materials and facades, reflection is sometimes preferred because it keeps the surface cooler. In practice, many coatings use a combination of both mechanisms depending on the application.

Where are infrared absorbing nanoparticles used?

IR absorbing coatings for glass

In automotive glass and architectural glazing, infrared shielding nanoparticles are added to coatings or laminated films to reduce heat transmission from sunlight. The coating blocks near-infrared radiation while maintaining visible light transmittance, keeping interiors cooler without darkening windows.

Near infrared absorbers in solar control

Solar control glass uses near infrared absorber nanoparticles to selectively block the heat-carrying part of sunlight. This reduces cooling costs in buildings and vehicles without reducing natural light.

Thin film coatings

In optical applications, nanoparticle-based thin film coatings provide infrared shielding on lenses, sensors and optical components where precise wavelength control is needed.

IR sensors and thermal imaging

Some nanoparticle coatings are used on IR sensors and detector surfaces to control which wavelengths reach the sensing element, improving measurement accuracy.

Radiation shielding in industrial applications

In high temperature industrial environments, infrared absorbing coatings reduce radiant heat transfer, protecting equipment and improving energy efficiency.

Why nanoparticles and not bulk materials?

Using nanoparticles rather than bulk infrared absorbing materials offers several advantages. Nanoparticles can be dispersed in a liquid carrier to form a stable ink or coating formulation, which can then be applied as a thin film using spray coating, dip coating or inkjet printing.

Because the nanoparticles are so small, typically between 5 and 20 nm for high-performance infrared coatings, a very thin layer contains enough material to achieve strong infrared absorption. This keeps coatings light, transparent and practical for large-scale applications. The particle size also affects the absorption coefficient and the specific wavelength range where absorption occurs. By controlling particle size during synthesis, the absorption properties of the nanoparticle can be tuned for a specific application.

For more on how LaB₆ nanoparticles work as an infrared absorber and how they can be applied as an ink, visit our LaB₆ nanoparticles page.

Summary

Infrared absorbing nanoparticles intercept near-infrared radiation before it passes through a coating or material, converting it into heat that dissipates harmlessly. The absorption coefficient and optical properties of the nanoparticle determine which wavelengths are more or less strongly absorbed. Materials such as LaB₆, cesium tungsten oxide and ITO are used in IR absorbing coatings for automotive glass, architectural glazing, solar control and optical applications. Nanoparticles are preferred over bulk materials because they can be formulated into thin, transparent coatings that work at very low concentrations.