Pillar-Pore-Structured Nanophotonic Coating with Superior Solar and Thermal Modulation Abilities

doi:10.21203/rs.3.rs-4389659/v1

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Pillar-Pore-Structured Nanophotonic Coating with Superior Solar and Thermal Modulation Abilities

https://doi.org/10.21203/rs.3.rs-4389659/v1

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There is a growing focus on the development of advanced materials to enhance energy efficiency in space heating and cooling, which accounts for 13% of the annual consumption of global energy. Wavelength-selective dynamic regulation is undisputedly requisite to solve this problem. Radiative cooling materials work in the wavelength span of 8-14 µm, which is effective in heat dissipation. However, their fixed emissivity results in unpleasant undercooling. Adaptive cooling materials can change emissivity in response to temperature shifts, solving the problem of overcooling, however they absorb too much solar heat in daytime, especially in summer. Hence, a material is requisite to have high reflectance in solar wavelengths (0.25 – 2.5 µm). To address this issue, we propose a temperature-adaptive radiative cooling coating with high solar reflectance that efficiently reflects solar radiation and automatically switches its sky-window emissivity from 0.19 to 0.74, delivering an 85.92% solar reflectance for optimal energy efficiency and all-year-round thermal comfort within a specific climate zone. The simulations demonstrate that our sample surpasses existing roofing coatings in terms of energy efficiency across different climate zones, particularly those with substantial seasonal variations.

Physical sciences/Optics and photonics/Optical materials and structures/Nanocavities

Physical sciences/Optics and photonics/Applied optics/Mid-infrared photonics

Maintaining building thermal environments with temperature in a certain range (18 ~ 24°C) is substantial for human society, which directly influences human body thermal comfort and work efficiency. The energy consumption of buildings accounts for up to 30–40% of the world's total primary energy ^1–5, approximately 51% of global electricity usage, and around 33% of black carbon emissions ⁶.

Radiative cooling (RC) allows for efficient heat dissipation by emitting thermal radiation into outer space through the atmospheric window ^{7, 8}. Various daytime radiative-cooling coatings have been developed over recent years and succeeded in cooling to sub-ambient temperatures ⁹, such as inorganic particles dispersion ^{10, 11}, porous polymers ^{12, 13}, micro-structures ¹⁴, and nano-fabricated layered structures ^15–17. These studies ^{18, 19} have demonstrated the important advancement in modulation of the full spectral range of solar and blackbody radiation ^20–22. However, these materials are beneficial for energy savings in summer but leads to high heating costs in winter ²³.

The development of self-switchable RC materials and structures that can dynamically adapt to ambient temperatures is crucial, particularly for optimizing energy efficiency in buildings during summer and winter ²⁴. Previous studies have made impressive progress (CaF₂/VO₂ core-shell microspheres (Δε = 0.36) ²⁵, pigments-VO₂-PE-Al (Δε = 0.6) ²⁶, VO₂-BaF₂-Ag (Δε = 0.7) ²⁷), whereas these coatings show a temperature increase and hindering effective dissipation of radiant heat ²⁸ because of a very high solar absorptivity when exposed to direct solar radiation. To enhance solar reflectance, multilayered structures were developed using a metal-film underneath ^{29, 30}, a thick (∼300–500 µm) loose porous film as overlayer ^31–34, or an overlayer with a dense inorganic nanoparticle composite film (Al₂O₃³⁵, SiO₂³⁶, TiO₂, MgO³⁷). Alternatively, these coatings not only meet the criteria for high solar reflectance but also results in high infrared absorption in the atmospheric transparency window.

We propose a temperature-adaptive radiative cooling coating with high solar reflectance (STMC) as a solution (Table 1). This flexible coating consists of an upper mid-infrared transparent pigment/PAN layer for solar reflectance, and a lower layer of vanadium oxide for modulation of thermal emittance. The STMC automatically switches the skylight emissivity (ε_sky) from 0.19 to 0.74 when the surface temperature exceeds a preset transition temperature, while maintaining a consistently high solar reflectance of 85.92%. Numerical simulations based on climate data further demonstrate the energy-saving potential of the STMC in tropical/subtropical monsoon climates.

We develop a category of STMC for all-year-round building heating and cooling energy savings, as illustrated in Fig. 1a. STMC demonstrates reflection characteristic to minimizes solar heat gain in the solar radiation (SR) wavelength range (mainly 0.25 to 2.5 µm). In thermal infrared wavelength ranges, in winter, it is a low emissivity state, suppressing the thermal emissions from building out-surfaces. And in summer, it demonstrates a high emissivity state to boost the radiative cooling with outer environments.

We developed the STMC based on bilayer construction with pillar pore structure solar reflector as upper, the periodic columnar arrangement VO₂ thermal emissivity modulation coating as lower. The solar reflectance and emissivity switching of STMC were investigated via FDTD. The micro- and nanopores in the solar reflector of STMC efficiently backscatter sunlight at any temperature (Fig. 1b). For the thermal emissivity modulation coating of STMC, when T is below T_MIT, the VO₂ columns are in the insulating state³⁸, most of the IR radiation is reflected by the Al, resulting in a high reflectance. When T is above T_MIT, VO₂ switches to the metallic state, two neighboring VO₂ columns will form an optical antenna³⁹, giving rise to high absorptance in the mid-IR (Fig. 1c).

A phase separation process was developed for the production of STMC, as illustrated in Fig. 2a. In brief, VO₂/PAN precursor (upper side of Fig. 2a), inorganic nanoparticles/PAN precursor (lower side of Fig. 2a) were formed by coating, and the precursors were immersed in deionized water to phase separation, transfer inorganic nanoparticles/PAN layer to VO₂/PAN layer in water ⁴⁰, dry naturally to obtain STMC. The optical photos and SEM morphology of the prepared STMCs are shown in Fig. 2b. Inorganic nanoparticle zinc oxide (ZnO), iron oxide (Fe₂O₃) and chromium oxide (Cr₂O₃) are utilized to generate primary colors (white, red, and green, respectively). The solar reflector is filled with pillar pore, as shown in the cross-sectional SEM. The pillar pore is 21 µm in height and 4 µm in diameter, as shown in Fig. 2c. The pores of the thermal emissivity modulation coating are blocked by the upper PAN, and the vanadium oxide in coating is still arranged in an isolated column, with 1.23 µm intervals, which is similar to the VO₂ photonic structure ²⁶. EDS elemental mappings of a cross section of the as-prepared ZnO-based STMC show that the Zinc oxide nanoparticles well dispersed in pillar hole wall (Fig. 2d), which effectively enhances the refractive index difference between air holes and hole walls, improving the scattering of solar light by STMC.

To investigate the influence of the pillar pore structure on the optical properties of the solar reflector, we further compared dense coatings (left column) obtained by directly solution coating with pillar pore structured coatings (right column) obtained by phase separation, as shown by the SEM images in Fig. 3a–c. Solar reflector with pillar pore presents the high average solar reflectance in UV-Vis-NIR wavelengths, while dense structure of the solar reflector shape drops to a low average solar reflectance in the near-to-short wavelength infrared range (0.25–2.5 µm) because only these inorganic nanoparticles are too small to effectively scatter such wavelengths. (Fig. 3d). The periodic structure with nano/microscale pillar pores is conducive to enhance the total scattering efficiency.

Figure 3e shows that the solar reflector with or without pillar pore show high transparency in the mid-IR region. Because PAN was composed of only aliphatic C-C and C-H bond and was IR transparent. The inorganic solids include chromium oxide (Cr₂O₃), iron oxide (Fe₂O₃), and zinc oxide (ZnO) with particle sizes of 100–300 nm, as shown in Figure S6a. This sub-microscale dimension is much smaller than the atmospheric transparent window 8–13 µm; almost no strongly scattering occurred for MIR infrared light. Fourier transform infrared (FTIR) spectroscopy (Figure S6b) illustrates that these inorganic solids have negligible absorbance in atmospheric transparent window. Compared with dense structure, the pillar hole structure slightly improves the transmittance of the solar reflector through the atmospheric transparent window. The presence of large holes permits direct transmission of partial MIR light, thereby mitigating the energy loss associated with light passing through the coating.

STMC efficiently reflects the incident solar radiation at any temperature. And STMC presents a low emission state at low temperature and a high emission state at high temperature, which is consistent with the working mechanism of the VO₂ inter-pillar photonic resonance ²⁶ (Fig. 4a). The VO₂/PAN thermal emissivity modulation coating with pillar hole structure, achieved comparable modulation performance (Δε_sky = 0.55) with a VO₂ concentration of 10% (Fig. 4b). By using ZnO (white), Fe₂O₃ (red), and Cr₂O₃ (green), we are able to broadly tune the color and solar reflectance of a STMC (Fig. 4C). The mid-IR spectral emissivity of the colored STMCs at both low and high temperatures was measured by FTIR spectroscopy (Figure S8), showing an evident insensitivity to the solar reflector added. ε_sky and solar reflectance were calculated and shown in Fig. 4d. The VO₂/PAN thermal emissivity modulation coating exhibits only 7.16% reflectance; the bilayer coatings (STMC) in white, red, and green colors show 85.92%, 44.30%, 36.52% reflectance in SR, respectively. The colorful coating not only maintains the aesthetic effect, but also provides additional thermal modulation.

Table 1

Comparison of representative radiative cooling technologies
Representative Materials	Working principle	Δε > 0.45	%R > 85%	color	Low cost
Pigment/VO₂/Al (This Work)	Optical micro cavity	√	√	√	√
Pigments/VO₂/PE/Al ²⁶	Optical antenna array	√		√	√
CaF₂@VO₂²⁵	core − shell structured
VO₂/PMMA/ITO/glass ²⁴	F-P resonator
VO₂/BaF₂/Ag ²⁷	F-P resonator	√
P(VdF-HFP)HP ¹²	porosity		√		√
cellulose nanofibers ⁴¹	regularly arranged fiber
Pigment/PE/Al ³⁰	Dense layer		√	√	√
Pigment/Al ²⁹	Dense layer		√	√	√
Silica particles/PP, Ag/Cu Zn/Cu ⁴²	Temperature shape memory effect				√
HfO₂ / SiO₂⁴³	Multilayer structure		√
TiO₂¹⁰	Hierarchical porous		√		√

The comprehensive performance of current state-of-the-art radiative cooling technologies is summarized in Table 1. The STMC prepared by phase separation process in this work exhibits very high solar reflectance and high modulation of thermal emittance, surpassing most of the reported results in Table 1.

Owing to the installation flexibility and versatility, our STMC are suitable to be used in warehouses for cold chain storage without adding extra weight and volume. As shown in Fig. 5a, two warehouse models with testing surface (upper) by commercial white wall coating (CWP) and ZnO based STMC were placed in cold environment (13.2°C) and an infrared baking lamp was used to radiate the CWP/STMC. Figure 5b shows the changes in ice mass during the testing process. It was observed that STMC can significantly slow down the ice melting speed. Storage of 50g of ice cubes in two warehouses, at the end of the test, the mass of ice cubes inside warehouse with STMC was 12.56g, while the mass of ice cubes inside CWP warehouse was 0g. Figure 5c exhibits the photographs of ice cubes before and after the 160 min testing period. It indicates that a great amount of cooling energy or phase change materials for temperature maintenance can be saved during storage. The experiment verifies that the high solar reflectance and emittance modulation of STMC can effectively alleviate radiative heat gain from the ambient.

The EnergyPlus software was used to calculate potential HVAC energy savings in midrise apartment buildings in cities along the Yangtze River, based on applying the STMC to walls, as shown in Fig. 5d. In cities with latitude 31, in terms of the HAVC, the use of STMC saves 1.12 tons of coal per year compared to TARC ²⁷ (Fig. 5e). In cities with latitude 29, in terms of the HAVC, the use of STMC saves 2.28, 3.29, 3.42 tons of coal per year compared to PEAC ²⁶, TARC ²⁷, coloured low-emissivity films³⁰, respectively (Fig. 5e). In cities with latitude 25, in terms of building cooling, the use of STMC saves 2.69 tons of coal per year compared to color low-emission radiation cooling coating²⁹ (Figure S7a); in terms of the HAVC, the use of STMC saves 9.31, 6.74 tons of coal per year compared to TARC ²⁷, PEAC ²⁶, respectively (Fig. 5e). In cities with latitude between 19 and 22, in terms of building heating, the use of STMC saves 1.15 tons of coal per year compared to color low-emission radiation cooling coating²⁹ (Figure S7b). Installation of STMC can achieve positive total HVAC energy savings penalties through high emittance modulation and solar reflectance, providing a comprehensive year-round energy-saving solution suitable for various regions.

In summary, we have prepared a feasible pillar pore structured coating STMC with high spectral selectivity and ultrabroadband modulations. The solar reflectance and thermal emissivity modulation of the STMC enable switching of it from the radiative cooling mode at high temperatures to the keep-warm mode at low temperatures in an outdoor setting. In the whole-building energy simulations, the STMC outperform the counterpart commercial building materials across the different climate zones. This study has revealed the effects of pillar pore on optical performance, which may provide deep insight into the crucial roles of various pores and their array in solar reflectance and thermal emittance and help us design and fabricate more efficient temperature-adaptive radiative cooling coating.

Synthesis of VO₂(M) nanoparticles

VO₂(M) nanoparticles were obtained by a novel molten-assisted liquid shielding method developed in our laboratory⁴⁴. Briefly, proper amounts of VOSO₄ (99.9%, in mass fraction, Cia reagent), KCl (99.9%, in mass fraction, Kermel), and 5% (in mass, compared to VOSO₄) of glucose (C₆H₁₂O₆, 99.9%, in mass fraction, Kermel) are mixed in an agate mortar and ground to get a uniform powder. To avoid the oxidization of vanadium precursor, are added in the sample pillars to introduce a slight reduction environment. Then, the mixed powder is pressed into small pillars with diameter of 20 mm and thickness of approximately 50 mm under 200 MPa, which is further placed in a corundum crucible and covered with extra amounts of KCl and LiCl mixed salts (KCl 0.4 mol and LiCl 0.6 mol). The weight ratio of sample pillars to chloride salts is around 1:10. Then the crucible is rapidly heated at 425 ℃ in a smelter hearth in air and held for 1h. After that, a washing and drying process is applied to obtain the final VO₂ powders.

Preparation of VO/PAN composite coatings on Al foil

Different amount of the above VO₂ powders and 0.3 g PAN (Mw: ~150,000, Aladdin) were added to 5 mL N,N dimethylformamide solution (DMF). Proper amounts zirconium beads were added to the above solution, then the solution was stirred and ground at 60°C for 4h. Al foil was cleaned by ultrasonication in deionized water, acetone and ethanol in turn. The solution was coated on the Al substrate to form a precursor coating, after that immersed it rapidly in water. Then, it is naturally dried in the air to obtain the VO₂/PAN composite coatings.

Preparation of STMC

Different category of the inorganic nanoparticles powders were added to 5 mL DMF and sonicated 60 minutes in an ultrasonic machine. 0.35 g PAN (Mw: ~150,000, Aladdin) was added into the above solution and stirring for 12 h. The solution is then casted onto the quartz glass substrates by spin coating method to form precursor coating, immersed it rapidly in water. The solar reflector formed by phase separation of the precursor in water detaches from the quartz glass substrate. Then, the VO₂/PAN composite coating is used to pull the solar reflector out of the water air interface. After evaporation at air atmosphere, the STMC is obtained.

Characterization

The surface morphology of the coating was determined by field emission scanning electron microscopy (SEM, Hitachi SU8220, Japan). Optical reflectance was measured on a UV-Vis-NIR spectrophotometer (Hitachi U-4100, Japan). The reflectance in the micron region was measured using a Fourier transform infrared spectrometer (FTIR, TESORII, Bruker, Germany) equipped with a diffuse reflectance gold integrating sphere. Gold standard coatings were used as a reference for reflectance measurements.

For the assessment of the optical properties of the coatings, the solar reflectance (R_sol, 250 − 2500 nm) and thermal emissivity (ε, 8 − 13µm) are calculated from the following equations:

\({R_{solar}}=\frac{{\int_{{0.25}}^{{2.5}} {{I_{solar}}(\lambda ) \cdot R(\lambda )d\lambda } }}{{\int_{{0.25}}^{{2.5}} {{I_{solar}}(\lambda )d\lambda } }}\)

\({\varepsilon _{\text{t}}}=\sum\nolimits_{8}^{{13}} {{G_t}} (\lambda ){E_t}(\lambda )\Delta \lambda \approx 1 - \sum\nolimits_{8}^{{13}} {{G_t}} (\lambda )R(\lambda )\Delta \lambda\)

where R(λ) is the reflectance of coating at the wavelength from 0.25 to 2.5 µm, I_solar is the ASTM G173 Global solar intensity spectrum, ε_t is the coating emissivity at temperature t, and G_t(λ) is the blackbody radiation at temperature t (CNS GB/T1895.2-2002).

The emissivity variation (∆ε) are acquired by the following formula:

\(\Delta \varepsilon ={\varepsilon _{100^\circ C}} - {\varepsilon _{30^\circ C}}\)

Optical simulations

The optical properties of the microcavity structure were simulated by a finite difference in time domain (FDTD) method. The spectral reflectance of STMC is numerically calculated using a finite difference in time domain (FDTD) method, with all the geometric parameters matching the original design. Material properties in IR and visible ranges are from Refs^45–48.

Heat Insulation Performance Demonstration in Artificial Hot Environments

We used high-density polystyrene foam (4–6 cm) to make foam box with an inner diameter of 5 cm × 5 cm × 5 cm and attached low-emission aluminium foil to the outer surface of the foam box to provide better thermal insulation. Quartz glass container are placed inside the foam box, and the dimension of the container is 5 cm ×5 cm × 5 cm. We added different coatings to the container upper surfaces. For the ice test, ice cubes of nearly the same mass and shape were put in a top-open container with other surfaces heat insulated by foam box. At certain time intervals, the ice container was taken out for photographs, and ice mass was measured by taking out and absorbing surface liquid water rapidly. For the warehouse simulants models with different surface coatings, we fixed their position in the artificial hot environments, as well as the position of heaters and ice cube containers, to make the measurements as parallel as possible to provide reasonable comparison.

Energy-Saving Calculation by EnergyPlus

EnergyPlus (version 9.4) was used to perform whole building energy simulation. We used the commercial reference building model (midrise apartment) defined by China DOE. The model building has four stories, including 32 apartments. The total floor area is 2952 m². The windows cover 15.52% of the total wall surface area. The building is isolated (i.e., no neighboring buildings/objects). The latitude of building is 20.03 degree, and the longitude of building is 110.35 degree. Internal gains and HVAC systems have been comprehensively designed in the models. For the HVAC systems in the building, DX cooling is employed for cooling (COP = 2.5) and electric heaters (efficiency = 1) are used for heating. The fan’s efficiency is 0.7. The indoor air temperature set-point was set constantly as 18 ~ 24°C, and the external weather utilized hourly weather data for a TMY3 of different cities. The weather data comprehensively include temperature, relative humidity, wind direction and speed, solar radiation, etc. The wall and roof insulation condition of the modeled building is defined in the downloaded EnergyPlus models for 15 cities (Haikou, Sanya, Hong Kong, Guangzhou, Macao, Shenzhen, Xiamen, Hangzhou, Nanjing, Suzhou, Shanghai, Wuhan, Chongqing, Chengdu, Nanning). The building insulation conditions in other cities are extrapolated from the above 15 cities. The baseline HVAC energy use, including cooling, heating, and fans, was calculated for the building model with conventional wall properties (as set in the downloaded EnergyPlus models). The initial solar reflectivity values of the building wall is 0.3, and MIR emissivity (thermal absorbance) values is 0.9. To calculate the HVAC energy use with installation of STMC, we modified optical properties of the wall (both inside and outside sides) in the building model, using experimental measured data (solar reflectivity: 0.85, thermal emissivity modulation: 0.38(Figure S5b ). Comparing the energy use difference between building models with and without STMC installation we obtained the all-year energy saving for cooling, heating, fans, and total HVAC. All annual energy-saving values are normalized by being divided by total floor area of the simulated building (2952 m²).

Acknowledgements

This study was supported in part by funding from the National Natural Science Foundation of China (NSFC, Contract Nos.: 51502268, 51325203), the Open Project of State Key Laboratory of Advanced Special Steel, Shanghai Key Laboratory of Advanced Ferrometallurgy, Shanghai University (SKLASS 2022-04), the Science and Technology Commission of Shanghai Municipality (No. 19DZ2270200) and the Provincial and Ministerial Co-construction of Collaborative Innovation Center for Resource Materials (Contract No.: zycl202008).

Author details

¹ School of Materials Science and Engineering, Zhengzhou University, Kexue Ave 100, Zhengzhou 450001, China

² School of Materials Science and Engineering, Shanghai University, Shangda Rd 99, Shanghai 200444, China

Author contributions

Y.G. and Z.Z. designed the experiment. J.L., C.Z., Y.L, and Y.C. performed the experiments. Y.G. and Z.Z. designed the framework of the manuscript. Y.G., Z.Z., and J.L. wrote the paper, M.X., J.T., S.Y., and L.Z. provided some guidance for the experiment, and all authors participated in the discussion of the paper.

Data availability

Data will be made available on request.

Conflict of interest

The authors declare no competing interests.

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Pillar-Pore-Structured Nanophotonic Coating with Superior Solar and Thermal Modulation Abilities

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Introduction

Results

Discussion

Materials and methods

Synthesis of VO₂(M) nanoparticles

Preparation of VO/PAN composite coatings on Al foil

Preparation of STMC

Characterization

Optical simulations

Heat Insulation Performance Demonstration in Artificial Hot Environments

Energy-Saving Calculation by EnergyPlus

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References

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Pillar-Pore-Structured Nanophotonic Coating with Superior Solar and Thermal Modulation Abilities

Status:

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Abstract

Figures

Introduction

Results

Discussion

Materials and methods

Synthesis of VO2(M) nanoparticles

Preparation of VO/PAN composite coatings on Al foil

Preparation of STMC

Characterization

Optical simulations

Heat Insulation Performance Demonstration in Artificial Hot Environments

Energy-Saving Calculation by EnergyPlus

Declarations

References

Additional Declarations

Supplementary Files

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Synthesis of VO₂(M) nanoparticles