Tailoring High-Entropy Oxides as Emerging Radiative Materials for Daytime Passive Cooling

In the framework of intense research about high-entropy materials and their applications in energy-oriented technologies, in the present work, we discuss the potential applicability of selected oxides and of the alloys they form at different concentrations for daytime radiative cooling implementation. In particular, by combining density functional theory and the finite difference method, we provide an unbiased, scattering-free description of structural, electronic, and dynamic features of the best candidates, showing the required strong radiative properties for passive cooling while offering the benefits of affordability and compatibility with commercial coating fabrication processes.


Experimental Section
Materials and Methods All the reagents are commercially available and used without further purification.All the reagents were purchased from Merck.X-ray diffraction (XRD) patterns of the samples were recorded using a Malvern PANalytical Empyrean diffractometer with BraggBrentano geometry.The diffractogram was recorded in 20-70°2Θ range with a step size of 0.013°and an integration time of 79 s/step using a PIXcel3D-Medipix3 solid-state detector and a Cu anode.The dimensions and morphology of the compounds were measured through a FE-SEM (FEG LEO 1525, Zeiss, Oberkochen, Germany).Thermogravimetric analysis were carried out with a Netzsch STA2500 Regulus thermoanalyzer under a 5 mL min -1 air flux with a heating rate of 10 °C min -1 .For the measurements of solar reflectance a JETI Specbos 160 1211UV spectroradiometer was used, while the FTIR spectra and MIR emittance measurements were obtained with a PerkinElmer Spectrum 3™ MIR/NIR/FIR system equipped with a 3-inch diameter highly reflective gold-coated Mid-IR IntegratIR™ sphere (PIKE Technologies).minutes, 1 mmol of citric acid is added to the solution.When the acid is dissolved completely, 3 g of glucose in 100 mL of deionized water and 2 mmol of acrylamide are added.Following the addition of glucose and acrylamide, the pH value of the solution is adjusted to ≈ 7 with NH 4 OH.The solution is then heated at 80°C in a water bath for 3h to become a gel, and then the obtained gel is dried at 120°C in an oven overnight.Finally, the gel precursor is grinded into powder and calcined at 900°C for 4h in a muffle furnace to achieve the final product.

Synthesis of Y
For the synthesis of Al 2 Ce 2 O 7 the same procedure has been followed, using Al(NO 3 was synthesized with a similar sol-gel method introducing some modifications.0.5 mmol of tetraethoxysilane (TEOS) are dissolved in 10 mL of ethanol and stirred for 24h.Then, the TEOS solution is added into a 0.05 M aqueous solution of Al(NO 3 ) 3 • 9H 2 O and stirred for 20 minutes.After this, the procedure is the same as described above for the synthesis of Y 2 Ce 2 O 7 .

2
Ce 2 O 7 and Al 2 Ce 2 O 7 .Y 2 Ce 2 O 7 and Al 2 Ce 2 O 7 were synthesized following the sol-gel procedure described by Dang et al. 1 0.5 mmol of Y(NO 3 ) 3 • 6H 2 O and 0.5 mmol of Ce(NO 3 ) 3 • 6H 2 O are put into 10 mL of deionized water and stirred.After 10

FigureFigure S8 :
Figure S7: (a): Optical spectrum calculated at the BSE level for SiO 2 ; (b) Absorption spectrum still calculated at the BSE level; (c) and (d) are refractive index, n(ω) and reflectivity, R(ω), respectively, obtained at the same level of theory.The red line in (a) and (b) shows the value of the QP bandgap calculated at the G 0 W 0 level of theory.Spectra are calculated using 15 (15) occupied (unoccupied) states in the BSE matrix.24 k-points were used to sample the Brillouin Zone.960 Bands were included in the calculations.All the remaining parameters are the same as reported in Computational Details section.

Figure S9 :
Figure S9: Same analysis of Figure S8 but for the case of SiO 2 (α-quartz).

Figure S10 :
Figure S10: Convergence test for (Y 0.25 Sc 0.25 Ga 0.25 In 0.25 ) 2 Si 2 O 7 .in terms of number of bands employed to calculate the imaginary part of the dielectric function, ϵ 2 .The plot reports the xx component calculated at the BSE level of theory.

Table S2 :
Calculated bandgaps (eV) at the PAW/PBE level for the ternary compounds.

Table S3 :
Convergence of Valence Band Maximum (VBM) and Conduction Band Minimum (CBM) Energy (eV) as function of the number of (NKPTS) in the Brillouin Zone in "one-shot" G 0 W 0 QP calculations.