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TiO2 Nanocrystalline Pigmented Polyethylene Foils for Radiative Cooling Applications: Synthesis and Characterization Y. Mastai,† Y. Diamant,‡ S. T. Aruna,‡ and A. Zaban*,‡ Department of Chemistry, Bar-Ilan University, Ramat-Gan, 52900, Israel, and Department of Materials and Interfaces, The Weizmann Institute of Science, Rehovot 76100, Israel Received March 12, 2001. In Final Form: July 10, 2001 Radiative cooling devices should ideally operate with a shield substrate that completely blocks solar radiation but is transparent in the “atmospheric window” (8-13 µm) region. In this paper, we introduce a new type of shield for radiative cooling applications based on the use of polyethylene foils pigmented with nanocrystalline TiO2. Homogeneous shields were prepared by spreading colloidal suspension of TiO2 (rutile ca. 60 nm) between two polyethylene sheets followed by hot pressing. Optical and structural properties of the films were investigated using X-ray diffraction, TEM, and UV/vis/NIR and FTIR spectroscopy. The shields show high IR transmittance and high solar reflectance, which are favorable characteristics for solar radiation shields in radiative cooling devices. The mechanism for obtaining improved optical properties of nanocrystalline foils (in comparison with submicrometer bulk foils) is discussed with respect to both the materials and the shield preparation method. The simplicity and low-cost preparation of nanocrystalline TiO2 films, with their superior optical characteristics, could find widespread use in radiative cooling and other environmental applications.
Introduction Passive cooling has many potential applications, such as the cold storage of foods, seeds, and medicines and the climatization of buildings.1-3 Radiative cooling refers to the preferential emission of radiation in the 8-13 µm “atmospheric window” region, resulting in cooling of the emitting surface. Radiative cooling relies on the fact that the thermal energy emitted by a clear sky in the “window” region is much less than this energy emitted in the same wavelength range by a blackbody having ground air temperature. Hence, a surface on the earth facing the sky experiences an imbalance of outgoing and incoming thermal radiation, which results in cooling below the ambient air temperature. This concept works well at night assuming a relatively dry atmosphere;4 however, during the day, when the solar energy input is usually much greater than the outgoing radiation, the surface does not cool below ambient temperature. To prevent solar heating, a shield is needed that can block the solar radiation (3003000 nm) but simultaneously be transparent in the “atmospheric window”. The ideal shield should secure complete reflection of the solar radiation and complete transmission of the IR window region (8-13 µm).1 The properties required for solar shielding may be achieved by two different approaches, using either submicrometer materials or nanosize particles. Submicrometer materials inherently reflect or absorb the solar radiation, and they are transparent in the “atmospheric window”. For example, ZnS and ZnO were recently reported as an optional shielding material.5,6 ZnS can block ca. 75% of solar radiation, but it is unstable in continuous † ‡
The Weizmann Institute of Science. Bar-Ilan University.
(1) Granqvist, C. G.; Eriksson, T. S. Materials for Solar Energy Conversion Systems; Granqvist, C. G., Ed.; Pergmon Press: London, 1991. (2) Martin, M. Passive Cooling; Cook, J., Ed.; MIT Press: Cambridge, MA, 1989; p 138. (3) Tormbe, F. Bull. Inst. Int. Froid 1964, 301. (4) Bartoli, B.; Silverstini, V. Scienze 1980, 70, 139.
sun illumination. ZnO is more stable, but it can reflect only ca. 65% of the sun’s radiation. The nanosize approach, on the other hand, makes use of materials that exhibit the desired behavior only by size effects. This approach utilizes mostly wide band-gap semiconductors that are transparent in the visible region but can be turned into scattering materials when their size is smaller than the light wavelength. Numerous studies have been performed in recent years on the effect of nanoparticles’ size on their optical properties. Many of these studies show that it results from changes in the band structure and from the significant increase of the surface area-to-volume ratio. The latter enhances surface properties with the decrease in size.7-9 The theoretical background including the equations that correlate the material parameters with its optical properties is established.10 The approach using the size effect of colloidal semiconductors for solar shielding has the advantage of using cheap, low-tech, environmental friendly materials. A highly investigated system in the field of nanocrystalline materials, colloidal TiO2, is a good candidate for three reasons: First, this wide band-gap semiconductor does not absorb either in the visible or in the IR “atmospheric window”. Second, the synthesis procedures of TiO2 allow fine control of the colloid size, shape, crystal structure, and purity. Finally, TiO2 has a high refractive index of 2.57 for anatase and 2.74 for rutile crystal structures (at 550 nm),11 preferable for such scattering. In most of the shields reported to date, mechanical stability is achieved by using polyethylene (PE) foils as (5) Nilsson, N. A.; Eriksson, T. S.; Granqvist, C. G. Sol. Energy Mater. 1985, 12, 327. (6) Niklasson, G. A.; Eriksson, T. S. Proc. Soc. Photo-Opt. Instrum. Eng. 1988, 1016, 89. (7) Brus, L. E. J. Phys. Chem. 1983, 79, 5566. (8) Lippens, P. E.; Lannoo, M. Phys. Rev. B 1989, 39, 10935. (9) Zunger, A. MRS Bull. 1998, 23, 35. (10) Yoffe, A. D. Adv. Phys. 1993, 42, 173. (11) Palik, E. D. Handbook of Optical Constants of Solids; Academic Press: New York, 1985; Vol. I.
10.1021/la010370g CCC: $20.00 © 2001 American Chemical Society Published on Web 10/06/2001
TiO2 Nanocrystalline Pigmented Polyethylene Foils
a substrate. The polyethylene is transparent to most of the relevant spectrum including the “atmospheric window”, thus allowing solar reflection with infrared transmission. Other materials either absorb strongly in the window region or are not feasible in large areas. Two different shield designs are commonly used: The first is the introduction of optical scattering materials into the PE substrate,6,12 and the second is the coating of the PE with the optically active films.13,14 We report here on the use of fine nanosize TiO2 crystal pigmented polyethylene foils as shields for radiative cooling. We prepared nanocrystalline TiO2 pigmented PE foils with various thicknesses of the TiO2 films by a new method: film spreading between two PE sheets. The optical properties in the UV/vis/NIR and IR reign of these foils are shown to possess excellent solar and IR characteristics in comparison to submicrometer shields made by the standard “mixture” method. The quality of the TiO2 colloids in terms of size, size distribution, and crystal structure contributes to their improved shield performance.
Langmuir, Vol. 17, No. 22, 2001 7119 properties, since the total radiation is importantsand not solely the direct amount of radiation. Diffuse transmittance and reflectance spectroscopy measurements in the UV/vis/NIR region were performed on a Jasco VERY-570 spectrophotometer equipped with an integrating sphere. Spectra were recorded at room temperature from 200 to 2000 nm with a scanning speed of 100 nm/min, using MgCO3 as a reference. Infrared spectra were recorded on a FTIR spectrophotometer. The measurements were performed with normal incidence of light between 2.5 and 25 µm. X-ray Diffraction. Powder X-ray diffraction (XRD) spectra of the powders were obtained using a Rigaku RU-200B Rotaflex diffractometer, operating in the Bragg configuration using Cu KR radiation. Data were collected at a counting rate of 0.25 deg/ min and sampling interval of 0.002 deg. The accelerating voltage was set to 50 kV with a 150 mA flux. The crystal size was estimated from the width of the diffraction peaks using the Debye-Scherrer relationship.18 Transmission Electron Microscopy. Conventional TEM brightfield (BF) imaging and electron diffraction (ED) were performed with Phillips CM-120 microscopes operating at 120 kV. For TEM analysis, a drop of the colloidal solution was placed on TEM copper grids coated with thin amorphous carbon.
Experimental Section
Results and Discussion
Pigmented Polyethylene Foil. For the preparation of submicrometer TiO2 pigmented foils, high-density polyethylene (HDPE) and TiO2 powders (purity of 97.5% Kemira, ca. 0.25 µm) were mixed together in an extruder and heated to 210 °C. Subsequently, the mixture was pressed into films at 165 °C and 10,000 psi for 3 min. In these films, the pigment volume fraction was 5% and the nominal film thickness was ca. 100 µm as measured with a micrometer. Colloidal TiO2 Suspension. Volumes of 18.5 mL of both titanium isopropoxide and 2-propanol were added dropwise to a vigorously stirred 150 mL of pH 0.5 nitric acid. The solution was stirred for 2 days forming a clear solution, followed by evaporation of the organic compounds at 82 °C for 2 h. A total volume of 150 mL was placed in a titanium autoclave (Parr Instrument), equipped with a homemade stirring system for a hydrothermal treatment at 250 °C for 54 h. The stirring during the hydrothermal process was crucial for the formation of 100% rutile-structured particles.15 Film Preparation. Commercial HDPE foils ca. 50 µm were used as the substrate. The foils were cut to 1 × 5 cm2, cleaned with detergent, and thoroughly rinsed in water. Thin films of colloidal TiO2 were prepared by spreading this TiO2 suspension on the PE foils using adhesive tape as spacers.16 The films were then air-dried for several hours. This TiO2 deposition method on the polyethylene substrate tended to provide homogeneous films with very poor adhesion. Therefore, the TiO2 films were covered with a second polyethylene foil and pressed at 110 °C and 2000 psi for 5 min. The thickness of the TiO2 layer was calculated from its weight using a thickness-to-weight value that was determined experimentally. The weight of the TiO2 layer was measured by weighing the PE foil before and after the TiO2 deposition. The thickness-to-weight ratio was determined using films that were deposited on glass, whose thickness was measured with a profilometer (Mitutoyo, Surftest SV 500). These measurements show ca. 60% porosity in the film, which agrees with published results.17 Characterization Methods. Optical Characteristics. All FTIR spectra are direct (specular) transmittance reflectance, whereas the UV/vis/NIR spectra are diffuse transmittance and reflectance. The diffuse spectra are more relevant for the shield
Two types of shields consisting of TiO2 embedded in PE were prepared in this study. First, the nanocrystalline TiO2 shields were hot-pressed between two PE films. Second, the commonly used submicrometer shields were made by pressing a hot mixture of PE with submicrometer TiO2 crystals. As described above, an ideal shield should reflect all the incoming UV/vis/NIR solar radiation, while transmitting all the outgoing 8-13 µm IR radiation. To determine the quality of the shields, one must measure their optical properties in both spectral regions. The shield transparency is the critical parameter in the IR window, because the transparency affects the cooling rate. Practically speaking, it is not important whether the blocked cooling radiation is absorbed by the shield or reflected back into the device. In contrast, in the UV/vis/NIR region, the shield transmission measurements must be accompanied by absorbance measurements. This measurement is needed to determine the fraction of solar radiation that will be absorbed by the shield, in addition to the fraction that will penetrate the device, since both fractions generate heat in the device. In other words, if the solar radiation blockage is achieved by absorption, the cooling will not be efficient, because the absorption will cause heating of the shield that will, in turn, heat the device. (The extent to which solar absorption will affect the device performance depends on the geometry of the device and the heat flow characteristics of the surrounding ambient.) To compare the optical properties of different films over a wide spectral range, the integrated transmittance absorbance and reflectance intensities are calculated for the UV/vis/NIR and the “atmospheric window” regions. Following the definition of Granqvist and coauthors,12 the solar reflectance is calculated by averaging the spectral reflectance of the shield over the solar spectrum.
(12) Nilsson, T. M. J.; Niklasson, G. A.; Granqvist, C. G. Sol. Energy Mater. Sol. Cells 1992, 28, 175. (13) Grenier, P. Rev. Phys. Appl. 1979, 14, 87. (14) Engelhard, T.; Jones, E. D.; Viney, I.; Mastai, Y.; Hodes, G. Thin Solid Films 2000, 2, 101. (15) Aruna, S. T.; Tirosh, S.; Zaban, A. J. Mater. Chem. 2000, 10, 2389. (16) Zaban, A.; Meier, A.; Gregg, B. A. J. Phys. Chem. B 1997, 101, 7985. (17) Kavan, L.; Gratzel, M.; Rathousky, J.; Zukal, A. J. Electrochem. Soc. 1996, 143, 394.
Rsol )
∫R(λ)W(λ) dλ ∫W(λ) dλ
(1)
Here W(λ) ) AM 1.5 solar spectrum19 and R(λ) ) spectral reflectance of the film. (18) Cullity, B. D. Elements of X-ray Diffraction, 2nd ed.; AddisonWesley Publishing Co., Inc.: London, 1978. (19) Maston, R. J.; Emery, K. A.; Bird, R. E. Sol. Cells 1984, 11, 2.
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Table 1. Optical Function Values of the Different TiO2 Pigmented Shieldsa sample
TiO2 film thickness (µm)
Tsol
Rsol
Asol
T813
R813
A813
A B C D E submicrometer TiO2 PE
0.12 0.45 0.75 1.08 1.82
0.610 0.412 0.273 0.220 0.166 0.323 0.891
0.381 0.581 0.712 0.684 0.763 0.601 0.078
0.009 0.007 0.015 0.096 0.071 0.076 0.031
0.765 0.731 0.681 0.651 0.546 0.391 0.813
0.172 0.154 0.162 0.153 0.178 0.174 0.151
0.063 0.115 0.157 0.196 0.276 0.435 0.036
a
R8-13, A8-13, T8-13, Rsol, Tsol, and Asol are defined in the text by eqs 1-3.
Figure 1. FTIR transmittance spectra of the submicrometer and three nanocrystalline TiO2 pigmented polyethylene shields. The nanocrystalline shields labeled a, c, and e differ by the thickness of the TiO2 film (see Table 1).
Figure 2. Vis/NIR total (direct + scattered) transmittance spectra of the submicrometer and the three nanocrystalline TiO2 pigmented polyethylene shields presented in Figure 1.
Similarly, the solar transmittance through the shield and the absorbance of the solar radiation by the film can be calculated.
∫T(λ)W(λ) d(λ) ∫W(λ) d(λ)
(2)
Asol ) 1 - (RSol + TSol)
(3)
Tsol )
Applying the same calculation in the “atmospheric window” is more complex, because the sky spectrum (“atmospheric window” range) is highly dependent on the atmospheric humidity. For simplicity, a straight transmittance average, T8-13, is used instead. Likewise, the average reflectance (R8-13) and absorbance (A8-13) are calculated in the “atmospheric window” range. In this study, five nanocrystalline pigmented polyethylene shields are compared to the submicrometer shield. The five nanocrystalline shields labeled “a” through “e” differ by the thickness of the TiO2 film embedded in the PE. Table 1 indicates the TiO2 thickness of the shields. Figure 1 shows the FTIR transmittance spectra of three nanocrystalline shields compared to the submicrometer TiO2 shield. For clarity, we present three spectra of the nanocrystalline shields (a, c, e). Table 1 shows the average integrated optical functions of all shields as calculated by the equations described above. The FTIR transmission measurements in the “atmospheric window” were performed without the use of an integrating accessory. Therefore, these measurements underestimate the transmittance values, since scattered transmitted radiation is not counted. Figure 1 shows that in the “atmospheric window” region all films are considerably more transparent than the submicrometer sample. The average transmittance in the window region decreases with increasing thickness of the TiO2 film. Nevertheless, even the lowest transmittance observed, 54.6% for film e, is a respectable value for such a shield, taking into account the scattered transmittance.
Figure 3. Vis/NIR IR total (specular + diffuse) reflectance of the submicrometer and the three nanocrystalline TiO2 pigmented polyethylene shields presented in Figure 1.
The optical characteristics of the shields discussed above in the visible/near-IR region were also measured. Contrary to the FTIR measurements, in this spectral region the total values are measured using an integrating sphere. The total transmittance in the visible/near-IR region is crucial since the incoming solar radiation flux is significantly larger than the outgoing cooling flux. Even a minimal percent of scattered radiation in the visible/nearIR region could have a major effect on a device’s performance. Similarly, the total absorbance measurements are important because of their contribution to the heating of the devicesalthough such heating is not as effective as the transmitted radiation. Figure 2 shows the visible/near-IR total transmittance spectra of the shields discussed above. The shield transmission decreases with increasing thickness of the TiO2 film. Figure 3 shows the total reflectance spectra (specular + diffuse) of the shields in the visible/near-IR region. As the TiO2 film becomes thicker, the measured reflection increases. Using eqs 1-3 referenced above, the average transmittance, reflectance, and absorbance values of all the films were calculated. Table 1 and Figure 4 present the values together with the transmittance values of the “atmospheric window”.
TiO2 Nanocrystalline Pigmented Polyethylene Foils
Figure 4. Average optical values of the nanocrystalline shields (filled signs) and submicrometer shields (empty signs) as a function of the TiO2 film thickness. The reflectance, transmittance, and absorbance at the solar radiation region and the “atmospheric window” transmittance are labeled Rsol, Tsol, Asol, and T8-13, respectively. The values of the submicrometer shield were placed at the thickness in which they fit to the nanocrystalline shields in the solar region.
Figure 4 and Table 1 both show that it is possible to control the properties of the shield by changing the thickness of the TiO2 layer. As the thickness increases, the blocking of the solar radiation improves at the expense of the transmission in the cooling spectral region. This finding opens the possibility of optimizing the system to specific atmospheric conditions; that is, on the basis of the ratio between the incoming radiation flux and the outgoing cooling flux at the working location, the optimal TiO2 thickness will provide maximum cooling. As noted in Figure 4, the comparison between the nanocrystalline and submicrometer shields shows that the nanocrystalline shields are superior. Figure 4 also shows that a nanocrystalline shield, having the same transmittance in the visible/near-IR region as a submicrometer shield, has almost twice as much transmittance in the cooling region. In addition, the nanocrystalline shield absorbs less solar radiation than the submicrometer shield. As reported previously,12,20 the nanocrystalline shields are also compared to submicrometer TiO2/PE-based shields. These researchers measured the reflectance and transmittance of 100 µm thick polyethylene foils pigmented with TiO2 (0.23 µm diameter, rutile) as a function of the pigment volume fractions. These foils show high solar reflectance; for example, foils with a volume fraction of 0.39 showed a solar reflectance of ca. 80% versus 76.3% in the nanocrystalline samples. However, the submicrometer foils suffer from low transmittance in the “atmospheric window” region (T8-13) 0.45). This figure is lower than the transmittance achieved with the nanocrystalline shields (T8-13 ) 0.54). Finally, a field test of the nanocrystalline and the submicrometer shields showed that the nanocrystalline is more efficient than the submicrometer shields. This field test was performed for the purpose of comparing the two shield types and not for a study of maximal performance. Thus, no optimization of the other system components, such as the IR emitter, was involved. Furthermore, the humidity in the test area is far from optimal for achieving good performance of radiative cooling systems. The testing configuration was based on a thermocouple located between the shield and a black metal plate that was placed in a thermally insulated box. The system (20) Nilsson, T. M. J.; Niklasson, G. A. Sol. Energy Mater. Sol. Cells 1995, 37, 93.
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Figure 5. FTIR transmittance and reflectance spectra of 100 µm high-density polyethylene foil.
Figure 6. (a) XRD spectrum of a TiO2 colloidal suspension used for the preparation of the nanocrystalline shields. (b) XRD spectrum of TiO2 films after pigmentation in polyethylene.
containing the nanocrystalline shield showed a decrease of more than 3 deg below ambient at noontime on a sunny day, while the submicrometer-based system, measured simultaneously, maintained ambient temperature. (Under these conditions, without a shield, the system heats above ambient.) Taking into account the experimental conditions, the improvement achieved by the new shield is significant. The difference in the optical properties between the nanocrystalline and submicrometer shields may be explained by the difference in crystal size, crystal purity, and preparation method. However, before this variance is discussed, it is necessary to describe the properties of the materials used to fabricate the nanocrystalline shields. In both cases, the shield is mechanical stability is achieved using PE as a substrate. For better strength and rigidity, the high-density form of PE is used, although the optical properties of the low-density polyethylene are superior. Figure 5 shows the IR transmittance and reflectance spectra of a 100 µm high-density PE foil. Except for characteristic bands at 2.4 (not shown), 3.4, 6.8, and 13.7 µm, the absorption of PE at all wavelengths is negligible. Table 1 presents the average transmittance, reflectance, and absorbance values for the spectral regions that are relevant to the radiative cooling systems. In both regions, the PE is more than 80% transparent (Tsol ) 0.891 and T8-13 ) 0.813), while most of the transmission losses are due to reflection. Figure 6a presents the XRD defractogram of the TiO2 colloids used for the preparation of the nanocrystalline shields. The XRD spectra correspond to pure rutile. The average crystallite size is 50.6 nm. This is calculated from the peak broadening using the Debye-Scherrer relationship. Figure 7a shows the corresponding TEM image of the TiO2. The particles are irregularly shaped, closer to prolate spheroids than to spheres, and the colloids are
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Figure 7. (a) TEM image. (b) Crystal size distribution of the TiO2 colloidal suspension used for the preparation of the nanocrystalline shields. Since the particles were often slightly oblate, an average size between the two in-plane dimensions was used to estimate each particle size.
separated with a small degree of aggregation. Figure 7b shows the crystals’ size and size distributions measured from the TEM image. The average size calculated from the histogram is 61.5 nm, which is larger than the crystallite size estimated from X-ray diffraction. The difference may indicate the presence of defects in the crystals (faults, strain), in which case the colloids are larger than the crystallites. However, we believe that this minimal difference primarily results from the way the TEM images are analyzed; that is, the size of the nonspherical particles is determined by averaging their length and width on the TEM pictures. High temperatures can lead to crystal growth by several mechanisms. Nanoparticles, particularly the small ones, might grow by coalescing at high temperatures.21 Another effect of high temperature is related to the thermodynamic instability of crystals that are smaller than a certain critical size. The kinetic stabilization of these colloids is expected to be less effective at high temperatures.22 Therefore, the TiO2 crystal structure and size were also measured after the hot pressing, although the colloidal suspension used for film formation was exposed to significantly more extreme conditions during the hydrothermal synthesis (250 °C). Electron microscopy, which is usually useful in the analysis of crystal size, cannot be (21) Hodes, G.; Albu-Yaron, A.; Decker, F.; Motisuke, P. Phys. Rev. B 1987, 36, 4215. (22) Gorer, S.; Hodes, G. In Semiconductor Nanoclusters: Physical, Chemical and Catalytic Aspects; Kamat, P. V., Meisel, D., Eds.; Elsevier Scientific: New York, 1997.
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employed in this case because the TiO2 is pressed between two PE foils. Nonetheless, X-ray diffraction and optical transmission can be used to examine changes in the structure and size of the crystals during film preparation. Figure 6b presents the X-ray diffraction of TiO2 after pigmentation in polyethylene was performed. Except for the additional peaks that correspond to crystalline PE, the XRD spectra are similar before (Figure 6a) and after (Figure 6b) the pigmentation process. This similarity indicates that the shield preparation processes do not affect the colloids concerning the crystal structure or size. In addition, the crystals were examined by visible spectroscopy with respect to possible size changes during the pigmentation. Changes in the size of nanocrystalline semiconductors are expressed as a variation of the semiconductor energy level structure and, therefore, as a variation of the optoelectronic properties. Of these properties, the optical transmittance (absorbance) spectra are the most common and simplest ones to measure. The optical band gaps, calculated from transmittance spectra of the TiO2 films as prepared and pigmented in polyethylene (not seen here), show identical values. Mie and Rayleigh derived a general theory of light scattering by spherical particles early in the 20th century.23 This theory calculates the intensity of light scattering from spherical particles as a function of their refractive index, crystal size, and the light wavelength. When the particle diameter is less than ca. 10% of the wavelength, one can simplify Mie’s equations and apply Rayleigh’s theory. By doing so, we show that the particles’ size strongly affects the wavelength at which the particles do not scatter, thereby allowing high transmission. The average crystal sizes of the nanocrystalline TiO2 used to fabricate the shields meet Rayleigh’s condition in the spectral range of 400 nm and above. The refractive ratio of indices of TiO2 (2.74) and PE (1.58) is 1.73, which is a major factor in Rayleigh’s equations. Using these values in the application of Rayleigh’s scattering theory shows that a shield containing 60 nm nanocrystalline TiO2 should scatter in the visible range (500 nm) ca. 20 times more than in the “atmospheric window” region (10 µm). The theory does not consider the effect of various parameters such as particles shape, absorption, or large refractive index differences. However, since the spectral regions of interest are well separated, the Rayleigh scattering model can be used as a good approximation of the scattering behavior of the solar shields. The submicrometer shields are made from commercial TiO2 crystals with an average diameter of 0.23 µm. Although, for this crystal size, the Rayleigh theory applies only for radiation with wavelengths longer than 2 µm, the general trend still exists; that is, scattering will decrease as the wavelength increases. However, the wavelength at which the large particles exhibit negligible scattering is longer than that of the nanocrystalline particles. The shields consisting of the large particles are thus expected to be less transparent than the nanocrystalline particles at the “atmospheric window”. Taking into account the presence of some particles that are larger than the 0.23 µm average, the difference between the submicrometer and the nanocrystals concerning the scattering becomes even more pronounced. The improved performance of the nanocrystalline shields is therefore attributed to the size of the colloids used in the shields. The importance of particle size for light scattering was discussed in detail on the basis of the advanced models (23) Kerker, M. The Scattering of Light and Other Electromagnetic Radiation; Academic Press: New York, 1969.
TiO2 Nanocrystalline Pigmented Polyethylene Foils
that incorporated more structural information about the scattered particles. Recently, Vargas and Niklasson24 considered different optimization criterion for achieving high light scattering from nonabsorbing particles. The optimization criteria are based on the maximization of the volumetric scattering cross section or the reduced scattering cross section/unit volume. The particle size have been optimized to give the highest light scattering in the visible region. In their paper, Vargas and Niklasson concluded that for particles of refractive indices between 2.25 and 2.90 (2.74 for rutile TiO2) the different optimization criteria provide almost identical results with respect to particle size. The optimum particle diameter for maximum scattering approximates 0.10 µm. This optimum particle size is more confined to the crystal sizes in the nanocrystalline sample than in the submicrometer samples, thus explaining the improved properties of the nanocrystalline shields. The aggregation size is another parameter which differentiates the colloids used in the submicrometer shields from those used in the nanocrystalline shields. When light travels through two blocks of similar material that are separated by a different refractive index medium, the effect of the refractive index changes, decreasing as the distance between the blocks becomes less than ca. half the wavelength. This phenomenon is often referred to as the frustrated total internal reflection.25 In other words, two particles will be optically considered as one medium if the distance between them is zero. The particles will be considered as two separated media if the distance between them is larger than half the light wavelength. Between these two extremes, partial separation in optical terms exists. This phenomenon is relevant to the colloidal system mainly with respect to aggregation. Although the surface of the particles is not flat, in some areas the distance between two crystals may be less than half the wavelength, which increases their optical effective size. The microstructure of the shield with respect to the presence of crystal aggregates can, therefore, be of fundamental importance for light scattering. The influence of aggregation on the optically effective size of the TiO2 can also explain the superiority of the (24) Vargas, W. E.; Niklasson, G. A. J. Colloid Interface Sci. 1995, 169, 497. (25) Drexhage, K. H. Sci. Am. 1970, 222, 108.
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nanocrystalline shields. In the submicrometer shields, aggregation probably occurs more frequently than in the nanocrystalline-based shields. Figure 7 indicates that the nanocrystals do not tend to form large aggregates because they are synthesized and processed in an acidic medium. The crystals used for the submicrometer shield, on the other hand, are usually synthesized by aerosol techniques and are processed as dry powder which usually results in significant agglomeration. The shield fabrication involves the mixing of the pigment in an extruder and hot pressing, processes that do not break existing agglomerates. As described above, aggregation can lead to an increase in the optically effective particle size, which in turn increases the reflectivity of the shield in the IR region. With this understanding of the system, it is possible to improve the properties of the shields using ca. 100 nm crystals that contain the minimal amount of aggregation. The nanocrystalline method is preferred because the particles are kept separated. However, this method still requires the complex synthesis of 100 nm with narrow size distribution. Further experiments in these directions are currently in progress. Conclusions Efficient radiative cooling under solar illumination requires the formation of a shield that reflects most of the radiation while being transparent to the IR radiation. A new method for the preparation of such a shield has been reported using rutile TiO2 particles of 60 nm diameter. Both the pigmentation method and the particle size contribute to the improved performance of the nanocrystalline shields in comparison to the submicrometer shields. This paper provides new insight about the role of the materials in the optical properties of the shields, expressing the importance of material design for this application. Further improvement in the shield performance should be achieved by the synthesis of larger TiO2 colloids that will be embedded in the PE substrate by the new pigmentation method. Acknowledgment. The authors express gratitude to the Israel Ministry of Science (MOS) for providing support for this work. LA010370G
