Monodisperse Surface Charge-Controlled Black Nanoparticles for

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Monodisperse Surface Charge-Controlled Black Nanoparticles for Near Infrared Shielding Nanami Hano, Makoto Takafuji, Hiroki Noguchi, and Hirotaka Ihara ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00555 • Publication Date (Web): 29 Apr 2019 Downloaded from http://pubs.acs.org on May 1, 2019

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ACS Applied Nano Materials

Monodisperse Surface Charge-Controlled Black Nanoparticles for Near Infrared Shielding

Nanami Hano,† Makoto Takafuji,†, ‡ * Hiroki Noguchi,† Hirotaka Ihara†, ‡ *

† Department

of Applied Chemistry and Biochemistry, Kumamoto University,

2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan ‡ Kumamoto

Institute for Photo-electro Organics (PHOENICS),

3-11-38 Higashimachi, Higashi-ku, Kumamoto 862-0901, Japan

KEYWORDS Microwave synthesis, -Conjugated polymer, Dispersion polymerization, Amphiphilic surface, Selective reflectance of NIR, Heat-shielding materials, Heat-insulating materials

ACS Paragon Plus Environment

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ABSTRACT A simple and quick preparation of black polymer nanoparticles by the microwaveassisted polymerization of 1,5-dihydroxynaphthalene (DHN) and 1,3,5-trimethyl-1,3,5triazinane (TA) under high temperature and pressure was demonstrated. The average diameter of the nanoparticles ranged from 20 to 700 nm, depending on the preparation conditions such as reaction solvent, monomer concentrations, and monomer ratio in the seed mixture. The surface charge of the nanoparticles was also determined by the monomer ratio used in the seed mixture. The nanoparticles could be dispersed in polar solvents such as water, methanol, and ethanol, probably because the phenolic hydroxyl groups and primary/secondary amine groups remained on the surface of the nanoparticles. The reflectance spectroscopic measurements from 200 to 2200 nm showed that the nanoparticles expressed selective reflectivity of NIR wavelengths. The absorption and reflectance properties could be tuned by microwave-assisted wet calcination of the nanoparticles. The particles were initially dark green, but when the particles were heated at 250 °C for 10 min in ethylene glycol, they absorbed light in the UV-visible region (reflectance was less than 3% from 200 to 750 nm), indicating the color of the particles had become perfectly black. Solar reflection in the NIR region from


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750 to 2200 nm, however, remained more than 25% after calcination. The calcinated nanoparticles could be dispersed in water, and the surface charge was positive at lower pH and negative at higher pH. The isoelectric point shifted slightly from 5.3 to 4.4 after wet calcination at 250 °C. The amount of nitrogen in the nanoparticles decreased remarkably after calcination, therefore the phenolic hydroxyl groups must have remained on the surface of the calcinated nanoparticles preferentially to the amine groups. These black nanoparticles with selective reflectance in the NIR region could be applied to black heat-shielding materials such as paints for buildings and automobiles.


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1. INTRODUCTION Recently, heat-shielding and heat-insulating have become important issues for energy efficiency and global warming. Various heat-shielding materials have been developed for suppressing the internal temperature rise of buildings and automobiles caused by sunlight.1-3 Because approximately half of sunlight is heat rays, which includes the electromagnetic radiation from near-infrared (NIR) to infrared (IR), materials with high reflectivity in the NIR-IR region can be used as heat-shielding materials. Heat-shielding coatings have been increasingly used for the roofs and walls of buildings, as well as the bodies of automobiles, because of their excellent convenience. The heat-shielding coatings usually contain inorganic pigments which selectively reflect wavelengths in the infrared region in addition to reflecting some visible light.4-8 The color of an IRreflective pigment is characterized by its reflectivity and absorptivity; therefore, it is possible to control the color of the pigments by changing the mixing ratio of the raw materials. Inorganic materials such as titania, iron oxide, zinc, alumina, and silica are often used as raw materials,9-16 and the pigments are synthesized by calcinating a mixture of them at very high temperature. However, the specific density of inorganic materials is generally large, requiring the use additives such as dispersants and stabilizers, or modification of the surface of the pigments, in order for them to be dispersed in solvent. There have been few reports on organic dyes with selective reflectivity for heat-shielding materials.17,18 Perylene derivatives and an azo pigment


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having an azomethine group have been reported as organic heat-shielding materials, both of which contain -conjugated aromatic hydrocarbons in their skeleton. These dyes reflect NIR light from 750 to 1500 nm and from 500 to 1500 nm, respectively. Very recently we reported the preparation of amorphous carbon-like microspheres by the polymerization of 1,5-dihydroxynaphthalene (DHN) and 1,3,5-trimethyl-1,3,5triazinane (TA) in hydrophilic solvent without any additives (dispersants), and the subsequent calcination at relatively low temperature.19-21 The amorphous carbon-like microspheres obtained are mono-disperse, and their size could be controlled in the range from submicron to a couple of microns. The color of the microspheres was dark brown to black, with a reflectance of less than 10% from 300 to 750 nm, and thus the microspheres effectively absorb visible light. The polymer structure of the microspheres has not yet been clarified because the mechanism of polymerization accompanied by crosslinking is complicated. However, it can be expected that the polymer contains a conjugated carbon-like structure with a nitrogen-doped heterocyclic aromatic moiety. Because such a structure may be similar to that of the above-described perylene derivatives and azo pigment having an azomethine group, it seemed likely that the microspheres would express selective reflectivity of NIR-IR light. In this study, we demonstrate the preparation of amphiphilic black particles with nanometer to submicron diameters from a mixture of DHN and TA, and evaluate their selective reflectivity. Microwave-assisted heating was applied to make the size of the spheres


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smaller, because rapid heating can cause rapid nucleation. The effects of the reaction solvent, monomer concentrations, and monomer ratio on the size and surface characteristic of the polymer particles were also investigated. Furthermore, the reflectivity of the obtained black nanoparticles in the UV-vis-NIR region was evaluated.

2. EXPERIMENTAL SECTION 2.1. Materials. DHN and TA were purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan), and used for copolymerization without purification. The chemical structures of DHN and TA are shown in Figure 1. Ethanol, tetrahydrofuran (THF) and ethylene glycol were purchased from Fujifilm Wako Pure Chemical Corporation (Osaka, Japan), and used as reaction solvents. Hydrochloric acid and sodium hydroxide were purchased from Nacalai Tesque Inc. (Kyoto, Japan), and used to adjust the solution pH.

2.2. Preparation of monodisperse polymer nanoparticles. DHN and TA were dissolved in the desired solvent, and the solution was put in a pressure-resistant glass vessel. Microwave irradiation was carried out in a microwave chemical reaction apparatus (Monowave 300, Anton Paar USA Inc., USA). After microwave irradiation, the obtained particles were collected by centrifugation, then washed with ethanol several times and dried under vacuum. A schematic of the preparation is shown in Scheme 1.


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2.3. Measurements. The size and the surface charge of the nanoparticles were measured in each solvent by dynamic light scattering techniques (DLS; Zetasizer Nano ZS, Spectris, UK). The pH of the aqueous dispersion of the nanoparticles was adjusted by using 0.1 mM HCl or 0.1 mM NaOH aqueous solutions. The nanoparticles were examined with a scanning electron microscope (SEM; JCM-5700, JEOL, Japan), fieldemission scanning electron microscope (FE-SEM; SU-8000, Hitachi, Japan) and transmission electron microscope (TEM; JEM-1400plus, JEOL, Japan). Elemental analysis (EA; Micro Corder JM10, J Science Co., Japan) was carried out to identify the elements in the nanoparticles. The reflectance spectra of the nanoparticles in powder form were measured using an ultraviolet-visible-near infrared spectrophotometer (UVvis-NIR; UV-3600 Plus, Shimadzu, Japan). Thermography images were obtained with a thermal imaging camera (FLIR A65, FLIR Systems, Inc., USA). The nanoparticles were irradiated for imaging with a white LED (LDL2-74X30SW2, λ = 467 nm, λ2 = 634 nm, CCS Inc., Japan) and an infrared (IR) LED (LDL-74X27IR1200, λ = 1200 nm, CCS Inc., Japan).

3. RESULTS AND DISCUSSION 3.1. Preparation of monodisperse polymer nanoparticles. For the initial experiment, 20 mL each of ethanol solutions of DHN (30 mM) and TA (30 mM) were put in the high-


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pressure glass vessel, the mixture was heated to 150 °C by microwave irradiation, and stirred at 150 °C for 3 min. The solution was initially transparent and colorless, and changed to a slightly turbid dark moss-green color after microwave irradiation. A dark moss-green powder (D30/T30-E10) was isolated from the reaction. The SEM image revealed that mono-dispersed spherical particles with a diameter of less than 1 m were formed (Figure S1). The particles were dispersed in ethanol using an ultrasonicator bath (240 W, 5 min), and the particle size was evaluated by DLS measurement to have an average diameter of 690 nm with an 18% coefficient of variation (CV). The particle size did not change significantly even when the reaction time was greater than 3 min (620 nm after 15 min and 625 nm after 60 min). According to the Arrhenius equation,22 the reaction rate at 150 °C for 3 min (microwave irradiation) corresponds to that at 80 °C for 6 h. When an ethanol solution of DHN and TA (30 mM each) was stirred at 80 °C, the solution became brownish purple after 1 h, then gradually darkened. After stirring for 6 h, the color of the solution finally became moss green and did not further change with additional heating. SEM images indicated that spherical particles were also obtained in the solution prepared at 80 °C without microwave irradiation, and DLS measurement of the aqueous dispersion indicated that the average diameter of the obtained particles was 1330 nm with a 7% CV. The yields of nanoparticles in the reaction mixtures were 86% (150 °C, 3 min) and 63% (80 °C, 6 h). Presumably, the polymerization of DHN accompanied by the crosslinking reaction with TA was rapidly progressed by heating,


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and as the polymer chain grew, the polymer became insolubilized and precipitated to form the particles. When the reaction mixture was quickly heated to a higher temperature by microwave irradiation, there was probably a dense nucleation of particles. As a result, a smaller size of spherical particles could be obtained from the same monomer concentrations (DHN/TA = 30 mM/30 mM) in the same reaction solvent (ethanol) by using microwave irradiation instead of conventional heating. It was noteworthy that the microwave reaction was effective not only for shortening the reaction time, but also for producing smaller size spherical particles by the polymerization of DHN and TA. Based on these results, investigations were performed using microwave irradiation at 150 °C for 3 min with different reaction solvents, monomer concentrations, and monomer ratios. Table 1 summarizes the preparation conditions, the average particle diameters, and CV values. In water, the solution became dark green after microwave irradiation, and spherical particles with an average diameter of 544 nm (CV = 16%) were obtained. In contrast, the THF solution of DHN and TA remained colorless and transparent after microwave irradiation. DLS measurements suggested that the particle size was less than 1 nm, and no precipitate was obtained by centrifugation at 50,000 × g. These results suggested that the average particle size of the microspheres composed of DHN-TA polymer could be controlled by using different reaction solvents. Various ratios of water/ethanol and water/THF mixtures were examined as reaction solvents for


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the preparation of the microspheres. In all cases, the solutions became dark moss-green to black after microwave irradiation. As shown in Table 1, it was found that the average particle diameter varied significantly depending on the composition of the reaction solvent. In the mixture of water and THF (D30/T30-WwTt, w = water composition, t = THF composition), the average particle diameter tended to decrease as the proportion of THF increased. In contrast, in the mixture of water and ethanol (D30/T30-WwEe, w = water composition, e = ethanol composition), smaller particles could be obtained at water/ethanol ratios of 7:3 to 5:5. The particle diameter changes are largely considered to be due to the difference in solubility parameter () based on solvent polarity. There have been a large number of reports on the control of particle size in dispersion polymerization using solubility parameter differences. For example, the size of polyacrylamide particles was controlled by changing the solvent ratio of water and tertbutyl alcohol,23 and the size of copolymer particles with styrene and glycidyl methacrylate was controlled by changing the water and ethanol ratio.24 All of the obtained particles prepared in the reaction solvents listed in Table 1 except those prepared in THF were dark moss-green or dark green. When the polymerization was carried out at lower monomer concentrations (10 mM (D10/T10-W5T5) and 20 mM (D20/T20-W5T5)) in water/THF (50:50), smaller sized (22 nm and 83 nm) nanoparticles were obtained after microwave irradiation. Figure 2 shows typical microscopic images (SEM, FE-SEM, and TEM) of the obtained nanoparticles prepared under the various


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conditions. It should be emphasized that the dark-colored spherical nanoparticles from 20 to 700 nm in diameter could be obtained by changing the reaction solvent in the microwave-assisted polymerization of DHN and TA.

3.2. Surface charge control of nanoparticles. The nanoparticles prepared from a mixture of DHN and TA (1:1) were easily dispersed in polar solvents such as water and ethanol, and no rapid aggregation and precipitation occurred. This suggested that the surface of the nanoparticles was covered by hydrophilic moieties. Formation of a hydrophilic surface may be caused by the hydroxyl groups from DHN and the primary/secondary amine groups from TA. Different monomer ratios of DHN and TA in the seed mixture were tested to control the surface charge of the nanoparticles. Ethanol was used as a reaction solvent, and the microwave-assisted polymerization was carried out with different monomer ratios (DHN/TA = 10 mM/50 mM (D10/T50-E10), 20 mM/40 mM (D20/T40-E10), 30 mM/30 mM (D30/T30-E10), 40 mM/20 mM (D40/T20-E10), and 50 mM/10 mM (D50/T10-E10)). The preparation conditions, particle size, and elemental analysis results are summarized in Table 2. All the solutions were dark green after microwave irradiation, with the exception of D10/T50-E10, which was deep yellow. The average particle size of the nanoparticles showed a maximum size at an equimolar concentration of DHN (30 mM) and TA (30 mM) (D30/T30-E10), and decreased as the monomer concentrations deviated from equivalence. When ethanol was used as the


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reaction solvent, the yield was also the highest (87%) when the nanoparticles were prepared with an equimolar concentration of monomers (D30/T30-E10). In the case of D10/T50-E10 (DHN = 10 mM, TA = 50 mM), the yield was less than 1%. As seen in the elemental analysis, as the ratio of TA decreased, the amount of N also decreased, resulting in an increase in the overall C/N ratio. However, it was confirmed that the C/N ratio was two to three times larger than the theoretical value calculated from the composition ratio of the monomers. Figure S2 shows the diffuse reflectance Fourier transform infrared (DRIFT-IR) spectra of the polymer nanoparticles prepared using different monomer ratios (D50/T10-E10, D40/T20-E10, D30/T30-E10, and D20/T40-E10). Key absorption peaks were observed at 1435 cm-1 (C–H bending), 1371 cm-1 (O–CH2–N wagging), 1265–1263 cm-1 (C–O antisymmetric stretching), and 1190–1178 cm-1 (C–N antisymmetric stretching). It was presumed that an oxazine ring was formed by the polymerization of DHN and TA in all nanoparticles.25 However, the structural differences between the polymer nanoparticles could not be detected in the DRIFT-IR spectra. TG analyses of D50/T10-E10, D40/T20-E10, D30/T30-E10, and D20/T40-E10 (Figure S3) showed that the decomposition temperature of the polymer nanoparticles increased as the amount of DHN increased, indicating the polymerization with cyclization and crosslinking reactions progressed accordingly. The reaction process is currently under investigation using NMR spectroscopy and the detailed reaction mechanisms have not yet been clarified.25 However, it was estimated that the polymerization of DHN and TA


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occurred with an appropriate monomer ratio, because the yields and particle size decreased as the difference in the monomer ratio increased. Furthermore, the particle size could be controlled by changing the monomer ratio (Figure S1). All nanoparticles synthesized from DHN-TA mixtures with different molar ratios were well dispersed in water. Figure 3 shows the pH dependency of the surface zeta potential of the nanoparticles in the aqueous dispersion. All nanoparticles were negatively charged at higher pH values and positively charged at lower pH values. Isoelectric points (IEP) were detected from pH 3 to 7. The IEPs were higher in nanoparticles prepared with an excess molar ratio of TA (D10/T50-E10) and lower in nanoparticles prepared with an excess molar ratio of DHN (D50/T10-E10). It was estimated that a significant number of phenolic hydroxyl groups existed on the surface of nanoparticles prepared with an excess molar ratio of DHN. These results proved that the surface charge could be tuned by changing the monomer ratio of DHN and TA in the seed mixture. Figure 4 shows the typical images and size distributions of D30/T30-E10 dispersed in various solvents such as water, N,N-dimethylformamide, methanol, ethanol, THF, and chloroform. The obtained nanoparticles could be dispersed in the polar to non-polar solvents presumably because the nanoparticles have both a hydrophobic part (aromatic rings) and a hydrophilic part (phenolic hydroxyl groups from DHN and the tertiary amine groups from TA). To our knowledge, amphiphilic monodisperse spherical blackcolored nanoparticles with controllable surface charge have not been reported. The


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obtained nanoparticles have great potential for use in various application fields such as inkjet ink, electrophotography toner, optical lenses, and environmentally friendly fillers.

3.3. Selective reflectivity in the NIR region. To investigate the reflectivity of the nanoparticles, UV-vis-NIR reflectance spectroscopic measurements were carried out for the dark-green nanoparticles (D30/T30-W5T5, average diameter: 85 nm). As shown in Figure 5, D30/T30-W5T5 in powder form had a reflectance of less than 3% from 200 to 650 nm, and increased gradually after 650 nm until it reached 73% at 1380 nm. The color of the nanoparticles was not perfectly black because a small reflection of less than 10% was detected in a portion of the visible wavelength range (650 to 750 nm). In our previous report, 1 m particles prepared by dispersion polymerization with DHN and TA were blackened by using an electric tube furnace and heating at 200 or 400 °C for 2 h under a nitrogen atmosphere.19 It was confirmed that the color of the particles became black (less than 3% reflectance from 300 to 650 nm) and the particles were blackened even with heating at a low temperature such as 400 °C. To avoid the coagulation of nanoparticles by thermal treatment in the powder form, in this study the treatment of nanoparticles with a diameter of 85 nm (D30/T30-W5T5) was carried out by a wet process (in solvent) with microwave-assisted heating. In order to perform the microwave-assisted wet calcination at a higher temperature than that used in the previous experiments, the solvent was replaced by ethylene glycol using centrifugations


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and decantations. The color of the solution before wet calcination was dark green; it changed to black after wet calcination for 10 min at 200, 250, or 300 °C. The color changes were also seen in the diluted aqueous solutions, in which a yellowish color gradually darkened as the calcination temperature increased, and changed to deep orange after wet calcination at 300 °C. DLS measurements and TEM observations indicated that no aggregation occurred even after microwave-assisted calcination. Figure S4 shows the diffuse reflectance Fourier transform infrared (DRIFT-IR) spectra of the polymer nanoparticles (D30/T30-W5T5) before calcination and after calcination at 200, 250, and 300 °C. Key absorption peaks were observed at 2931–2927 cm-1 (C–H alkane stretching), 1444–1435 cm-1 (C–H bending), 1371–1367 cm-1 (O–CH2–N wagging), 1265–1263 cm-1 (C–O antisymmetric stretching), and 1205–1203 cm-1 (C–N antisymmetric stretching). The intensity of the peaks at 2931–2927 cm-1 (C–H alkane stretching) was increased by calcination of the polymer nanoparticles. It was presumed that calcination extended the alkyl chain length through cross-linking and the ring structure was formed. TG analyses (measured in air flow) showed almost no differences in all the black nanoparticles before and after calcination (Figure S5). This is probably due to the formation of an amorphous carbon-like structure rather than a graphite-like structure.26-28 As shown in Figure 5, the reflectance from 650 to 750 nm gradually decreased with increasing calcination temperature, indicating the color became perfectly black (the reflectance was less than 3% from 200 to 750 nm after wet


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calcination at 250 °C). The reflectance in the NIR region also decreased with heat calcination, but the total reflectance from 1250 to 2150 nm was maintained at greater than 50%. These results indicate that the -conjugated structure was extended by heat calcination.29 However, even when heated at 300 °C, the reflectance was more than 50% at wavelengths of 1800 nm and above. The obtained black nanoparticles were observed by a thermal camera under a white LED (λ = 467 nm, λ2 = 634 nm) and an infrared LED (λ = 1200 nm). As shown in Figure 5, the images suggest that the surface temperatures of the black particles were comparably higher than that of the black paper used as a reference under the white LED, but comparably lower than the black reference paper under the IR LED. These results suggest that the black particles adsorbed visible light more, resulting in a higher temperature than the reference paper, but adsorbed IR light less, resulting in a lower temperature than the reference paper. Figure 6 shows the average particle diameters were 86 (before calcination), 86, 85, and 104 nm (after calcination for 10 min at 200, 250, and 300 °C, respectively). As shown Figure S2, TEM observations indicated that the particle size was not significantly changed after calcination. A slight increase of the particle diameter after calcination at 300 °C was caused by aggregation of the particles. Further calcination at 300 °C for 30 min induced aggregation of the particles (135 nm diameter), as seen in both the DLS measurements and the TEM images. The elemental analyses of the particles (Table 3) suggested that the amount of C increased gradually and the amount of N decreased rapidly with


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increasing calcination temperature. As a result, the C/N ratio of the particles after calcination at 300 °C for 10 min more than doubled compared with that before calcination. The surface charges of the nanoparticles were mostly maintained in the wide pH range of 2 to 11, and the IEP was slightly shifted from 5.3 to 4.3 after calcination at 300 °C (Figure 7). Since the N component decreased significantly with calcination, it was concluded that the phenol group had the dominant effect on the surface of the nanoparticles. As shown in Table S1, the composition of the black nanoparticles did not change after aging in ambient conditions at room temperature for more than 6 months. Furthermore, TEM observations and DLS measurements indicated that no agglomeration or decomposition occurred after aging (Figure S7). These results suggest that the black nanoparticles are relatively stable under ambient conditions.

4. CONCLUSIONS Amphiphilic monodisperse black nanoparticles were prepared by microwave-assisted heating of a mixture of DHN and TA. The diameter of the particles could be controlled from a couple of tens of nanometers to the submicron range by changing the reaction solvent, monomer concentration, and monomer ratio. Compared with previously reported carbon-based nanoparticles,17,18,30-33 the black nanoparticles presented in this paper have several advantages: 1) the nanoparticles can be prepared in a short amount of time by the simple method of one-pot microwave irradiation, 2) the obtained


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nanoparticles are a monodispersed spherical shape, and the size is controllable from a couple of tens of nanometers to the submicron range, 3) the nanoparticles can be carbonized (blackened) in a solvent at a comparably lower temperature, 4) the polymer particles

could

be

dispersed

in

various

solvents

such

as

water,

N,N-

dimethylformamide, methanol, ethanol, THF, and chloroform without any additives or surface treatments, and 5) the nanoparticles exhibit selective reflectivity for near infrared light. The obtained nanoparticles were dark brown, and further blackening of the nanoparticles accompanied with carbonization occurred during wet calcination. These color changes were probably caused by the extension of -conjugated structures in the nanoparticles during the dispersion polymerization and wet calcination processes. Since the nanoparticles dispersed very well, and are negatively charged in acidic conditions (low pH) and positively charged in basic conditions (high pH), it was concluded that the phenolic hydroxyl groups and primary/secondary amine groups remained on the surface of nanoparticles. Further investigation is needed to clarify the chemical structure of the obtained black nanoparticles and the polymerization process. Because the polymer particles could be dispersed in various solvents without any additives or surface treatments, which is important to realize dispersant-free suspension as well as high coatability on substrates, the black nanoparticles are applicable as environmentally


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friendly materials to a variety of fields such as inkjet ink, electrophotography toner, optical lenses, and fillers. Furthermore, it was confirmed that the polymer nanoparticles absorbed visible light and selectively reflected NIR light, and the absorption and reflection properties could be controlled by microwave-assisted wet calcination at a relatively low temperature. The nanoparticles are totally organic and carbon-rich black-colored materials, yet their high dispersibility was maintained in various solvents, including hydrophilic solvents. The obtained black nanoparticles are therefore considered to have high potential for use in heat-shielding and heat-insulating coatings with selective reflection for NIR light.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge at the ACS publications website at DOI: Elemental analysis; SEM images; TEM images; DRIFT-IR spectra; TG curves; Particle size distributions AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENTS This research was supported by a Grant-in-Aid for Scientific Research (KAKENHI) from the Japan Society for the Promotion of Science (JSPS).


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REFERENCES (1) Xing, Z.; Tay, S. W.; Ng, Y. H.; Hong, L. Porous SiO2 Hollow Spheres as a Solar Reflective Pigment for Coatings. ACS Appl. Mater. Interfaces 2017, 9, 15103–15113. (2) Xiang, B.; Yin, X.; Zhang, J. A Novel Cool Material: ASA (Acrylonitrile-styreneacrylate) Matrix Composites with Solar Reflective Inorganic Particles. Composites Science and Technology 2017, 145, 149–156. (3) Levinson, R.; Berdahl, P.; Akbari, H.; Miller, W.; Joedicke, I.; Reilly, J.; Suzuki, Y.; Vondran, M. Methods of Creating Solar-reflective Nonwhite Surfaces and Their Application to Residential Roofing Materials. Solar Energy Materials & Solar Cells 2007, 91, 304–314. (4) Huang, B.; Xiao, Y.; Zhou, H.; Chen, J.; Sun, X. Synthesis and Characterization of Yellow Pigments of Bi1.7RE0.3W0.7Mo0.3O6 (RE = Y, Yb, Gd, Lu) with High NIR Reflectance. ACS Sustainable Chem. Eng. 2018, 6, 10735−10741. (5) Thara, T. R. A.; Rao, P. P.; Divya, S.; Raj, A. K. V.; Sreena, T. S. Enhanced Near Infrared Reflectance with Brilliant Yellow Hues in Scheelite Type Solid Solutions, (LiLaZn)1/3MoO4−BiVO4 for Energy Saving Products. ACS Sustainable Chem. Eng. 2017, 5, 5118−5126.


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(6) Raj, A. K. V.; Rao, P. P.; Divya, S.; Ajuthara, T. R. Terbium Doped Sr2MO4 [M = Sn and Zr] Yellow Pigments with High Infrared Reflectance for Energy Saving Applications. Powder Technology 2017, 311, 52–58. (7) Sameera, S.; Rao, P. P.; Divya, S.; Raj, A. K. V., Thara, T. R. A. High IR Reflecting BiVO4-CaMoO4 Based Yellow Pigments for Cool Roof Applications. Energy and Buildings 2017, 154, 491–498. (8) Sameera, S.; Rao, P. P.; Divya, S.; Raj, A. K. V. Brilliant IR Reflecting Yellow Colorants in Rare Earth Double Molybdate Substituted BiVO4 Solid Solutions for Energy Saving Applications. ACS Sustainable Chem. Eng. 2015, 3, 1227–1233. (9) Oka, R.; Masui, T. Synthesis and Characterization of Black Pigments Based on Calcium Manganese Oxides for High Near-infrared (NIR) Reflectance. RSC Adv. 2016, 6, 90952–90957. (10) Li, Y. Q.; Mei, S. G.; Byon, Y. J.; Wang, J. L.; Zhang, G. L. Highly Solar Radiation Reflective Cr2O3−3TiO2 Orange Nanopigment Prepared by a Polymer-pyrolysis Method . ACS Sustainable Chem. Eng. 2014, 2, 318−321. (11) Ianoş, R.; Muntean, E.; Lazău, R.; Băbuță, R.; Moacă, E. A.; Păcurariu, C.; Dabici, A.; Hulka, I. One-step Synthesis of Near-Infrared Reflective Brown Pigments


22

Page 23 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Based on Iron-doped Lanthanum Aluminate, LaAl1-xFexO3. Dyes and Pigments 2018, 152, 105–111. (12) Sharma, R.; Tiwari, S.; Tiwari, S. K. Highly Reflective Nanostructured Titania Shell: A Sustainable Pigment for Cool Coatings . ACS Sustainable Chem. Eng. 2018, 6, 2004−2010. (13) Yang, R.; Han, A.; Ye, M.; Chen, X.; Yuan, L. Synthesis, Characterization and Thermal Performance of Fe/N Co-doped MgTiO3 as A Novel High Near-infrared Reflective Pigment. Solar Energy Materials & Solar Cells 2017, 160, 307–318. (14) Thongkanluang, T.; Chirakanphaisarn, N.; Limsuwan, P. Preparation of NIR Reflective Brown Pigment. Procedia Engineering 2012, 32, 895–901. (15) Jeevanandam, P.; Mulukutla, R. S.; Phillips, M.; Chaudhuri, S.; Erickson, L. E.; Klabunde, K. J. Near Infrared Reflectance Properties of Metal Oxide Nanoparticles. J. Phys. Chem. C 2007, 111, 1912–1918. (16) Mastai, Y.; Diamant, Y.; Aruna, S. T.; Zaban, A. TiO2 Nanocrystalline Pigmented Polyethylene

Foils

for

Radiative

Cooling

Applications:

Synthesis

and

Characterization. Langmuir 2001, 17, 7118–7123. (17) Kaur, B.; Bhattacharya, S. N.; Henry, D. J. Interpreting the Near-infrared Reflectance of A Series of Perylene Pigments. Dyes and Pigments 2013, 99, 502–511.


23


Page 24 of 38

(18) Okamoto, H.; Iguchi, K.; Ito, H. Near Infrared-Reflecting/Transmitting Azo Pigment, Method for Manufacturing Near Infrared-Reflecting/Transmitting Azo Pigment, Colorant Composition Using Said Azo Pigments,  Item-coloring Method and Colored Item. US9822266B2, Nov. 21, 2017. (19) Murakami, A.; Noguchi, H.; Kuwahara, Y.; Takafuji, M.; Nozato, S.; Sun, R. D.; Nakasuga, A.; Ihara, H. Non-conductive, Size-controlled Monodisperse Black Particles Prepared by a One-pot Polymerization and Low-temperature Calcination. Chem. Lett. 2017, 46, 680–682. (20) Noguchi, H.; Liu, T.; Nozato, S.; Kuwahara, Y.; Takafuji, M.; Nagaoka, S.; Ihara, H. Novel Black Organic Phase for Ultra Selective Retention by Surface Modification of Porous Silica. Chem. Lett. 2017, 46, 1233–1236. (21) Khan, M. N.; Orimoto, Y.; Ihara, H. Amphiphilic spherical nanoparticles with a nitrogen-enriched carbon-like surface by using -lactoglobulin as a template. Chem. Commun., 2018, Advance Article. (22) Lidström, P.; Tierney, J.; Wathey, B.; Westman, J. Microwave Assisted Organic Synthesis – A Review. Tetrahedron 2001, 57, 9225–9283. (23) Ray, B.; Mandal, B. M. Dispersion Polymerization of Acrylamide. Langmuir 1997, 13, 2191–2196.


24

Page 25 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(24) Yang, W.; Hu, J.; Tao, Z.; Li, L.; Wang, C.; Fu, S. Dispersion Copolymerization of Styrene and Glycidyl Methacrylate in Polar Solvents. Colloid Polym. Sci. 1999, 277, 446–451. (25) Uyar, T.; Hacaloglu, J.; Ishida, H. Synthesis, Characterization, and Thermal Properties of Alkyl-Functional Naphthoxazines. J. Appl. Polym. Sci. 2013, 127, 3114–3123. (26) Chen, S.; Xin, Y.; Zhou, Y.; Zhang, F.; Ma, Y.; Zhou, H.; Qi, L. Branched CNT@SnO2 Nanorods@carbon Hierarchical Heterostructures for Lithium Ion Batteries with High Reversibility and Rate Capability. J. Mater. Chem. A 2014, 2, 15582–15589. (27) Viculis, L. M.; Mack, J. J.; Mayer, O. M.; Hahn, H. T.; Kaner, R. B. Intercalation and Exfoliation Routes to Graphite Nanoplatelets. J. Mater. Chem. 2005, 15, 974– 978. (28) Lin, N.; Xu, T.; Li, T.; Han, Y.; Qian, Y. Controllable Self-Assembly of MicroNanostructured Si-Embedded Graphite/Graphene Composite Anode for HighPerformance Li-Ion Batteries. ACS Appl. Mater. Interfaces 2017, 9, 39318−39325. (29) Beaujuge, P. M.; Reynolds, J. R. Color Control in -Conjugated Organic Polymers for Use in Electrochromic Devices. Chem. Rev. 2010, 110, 268–320.


25


Page 26 of 38

(30) Liao, Y.; Li, X. G.; Kaner, R. B. Facile Synthesis of Water-Dispersible Conducting Polymer Nanospheres. ACS Nano 2010, 4, 5193–5202. (31) Ning, X.; Zhong, W.; Li, S.; Wan, L. A Novel Approach for The Synthesis of Monodispere Polypyrrole Nanospheres. Materials Letters 2014, 117, 294–297. (32) Dugan, F.; Temizkan, K.; Kaya, I. A Novel Shape-controlled Synthesis of Bifunctional Organic Polymeric Nanoparticles. Polymer 2015, 70, 59–67. (33) Yan, A.; Lau, B. W.; Weissman, B. S.; Kulaots, I.; Yang, N. Y. C.; Kane, A. B.; Hurt, R. H. Biocompatible, Hydrophilic, Supramolecular Carbon Nanoparticles for Cell Delivery. Adv. Mater. 2006, 18, 2373–2378.


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Table 1 Preparation conditionsa of polymer nanoparticles DHN Name

TA

Solvent (mL)

(mM (mM Ethano Water THF ) ) l 10 10 20 20 10 – 10

Yield

Particle diameterb

CVc

(%)

(nm)

(%)

D10/T10-W5T5 70 22 34 D20/T20-W5T5 83 83 22 D30/T30-W5T5 87 85 26 30 30 D30/T30-W7T3 14 – 6 46 110 64 D30/T30-W5E5 10 10 – 69 240 21 D30/T30-W3E7 30 30 6 14 – 69 210 13 D30/T30-W7E3 14 6 – 87 350 23 D30/T30-W10 30 30 20 – – 75 550 16 D30/T30-E10 30 30 – 20 – 87 690 18 a Reaction temperature: 150 °C, reaction time: 3 min. b Particle diameter was measured by DLS. (Solvent: ethanol) c Coefficient of variation (CV) was calculated as standard deviation divided by average particle diameter from DLS measurements.


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Table 2 Preparation conditionsa and characterization of polymer nanoparticles

Name

Particle Yiel CV diamete c Elemental analysis DHN TA Solvent (mL) d rb (mM (mM Wate Ethan TH C/ (%) (nm) (%) H% C% N% ) ) r ol F N

D10/T5010 50 – –

Monodisperse Surface Charge-Controlled Black Nanoparticles for

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