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Cite This: ACS Appl. Nano Mater. 2019, 2, 3597−3605
Monodisperse Surface-Charge-Controlled Black Nanoparticles for Near-Infrared Shielding Nanami Hano,† Makoto Takafuji,*,†,‡ Hiroki Noguchi,† and 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
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ABSTRACT: A simple and quick preparation of black polymer nanoparticles by the microwave-assisted polymerization of 1,5dihydroxynaphthalene and 1,3,5-trimethyl-1,3,5-triazinane 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. Reflection in the NIR region from 1250 to 2150 nm, however, remained more than 50% 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. KEYWORDS: microwave synthesis, π-conjugated polymer, dispersion polymerization, amphiphilic surface, selective reflectance of NIR, heat-shielding materials, heat-insulating materials
1. INTRODUCTION Recently, heat shielding and heat insulation have become important issues for energy efficiency and global warming. Various heat-shielding materials have been developed for suppressing the internal temperature increase of buildings and automobiles caused by sunlight.1−3 Because approximately one-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 © 2019 American Chemical Society
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 IR-reflective pigment is characterized by its reflectivity and absorptivity; therefore, it is possible to control the color of the pigments by changing the Received: March 26, 2019 Accepted: April 29, 2019 Published: April 29, 2019 3597
DOI: 10.1021/acsanm.9b00555 ACS Appl. Nano Mater. 2019, 2, 3597−3605
Article
ACS Applied Nano Materials Scheme 1. Schematic Illustration of the Preparation Procedure for the Black Nanoparticles
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, washed with ethanol several times, and dried under vacuum. A schematic of the preparation is shown in Scheme 1. 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), field-emission 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 (UV−vis−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 light-emitting diode (LED; LDL2-74X30SW2, λ1 = 467 nm, λ2 = 634 nm, CCS Inc., Japan) and an infrared (IR) LED (LDL74X27IR1200, λ = 1200 nm, CCS Inc., Japan).
mixing ratio of the raw materials. Inorganic materials such as titania, iron oxide, zinc oxide, 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 heatshielding materials.17,18 Perylene derivatives and an azo pigment having an azomethine group have been reported as organic heat-shielding materials, both of which contain πconjugated 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 1,5dihydroxynaphthalene (DHN) and 1,3,5-trimethyl-1,3,5-triazinane (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 monodisperse, 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 π-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 sub-micrometer 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 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.
3. RESULTS AND DISCUSSION 3.1. Preparation of Monodisperse Polymer Nanoparticles. For the initial experiment, 20 mL of each ethanol solution of DHN (30 mM) and TA (30 mM) was placed in the high-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/T30E10) was isolated from the reaction. The SEM image revealed that monodispersed spherical particles with a diameter of less than 1 μm 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 being stirred 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
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 Scheme 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 3598
DOI: 10.1021/acsanm.9b00555 ACS Appl. Nano Mater. 2019, 2, 3597−3605
Article
ACS Applied Nano Materials Table 1. Preparation Conditionsa of Polymer Nanoparticles solvent (mL) name
DHN (mM)
TA (mM)
water
D10/T10-W5T5 D20/T20-W5T5 D30/T30-W5T5 D30/T30-W3T7 D30/T30-W7T3 D30/T30-W5E5 D30/T30-W3E7 D30/T30-W7E3 D30/T30-W10 D30/T30-E10
10 20 30 30 30 30 30 30 30 30
10 20 30 30 30 30 30 30 30 30
10 10 10 6 14 10 6 14 20
ethanol
10 14 6 20
THF
yield (%)
particle diameterb (nm)
CVc (%)
10 10 10 14 6
70 83 87
