Super Black Material from Low-Density Carbon Aerogels with

Super Black Material from Low-Density Carbon Aerogels with Subwavelength Structures Wei Sun,†,‡,§ Ai Du,*,†,‡,§ Yu Feng,† Jun Shen,†,‡ Shangming Huang,†,‡ Jun Tang,†,‡ and Bin Zhou*,†,‡ †

School of Physics Science and Engineering and ‡Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, Tongji University, Shanghai 200092, China S Supporting Information *

ABSTRACT: Many scientists have devoted themselves to the study of the interaction between subwavelength structures and electromagnetic waves. These structures are commonly composed of regular arrays of subwavelength protuberances, which can be artificially designed. However, extending from 2D periodic patterns to 3D disordered subwavelength structures has not been studied yet. In this study, we studied the total diffuse reflectivity of carbon aerogels with various 3D networks of randomly oriented particle-like nanostructures by using normally incident visible light (430−675 nm). We observed that the different 3D network nanostructures of carbon aerogels, especially for the structures with the minimum size, reduced the reflectivity effectively. It was found that the key mechanism for the subwavelength-structure-induced ultralow reflectivity property is due to the decrease of the amplitude of electron vibration forced by the electromagnetic wave, which provides a simple method for designing perfect black materials. KEYWORDS: carbon aerogels, subwavelength, electron−microstructure interaction, low reflectivity films have been used as ultralow black materials.18−20 Recent theoretical calculations suggested that an extremely low index of refraction (n = 1.01−1.10) can be reached by using a lowdensity nanotube array structure,21 and Yang et al. proved the extremely low reflectance (0.045%) of aligned nanotube arrays at four specific wavelengths.22 Among various carbon-based materials, carbon aerogels (CAs), from our observations, seem to be the best candidate medium for studying the interactions between disordered subwavelength structures and light. First of all, CAs with random structures exhibit angle-independent reflectivity.23,24 What is more important, the size of the most of pores and skeletons in CAs (normally smaller than 50 nm) is much smaller than the wavelength of visible light, making the CAs a suitable medium for exploring the interaction between subwavelength structures and light. Additionally, the preparation process of CAs has been well studied ever since CAs were first developed by Pekala,25,26 and nowadays it is easy to obtain specific hierarchical porous nanostructures by changing the concentrations of the reactants and catalysts in precursor solutions.27,28 In this paper, we reported the visible light reflectance properties of CAs and compare the reflectance of different porous-type CAs. The CAs possessed a low diffuse reflectivity

A

perfect black material absorbs all the light that hits the surface, without any reflection or transmission. Since the introduction of a “black body” by Kirchhoff and the law of black-body radiation by Plank, the concept of a perfect absorber has been proposed over one century.1,2 The highabsorption property of black materials makes them valuable for many applications, ranging from solar energy conversion,3 optical instruments,4 thermal detectors,5 to pyroelectric sensors.6 Subwavelength structures, which are commonly composed of artificial periodic structures, have raised a great deal of attention because of their effectiveness in reducing reflectance.7−9 Lalanne et al. etched subwavelength surfaces on silicon wafers and found antireflection characteristics for visible light.10 Hao et al. showed that the highest absorption of the subwavelength metallic nanostructures reached to ∼88% at the wavelength of 1.58 μm.11 As it is well-known, the unique properties of low reflectivity or high absorbance of these materials are derived from the artificial periodic structures rather than directly from the materials’ natural structures. Fan et al. obtained double negative materials with three-dimensional (3D) disordered metallic ring networks12−14 instead of with arrays of periodic building blocks.15,16 Following Fan’s idea, it is interesting to study the possibility of realizing ultralow reflectivity from disordered subwavelength structures. Carbon is a good absorber due to the π-band’s optical transitions and nearly flat reflection response.17 Therefore, many kinds of carbon materials including carbon nanotubes and carbonous © 2016 American Chemical Society

Received: March 24, 2016 Accepted: September 2, 2016 Published: September 2, 2016 9123

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shown in Figure 1c, the reflectivity of the sample from left to right decreased sharply. The reflectivity of the graphite was obviously higher than that of the other samples (CAs). The middle two samples (CA-800-15 and CA-800-5), which had similar skeleton sizes (the same R/C) but different densities, showed different reflectance. Also, the right-two samples (CA800-5 and CA-300-5), which had similar densities but different skeleton sizes, exhibited different reflectance, which means the subwavelength structure could strongly affect the reflection behavior. Because the mirror reflectance of many of the CA samples was below the limit of the spectrophotometer, diffuse reflectance measured by an integrating sphere was used to evaluate the reflection performance. A schematic of the measuring principle was shown in Figure S1. The reflectance with wavelengths λ of 430−650 nm is shown in Figure 1d. All samples exhibited weak λ dependence. The measurement results were consistent with the phenomena observed in Figure 1c. The variation in reflectance among CAs was unlikely derived from differences in chemical composition. As shown in Figure S3, the positions of adsorption peaks for RF aerogels or CAs with different W% and R/C were consistent, which indicated that different W% and R/C do not influence their chemical compositions. The type of carbon in CAs was analyzed according to the Raman spectra (Figure S4). The main peaks in the spectra included a G band at 1588 cm−1 and a D1 band at 1335 cm−1. The D1/G band intensity (peak area) ratios ID1/IG was about 3, indicating that all the CAs were composed of the amorphous carbon since the amount of sp2 bonding was much higher than that of sp3 bonding.31 Thus, not the chemical composition but the subwavelength structure and density mainly affect the reflection behavior of CAs. The influences of density and subwavelength structure (R/ C) on the reflectance are shown in Figures 2a and 3a, respectively. With the increase of the density or W%, the reflectance of the samples increased sharply from 0.19% to 0.48%, as shown in Figure 2a. These results were similar to those of the other three groups (Figure S2a). As we know, the key to suppressing light reflection was to reduce the refractive index difference between the material and air.20,32 Lower densities would lead to a higher concentration of the air, resulting in a lower refractive index and reflectance. Furthermore, the increase in dilution (low W%) increased the distance between the sol particles,26 which increased the size and volume of the pores. This also resulted in the increase of penetration of the incident light. To gain deeper insights into the origin of CAs black-body behavior, we compared the reflectance of CA blocks with that of the roll-pressed CA sheets of the same W% and R/C. The reflectance spectra in Figure S6 showed that the average reflectance of the pressed CA-300-5 sheet was approximately 0.52%, almost three times higher than that of the CA-300-5 block (0.19%). The reflectance of the CA300-15 block increased from 0.49% to 0.61% after being pressed. The density of CA-300-5 increased from 54 to 164 mg cm−3, while the density of CA-300-15 increased slightly after pressing. Since the pressing would not affect the subwavelength structure, the reflectance increase might be due to the density increase. Actually, considering the reflectance spectra of CAs with the same W% but different R/C in Figures 2a or S2, the reflectance did not monotonically increase with the density. The lowest reflectance of all samples with the same R/C was obtained at W % of 5% instead of 4%, although the samples had similar densities. Thus, the reflectance of CAs may strongly rely on

of 0.19% across a wide spectral range from 430 to 650 nm. Specifically, from optical studies, we found that the black-body behavior of CAs was associated primarily with their minimal subwavelength structures, which can strongly affect the forced vibration of the electrons in CAs skeletons under the irradiation of the electromagnetic wave. The CAs in this study were derived from pyrolysis of a resorcinol-formaldehyde (RF) aerogels by the sol−gel polycondensation of resorcinol with formaldehyde under alkaline conditions.25,27,28 Here, the microstructure of the CAs was controlled by adjusting the amount of resorcinol and formaldehyde in precursor solution (W%) and the molar ratio of resorcinol-to-catalyst (R/C), which mainly determine the density and skeleton size, respectively.29,30 High W% results in a denser formation of the resorcinol-formaldehyde crosslinked clusters leading to higher density of the resultant CAs. Moreover, low R/C ratios results in small polymer particles that are interconnected with large necks (giving the CA a fibrous appearance). The samples hereafter were donated as “CA-R/CW%.”

RESULTS AND DISCUSSION As shown in Figure 1a,b, the CA-300-5 carbon aerogel was very black with and without flash-light illumination. To directly show the different reflection performances, a line laser was used to illuminate samples with different densities and structures. As

Figure 1. Photographs of the CA-300-5 sample taken (a) without and (b) with flash-light illumination. (c) Photograph of graphite, CA-800-15, CA-800-5, and CA-300-5 taken under line laser illumination. The measured samples had different densities and nanostructures. (d) Reflectance spectra of the samples shown in (c). The average reflectance of graphite, CA-800-15, CA-800-5, and CA-300-5 is 9.0%, 1.1%, 0.37%, and 0.19%, respectively. 9124

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Figure 2. (a) Reflectance spectra of CAs with the same R/C = 300 but different W%. The reflectance of the samples in generally increased sharply when W% increased. The average reflectance of these samples were 0.31% (W% = 4%), 0.43% (W% = 10%), 0.48% (W% = 15%), significantly higher than 0.19% (W% = 5%). (b) Plot of reflectance as a function of the density of all the samples. There was a roughly positive correlation between reflectance and density (42−328 mg cm−3). (c−f) Scanning electron micrographs and transmission electron micrographs (the scale bar is 50 nm) (inset) of CA-300-4, CA-300-5, CA-300-10 and CA-300-15, respectively. High W% resulted in the densification of the carbon skeleton particles.

300-4 and CA-200-5), the content of micropores was decreased, and the reflectance was the reverse. It was found that the more micropores a sample contained, the lower the reflectance was. It seems difficult to understand why aerogels composed of more micropores (subwavelength scale) could obviously decrease the reflection of visible light. Here, we describe an indirect interaction mechanism that includes electromagnetic wave−electron interaction and electron−microstructure interaction. As described in classical electrodynamics, electrons in a conductor will vibrate according to the alternating electric-field component of an electromagnetic wave.33 The vibration amplitude, Afree, of electrons in the free state is determined by the incident light and the resistance of the conductor, which is given as

their subwavelength structure as well as on their density. Figure 3a showed the reflectance spectra of samples prepared using different R/C values with a fixed W% of 5%. With increasing concentration of basic catalyst, the pH of the gel solution increased, leading to the formation of smaller CA particles. This in turn led to the formation of a uniform porous nanostructured network. The lowest-reflectance CA was CA-300-5, whose reflectance was only 0.19%, approaching the measuring limits of the integration sphere. From the reflectance results, a roughly positive correlation was found between reflectance and W% or R/C, indicating that a lower density and smaller skeleton size results in lower reflectivity. However, reflectance did not strictly follow the positive correlation. Though CA-300-4 and CA-200-5 had smaller skeleton nanoparticles (as shown in the transmission electron micrographs in Figures 2 and 3), the minimal reflectance was found in CA-300-5. We therefore used nitrogen adsorption−desorption isotherms to analyze the pore properties of these samples. The corresponding pore-size distributions were shown in Figure 4. The pore-size distribution curves for the three samples indicated that the samples contained many micropores with diameters of 1−2 nm located within the skeleton nanoparticles. As shown in Figure 4a, the adsorption peak of the CA-300-5 pore volume was higher than that of CA300-4, indicating that CA-300-5 contains many more micropores. This result was also observed in Figure 4b, which similarly indicated that the number of micropores in CA-200-5 was less than that in CA-300-5. CA-300-5 had a much higher Smicro value (940 m2 g−1) than CA-300-4 and CA-200-5. Furthermore, with decrease of skeleton nanoparticle sizes (CA-

A free =

eE0

(

Ne 2

m ω 2 + iω mσ

)

where e and m are the electric quantity and mass of the electron, respectively, E0 is the electric-field amplitude of incident light, ω is the angular frequency of incident light, N is the number density of the electrons, and σ is the electrical conductivity. Normally the mean free path of electrons in the conductor is approximately 10−100 nm, which is much larger than its amplitude (about several nanometers). Thus, the collision of the electrons cannot affect the vibration. However, in the current study, there were many micropores located within the carbon skeleton particles, which were estimated by liquid N2 9125

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Figure 3. (a) Measured reflectance spectra of the CAs synthesized at a fixed W% = 5%. The reflectance of these samples tended to decrease with increasing wavelength and 0.24% (R/C = 200), 0.26% (R/C = 500), 0.38% (R/C = 800), values that are obviously higher than the 0.19% for R/C = 300. (b) Plot of reflectance as a function of the R/C of all samples: (■)W% = 4%, (●)W% = 5%, (▲)W% = 10%, (▼)W% = 15%. (c−f) Scanning electron micrographs and transmission electron micrographs (the scale bar is 50 nm) of CA-200-5, CA-300-5, CA-500-5, and CA-800-5, respectively. With increasing R/C from 200 to 800, the size of the CA nanoparticles increased, and the basic network of CAs changed from loose to dense. When the R/C ratio was 200, small particles became interconnected with large necks, giving the CAs a fibrous appearance.

Figure 4. Pore-size distribution of (a) CA-300-4 and CA-300-5 and (b) CA-200-5 and CA-300-5. The pore-size distribution curves for the three samples indicate that the samples contained a large number of micropores with diameters of 1−2 nm. In comparison to CA-300-4 and CA-200-5, CA-300-5 showed that much higher adsorption peak of pore volume line in 1−2 nm, and the micropore specific surface area (Smicro) of CA-300-5 was 940 m2 g−1, which was much larger than that of CA-300-4 and CA-200-5.

smaller than Afree, the collisions between electrons strongly affect the vibration, making the vibration of one electron is confined between successive collisions with other electrons. The sharp decrease of the amplitude of the electrons will lead to the decrease of the wavelet amplitude induced by the electron vibration and ultimately to the decrease of the reflection. In other worlds, when E0 is a constant, the decrease of the amplitude will lead to a decrease of the refractive index, n. According to the Fresnel formula, the reflectivity of the conductor will decrease. This theory could explain why CA-300-5, CA-300-4, and CA200-5 exhibited similar densities but different reflectance values

isotherms to be in the range of 0−2 nm. Considering the electrons near the interface of the micropores (the distance less than the amplitude), the vibration is limited by the interface of micropores, and the amplitude decreases. Moreover, when the structure is smaller than the mean free path of the electrons, the microstructure will force the electrons to collide with others in the tiny carbon structure which is divided by the micropores, decreasing the mean free path (Figure 5). The smaller the structure is, the stronger the confinement and thus the shorter the mean free path. The restricted movement of the electrons decreases the mean free path to its 1−2 original orders of magnitude. When the confined mean free path, λconfined is much 9126

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from 200 to 800, and the amount of resorcinol and formaldehyde (W %) in precursor solution was controlled at 4%, 5%, 10% and 15%, respectively. In a typical synthesis, 4.5 g of resorcinol was dissolved in 128 mL deionized water, and 6.15 g of formaldehyde was added. The solution was stirred for an additional 5 min, and then 0.0144 g of sodium carbonate was added. After further stirring, the precursor solution was poured into glass molds, sealed, and cured in an oven at 85 °C for 3 d. The obtained hydrogel was immersed in acetone for 72 h to completely remove residual water and solvents. Then, the wet gel was subsequently dried using supercritical CO2 and pyrolyzed at 1000 °C under N2 atmosphere for 3 h. The carbon aerogel was isolated as black cylindrical monoliths. The samples were donated as “CA-R/C-W %.” Activation of Carbon Aerogels with CO2. CAs were heated in tubular furnace as pyrolysis from the room temperature to 1000 °C under N2 atmosphere with the flow rate of 200 mL min−1, retained at 1000 °C for 2 h under CO2 flow of 75 mL min−1, and then cooled down to the room temperature under N2 flow of 200 mL min−1. The obtained CO2 activated samples were donated as “ACA-R/C-W%.” Reflectance Measurement. Optical reflectance of CAs was measured under normal incident light by a UV−vis/NIR spectrometer (V-570, Japan) in the wavelength range of 430−650 nm. Spot size of the incident beam is about 5 × 10 mm. Figure S1 illustrates the schematic of the reflectance measurement setup. The sample was set at the back of the integrating sphere, and the light was incident normal to the sample with grating monochromator in spectrometer. The reflected light was totally collected by the integrating sphere and detected by the photomultiplier tube. The samples were processed into black cylindrical monoliths for reflectance measurement; the diameter and height of the samples were 20 mm and 20 mm, respectively. Since the thickness is about 10 mm, the transmittance could be ignored. Thus, the lower the reflectance is, the higher the absorption is. Characterizations. The morphology of the sample was characterized by a scanning electron microscope (XL30FEG, Netherland) and a transmission electron microscope (JEOL-1230). Raman spectra (Jobin-Yvon HR800) were recorded from 1000 to 2000 cm−1 using a 514 nm argon ion laser (source power 17 mW). Organic groups were investigated by a Fourier transform infrared spectroscope (TENSOR27, Germany). The electrical conductivity at room temperature was measured using a two-probe method. The pore size distribution and specific surface area were measured by a N2 adsorption analyzer (TriStar 3000, USA) using the BET nitrogen adsorption/desorption technique.

Figure 5. Schematic diagram of CA reflection.

attributed to the micropores (with sizes smaller than 2 nm). In order to further demonstrate our specular, we compared the reflectance of CA-800-15 with that of CA-800-15 activated with CO2 (ACA-800-15). The reflection spectra curve for CA-80015 sharply shifted to low values (Figure S7), and its higher average reflectance value of 1.1% decreased to 0.24% after activation. These experimental results proved that increasing the number of micropores can effectively decrease the reflectance of CAs. Therefore, fundamentally, a microporous structure (subwavelength scale) is essential for the CAs to show super low reflection behavior. Interestingly, as shown in the Figure S8, the electrical conductivity is irrelevant to the microstructure or the mean free path of electrons, but only scaled with the density, which agrees well with Lu’s results.33 This is because the collisions between the electrons only affect the electromagnetic loss, but do not affect the conductivity. As demonstrated in Drude model, the conductivity is mainly derived from electron−phonon interactions.34

CONCLUSION In summary, we have demonstrated that hierarchical porous CAs that contain randomly abundant subwavelength structures (micropores) can dramatically change their optical reflection. When the electrons of CAs couple with the electromagnetic resonance of incident light and forcedly vibrate, the interface composed of these three-dimensional disordered micropores will force the electrons to vibrate in a narrow space instead of allowing them freedom of movement. Moreover, the value of the forced vibration amplitude (A) effectively decreases with the increase in the collisions of electrons in the narrow space. As described by Fresnel’s law, a decrease in A results in a decrease of the refractive index and thus a drop in reflectance. Hence, by tuning the nanostructure of CAs, we obtained a minimum reflectance of approximately 0.19%, which approximated to the limit of the test equipment. Inspired by our work, similar or even darker materials could be fabricated by inducing subwavelength structures in the materials.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b02039. Schematic illustration of setup for measuring reflectance, additional reflectance spectra, FTIR spectra, Raman, and nitrogen adsorption−desorption isotherm and electrical conductivity for CAs samples are given in Figures S1−S8 (PDF)

AUTHOR INFORMATION

EXPERIMENTAL SECTION

Corresponding Authors

All reagents, including resorcinol (99%), formaldehyde (37% in water), alkaline sodium carbonate, and acetone, were purchased from Shanghai Chemical Reagent Company with their purity being of analytical grade and used as received. Preparation Procedures for CAs. The CAs in this study were derived from pyrolysis of resorcinol-formaldehyde aerogels, and the synthesis procedures have been reported in the previous articles. The molar ratio of resorcinol-to-formaldehyde (R/F) was fixed at 0.5 with deionized water as solvent and alkaline sodium carbonate (C) as base catalyst. The molar ratio of resorcinol-to-catalyst (R/C) was adjusted

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

§ W.S. and A.D. contributed equally to this work. A.D. initiated the concepts. W.S., B.Z., and A.D. designed the experiments. W.S., S.H., J.S., and J.T. conducted the experiments. Y.F. and A.D. performed the theoretical derivation. W.S. and A.D. analyzed the data and wrote the manuscript.

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The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors thank Liping Zou for Raman spectroscopy measurements and Fei Zhou for the language assistance. W.S., A.D., B.Z., and S.H. acknowledge financial support from a China National Natural Science Foundation grant number 51172163 and a National High Technology Research and Development of China (2013AA031801). J.S. acknowledges support from Science and Technology Innovation Fund of Shanghai Aerospace, China (SAST201321). REFERENCES (1) Robitaille, P.-M. On the Validity of Kirchhoff’s Law of Thermal Emission. IEEE Trans. Plasma Sci. 2003, 31, 1263−1267. (2) Gearhart, C. Black-Body Radiation. In: Compendium of Quantum Physics; Greenberger, D., Hentschel, K., Weinert, F., Eds.; Springer: Berlin and Heidelberg, 2009; pp 39−42. (3) Yoon, J.; Baca, A. J.; Park, S.-I.; Elvikis, P.; Geddes, J. B.; Li, L.; Kim, R. H.; Xiao, J.; Wang, S.; Kim, T.-H.; Motala, M. J.; Ahn, B. Y.; Duoss, E. B.; Lewis, J. A.; Nuzzo, P. G.; Ferreira, P. M.; Huang, Y.; Rockett, A.; Rogers, J. A. Ultrathin Silicon Solar Microcells for Semitransparent, Mechanically Flexible and Microconcentrator Module Designs. Nat. Mater. 2008, 7, 907−915. (4) Ibn-Elhaj, M.; Schadt, M. Optical Polymer Thin Films with Isotropic and Anisotropic Nano-Corrugated Surface Topologies. Nature 2001, 410, 796−799. (5) Fox, N. P. Trap Detectors and their Properties. Metrologia 1991, 28, 197−202. (6) Theocharous, E.; Deshpande, R.; Dillon, A. C.; Lehman, J. Evaluation of a Pyroelectric Detector with a Carbon Multiwalled Nanotube Black Coating in the Infrared. Appl. Opt. 2006, 45, 1093− 1097. (7) Wilson, S. J.; Hutley, M. C. The Optical Properties of ’Moth Eye’ Antireflection Surfaces. Opt. Acta 1982, 29, 993−1009. (8) Kanamori, Y.; Sasaki, M.; Hane, K. Broadband Antireflection Gratings Fabricated upon Silicon Substrates. Opt. Lett. 1999, 24, 1422−1424. (9) Yu, Z.; Gao, H.; Wu, W.; Ge, H.; Chou, S. T. Fabrication of Large Area Subwavelength Antireflection Structures on Si Using Trilayer Resist Nanoimprint Lithography and Liftoff. J. Vac. Sci. Technol., B: Microelectron. Process. Phenom. 2003, 21, 2874−2877. (10) Lalanne, P.; Morris, G. M. Antireflection Behavior of Silicon Subwavelength Periodic Structures for Visible Light. Nanotechnology 1997, 8, 53−56. (11) Hao, J.; Wang, J.; Liu, X.; Padilla, W. J.; Zhou, L.; Qiu, M. High Performance Optical Absorber Based on a Plasmonic Metamaterial. Appl. Phys. Lett. 2010, 96, 251104. (12) Shi, Z.; Fan, R.; Zhang, Z.; Qian, L.; Gao, M.; Zhang, M.; Zheng, L.; Zhang, X.; Yin, L. Random Composites of Nickel Networks Supported by Porous Alumina Toward Double Negative Materials. Adv. Mater. 2012, 24, 2349−2352. (13) Shi, Z.; Fan, R.; Yan, K.; Sun, K.; Zhang, M.; Wang, C.; Liu, X.; Zhang, X. Preparation of Iron Networks Hosted in Porous Alumina with Tunable Negative Permittivity and Permeability. Adv. Funct. Mater. 2013, 23, 4123−4132. (14) Yan, K.; Fan, R.; Shi, Z.; Chen, M.; Qian, L.; Wei, Y.; Sun, K.; Li, J. Negative Permittivity Behavior and Magnetic Performance of Perovskite La1−xSrxMnO3 at High-Frequency. J. Mater. Chem. C 2014, 2, 1028−1033. (15) Smith, D. R.; Padilla, W. J.; Vier, D. C.; Nemat-Nasser, S. C.; Schultz, S. Composite Medium with Simultaneously Negative Permeability and Permittivity. Phys. Rev. Lett. 2000, 84, 4184. (16) Fu, Y. H.; Liu, A. Q.; Zhu, W. M.; Zhang, X. M.; Tsai, D. P.; Zhang, J. B.; Mei, T.; Tao, J. F.; Guo, H. C.; Zhang, X. H.; Teng, J. H.; Zheludev, N. I.; Lo, G. Q.; Kwong, D. L. A Micromachined 9128

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