Enhanced Photothermal Effect in Ultra-Low-Density Carbon Aerogels

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Functional Nanostructured Materials (including low-D carbon)

Enhanced Photothermal Effect in Ultra-Low-Density Carbon Aerogels with Microporous Structures for Facile Optical Ignition Applications Fan Yang, Yingjuan Zhang, Xi Yang, Minglong Zhong, Zao Yi, Xichuan Liu, Xiaoli Kang, Jiangshan Luo, Jia Li, Chao-Yang Wang, Hai-Bo Zhao, Zhi-Bing Fu, and Yong-Jian Tang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17803 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019

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ACS Applied Materials & Interfaces

Enhanced Photothermal Effect in Ultra-Low-Density Carbon Aerogels with Microporous Structures for Facile Optical Ignition Applications Fan Yang,1,2 Yingjuan Zhang,1 Xi Yang,1 Minglong Zhong,1,2 Zao Yi,3 Xichuan Liu,2 Xiaoli Kang,1 Jiangshan Luo,1 Jia Li,2 Chaoyang Wang,1 Haibo Zhao,1 Zhibing Fu,1,* and Yongjian Tang2,*

1Research

Center of Laser Fusion, China Academy of Engineering Physics, Mianyang

621900, China.

2Institute

3Joint

of Modern Physics, Fudan University, Shanghai, 200082 China.

Laboratory for Extreme Conditions Matter Properties, Southwest University of

Science and Technology, Mianyang 621900, China.

ABSTRACT

ACS Paragon Plus Environment

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The exact mechanism responsible for the phenomenon known as photoignition with enhanced photothermal effect in a high-surface-area carbon with the addition of a metal catalyst is an open issue. Here, we report the first successful flash ignition of a pure carbon material in ambient air-microporous carbon aerogels (CAs) with ultra-low density and high surface area. Under flash exposure, CAs show strong local heat confinement effect near microporous structures (0.6-2 nm), and the graphite crystallite structures existed in single carbon nanoparticle (~15 nm) are damaged. The local heat confinement effects are mainly derived from the low gaseous thermal conductivity in micropores and low solid thermal conductivity in low-density CAs. In addition, the limiting effects of the microporous structure on the vibration amplitude of free-state electrons in low-density CAs result in dramatic increase in optical absorption. Numerical simulations of unsteady temperature fields of CAs with different densities and thicknesses are also performed, and the calculated maximum temperature of a 17 μm thick 20 mg/cm3 CA bed is 1782 ℃. CAs with higher density can also give rise to enhanced photothermal response and ignition with the addition of metal Fe nanoparticles. The metal catalyst increases both the light absorption capacity in the visible-light range and the heat


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accumulation capacity. These results are important for understanding the mechanism of flash ignition, especially the local high temperature and effects of metal catalyst in carbon materials during photothermal process.

KEYWORDS:

Carbon

aerogels,

Microporous

structures,

Photothermal

effect,

Photoignition, Mechanism

The ignition in single-walled carbon nanotubes (SWNTs) after exposure to a conventional photographic flash was first discovered in 2002,1 and the phenomenon known as photoignition has been confirmed in other high-surface-area carbons that bear a well-dispersed metal catalyst.2-4 Over the past decade, enhanced photothermal responses

have

been

realized

with

formulations

containing

graphene

oxide

nanosheets,5 silicon nanowires/films/particles,6-8 polyaniline nanofibers,9 and nanosized aluminum particles,10-13 and these materials have been widely used for optical ignition, nanomaterial welding, nanoreactors, solar/photo energy conversion materials and other applications. However, the effect of the metal catalyst and the exact mechanism of flash ignition in carbon-based materials are still not understood. Carbon-


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based photothermal materials, especially carbon nanotubes, stand out as optical ignition and initial materials due to their broadband absorption and excellent chemical stability.14-17 Several studies have shown that the catalytic particles (e.g., Fe/Pd) in SWNTs play a favorable even an indispensable role in the ignition process,3, 4 and the key to successful flash ignition is the addition of metal catalysts (13.8-30 wt% Fe).18 Scott et al. concluded that SWNTs themselves play little role in the ignition process, instead it is the iron nanoparticle catalyst used to grow them, along with oxygen, that supports combustion.19 Bockrath et al. hypothesized that it arises from photophysical effects associated with metal and carbon in chemical contact. The common features of carbon-based flash ignition materials are the combination of well-dispersed metal catalyst in intimate contact with a high-surface-area carbon.2, 20 Nevertheless, evidence indicates that the enhanced photothermal effect is derived from the nature and density of carbon, and the metal catalyst is irrelevant because the structural reconstruction of SWNTs occurs throughout the sample not just near the metal catalyst particles.1 However, to the best of our knowledge, no pure carbon materials can be flash ignited. Most of the heat produced during the enhanced photothermal process is dissipated without the


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addition of metal catalyst. Moreover, the mechanism responsible for the local high temperature and structural reformations in the enhanced photothermal process is still lacking. Carbon aerogel (CA) materials are highly porous, pure carbon materials with characteristics including low density, rich porosity, high conductivity, and extreme environmental stabilities, which allow wide applications such as environmental protections, energy storages, sensors, and catalysts etc.21-26 CAs have a network structure of interconnected, nanoscale primary particles with characteristics including utra-low density and rich microporous structure, which can be perfect light absorbers.27-29 The smaller pores and skeleton size of CAs (normally smaller than 50 nm) make them a suitable medium to explore photothermal effects between subwavelength structures and light.30 Recently, several studies have shown that as super black materials, lowdensity CAs exhibit extremely low broadband reflectivity and high absorption to incident light with wavelengths in the range of 400-2000 nm. Additionally, CAs with a specific porous structure, surface area, and density can be easily obtained by changing the concentrations of the reactants and catalysts in precursor solutions.31 Ultra low density down to 0.16 mg/cm3 and porosities up to about 99% can be achieved.21 These properties make CAs as ideal thermal insulation materials.32-35 The special structure of low-density CAs provides two advantages as a possible optical ignition material: first, the high porosity and high surface-to-volume ratio in low-density CAs provide the maximum surface area for energy absorption; second, the interconnected nanoparticles provide enough thermally conductive pathways through which absorbed energy can diffuse are reduced.36-42 The optical ignition and initial method is a novel and promising ignition method with extensive applications in energetic materials.43-49 Compared with that of conventional ignition methods, especially laser


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ignition, the ignition energy required for optical flash ignition is approximately two orders of magnitude lower. Ignition can easily be achieved on a large surface area which is immune to varying interferences and surrounding environmental conditions.50 Here, we report for the first time that low-density and high-surface-area CAs with microporous structures can be flash ignited in ambient air without any metal catalyst addition. Numerical simulations of unsteady temperature fields of CAs with different densities and porosities have been performed to support the experimental observations. The temperature rise is higher for lower density CAs because the high porosity reduces both the heat capacity and thermal conductivity. The unique porous structure, low density and high surface area of CAs are the primary factors that give rise to photothermal response and ignition. These findings have implications for applications of CAs as nanoreactors and optical ignition material for their local high temperature by exposing the mixture of CAs and initial materials to a bright flash light. Results and Discussion Morphology and Microstructure Analyses. The synthesis of cross-linked RF polymers to CAs is illustrated in Figure 1A. CAs without any metal catalyst can be obtained from


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the carbonization of resorcinol (R) and formaldehyde (F) gels, which are prepared by a sol–gel polycondensation process. The lowest density CAs (approximately 20 mg/cm3) can be obtained with a 1% RF ration. Four different density CAs with different pore structures and surface areas were synthesized: 20, 50, 100, and 200 mg/cm3. The surface morphology and textures, elemental distribution, and porous attributes of the CAs were characterized by scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS), transmission electron microscopy (TEM) and nitrogen sorption tests. The CAs with the lowest density of 20 mg/cm3 have a honeycomb-like three dimensional network of interconnected carbon nanoparticles with diameters of 1015 nm (Figure 1B, C). The high-resolution TEM image of the individual carbon nanoparticles indicated that they are mainly composed of graphite sheets (dspacing=0.34 nm) and microporous structures (Figure 1D). Based on some of HRTEM (Figure S1), there are also some individual graphene nanosheets existed. The quasi-spherical pure carbon beads were highly porous with a Brunaur-Emmett-Teller (BET) surface area as high as 1784 m2/g. The other CAs with a higher density have a similar three dimensional network structure as shown in Figure S1. The absorption peak of the pore


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volume for the 20 mg/cm3 CA is mainly in the range of 0.6-2 nm (Figure 1E). When the CA density increases to 200 mg/cm3, the BET surface area decreases by nearly half, and a large number of micropores (0-2 nm) disappear. The micropore volume and micropore surface area decrease to 0.348 cm3/g and 980 m2/g, respectively (Figure 1E). The nitrogen sorption isotherms and pore size distribution of CAs with densities of 50 and 100 mg/cm3 are shown in Figure S2. As the density increases, the BET surface area and micropore volume greatly decrease. The BET surface areas and pore structures of the CAs prepared at different densities are listed in Table S1. The porosity

P of the CAs is defined as the volume fraction of air in the pile and is evaluated by the following equation:

p  1

CAs c

(1) where ρCAs is the density of the CA and ρc is the density of the carbon skeleton (~2 g/cm3).


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XPS

was

employed

to

characterize

the

elemental

composition

and

the

chemical/electronic state of each element in the surface of the CAs with different densities, as shown in Figure S3. All CAs exhibit two main groups of spectral peaks in the XPS spectra, corresponding to C1s and O1s. The atomic percent of oxygen contents present in the CAs are estimated by the area ratio of the oxygen and carbon peaks. Analysis of C1s spectra of the CAs was carried out to evaluate the electronic state and chemical activity of carbon. The C1s spectra for the CAs are deconvoluted into four components. The contribution at around 284.6 eV can be ascribed to the presence of C-C bonds in graphitic carbon. A peak at around 285.3 eV is related to the presence of sp3-like defects in the main graphitic structure of CAs. The peak at around 286.3 eV can be related to single C-O bonds, and that at around 288.7 eV is assigned to double C=O bonds. Facial Optical Ignition and Crystal Structure Damage. The experimental setup for flash ignition of CA powders is schematically illustrated in Figure 1F. Specifically, CA powders were placed on top of a 1 mm thick glass slide located less than 2 cm above the camera flash in air (Figure 1G). The dimension of the CA samples was


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approximately 1 cm, which is much smaller than the length of the camera flash (~7.5 cm), so that the incident light energy on the samples can be assumed to be the same. The camera flash was a Xe flash with wavelength range from 200-2000 nm. The spectrum analysis shows that the camera flash energy is mainly distributed between 400-2000 nm. It can be simplified as a 450 nm (where the Xe flash intensity peaks) single wavelength light source with duration of 1.1 ms (Figure S4). The output energy and pulse duration were adjustable, and the maximum pulse energy was 2.1 J/cm2, as shown in Table S2. In fact, the area energy density on the samples varied with the distance between the Xe flash and samples (Figure S4). For experimental consistency, all CA samples were placed 1 mm above the camera flash for the optical ignition experiments. In addition, the absorption of glass slide to the Xe flash is not negligible. The actual power density of the flash with glass is approximately 90% of that without the glass. The transmittance (T) spectrum of glass slide in the range of 200-2200 nm is shown in Figure S4. The CAs with different densities all exhibited a large photoacoustic effect during the flash, which was caused by the expansion and contraction of trapped gases. Only the


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CA powders with a density lower than 50 mg/cm3 exhibited an enhanced photothermal effect and could be successfully flash ignited. The dynamic flash ignition process of the CAs was recorded by a high speed camera, and the combustion process was very vigorous with an intense glow, as shown in Figure 2A and Figure 2B (Movie S1, S2). The CAs began to ignite with red hot spots at 0.34 ms, and the whole combustion process lasts approximately several seconds. The CA powders were almost transformed into CO2 gas after flash ignition. The minimum flash ignition energy (Emin) of the low-density CAs was investigated, and Emin was measured by increasing the flash energy until ignition occurred. The results show that the Emin of the 20 mg/cm3 CA is approximately 0.63 J/cm2, while that of the 50 mg/cm3 CA is nearly doubled. With high temperature in local area, the low-density CAs are excellent optical ignition materials. The combustion process of 30 mg KClO4/Zr energetic pyrotechnics placed on top of 1 mg sample of low-density CAs was recorded by a high-speed camera (Figure S5). To determine the photothermal effect and structural changes of the CAs after flash ignition, the as-synthesized 20 mg/cm3 CAs, the flashed samples in an Ar atmosphere, and the flashed residues in ambient air were analyzed with X-ray diffraction (XRD) and


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Raman spectroscopy. The XRD spectrum (Figure 2C) shows that the CAs can be regarded as partly graphitized carbon with broad (002) and (101) diffraction peaks at 2θ of 23.5º and 43.8º, respectively.51-53 For the flashed residues in ambient air, the broad bands corresponding to the carbonaceous matrix become broader with a lower intensity. This change is due to both the destruction of graphite structure and reduction of the graphite carbon content. Raman spectrums of the as-synthesized 20 mg/cm3 CAs and flashed residues in air are shown in Figure 2D. The G' band at ~2700 cm-1 is a second-order process related to a phonon near the K point in graphene, activated by double resonance (DR) processes.54,

55

The intensity ratio of the G band (1580 cm-1)

corresponding to graphite and D band (1360 cm-1) representing amorphous carbon becomes lower after flash. In the range of 0 to 9 eV (wavelength greater than 140 nm), graphite is a good absorber due to the π-band optical transition.56 When the CAs are flashed in Ar, graphitized carbon is more likely to absorb the flash light energy for the restricted reconstruction. When the CAs are flashed in air, the destruction of the graphite structure and consumption of graphite carbon due to ignition result in broader diffraction bands and a lower IG/ID intensity ratio.


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Numerical Simulation of Unsteady Temperature Fields. Considering the dependence of the temperature rise caused by flash exposure (the key to successful optical ignition) on the CA density and porosity, the relationships between dynamic temperatures and the distances from the flash lamp were calculated by COMSOL Multiphysics software. As shown in Figure 3A, based on the morphology and texture characterizations, CAs with different densities can be regarded as a pile of carbon nanoparticle (~15 nm) bed with different porosities. A 100 μm thick carbon nanoparticle bed is placed on 1 mm thick transparent glass. The flash light irradiates on the carbon bed through the glass slide with 10% loss, and the carbon bed absorbs the flash energy according to the BeerLambert law. The bottom part of the carbon bed loses heat to the glass slide through conduction, and the top part loses heat to the air through convection. By solving the 1-D unsteady heat transfer equation, the time-dependent temperature profiles of the different density CAs were calculated (see detailed derivation in Supplementary Materials). Figures 3B, 3C, and 3D plot the time-dependent temperature profiles within carbon nanoparticle beds of different densities (100, 50, and 20 mg/cm3) which are exposed to


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the maximum incident flash fluence 1.89 J/cm2. The time-dependent temperature fields of CAs with higher density and lower porosity are shown in Figure S7. Because the combustion of CAs is complete at approximately 700 ℃ (Figure S9), the samples must reach such temperatures at the light power threshold necessary for ignition. For CAs with density lower than 50 mg/cm3, the temperature can reach more than 700 ℃ for ignition after a 0.3 ms Xe flash exposure. The simulation results correspond to the highspeed camera results in which the ignition time is around 0.34 ms. The calculated maximum temperature (Tmax) of CAs with an ultra-low density around 20 mg/cm3 is close to 1800 ℃, while that of CAs with a density of 100 mg/cm3 is below 700 ℃. Although the high temperature is very transient (hundreds of microseconds) and localized, bond breakages and rearrangements of carbon atoms can still happen in this situation. In the case of low-density CAs (100 mg/cm3), the Tmax decreases slowly, and the Tmax of CAs with a density of 400 mg/cm3 is higher than 400 ℃ (Figure S7). For comparison purposes, the adiabatic flame temperatures were calculated by HSC Chemistry 6 software (see detailed derivation in Supplementary Materials). The


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maximum adiabatic flame temperature 2189.2 ℃ occurs at a stoichiometric mixture of C-to-O2 (Figure S6), which is approximately 400 ℃ higher than the maximum temperature Tmax calculated by the 1-D unsteady heat transfer equation. In addition to the density and porosity, the thickness of carbon nanoparticle bed is another essential impact factor to maximum temperature. Interestingly, the thickness of the carbon bed had little influence on the maximum temperature when the CA porosity was lower than 0.975 (density>50 mg/cm3), and the maximum temperature difference in different thickness carbon bed was less than 100 ℃. For CAs with a porosity of 0.99 (density=20 mg/cm3), the maximum temperature difference in different thickness carbon bed was nearly 500 ℃, and the maximum temperature (~1782 ℃) appeared at the thickness of 17 μm (Figure 3E). In addition, the minimum ignition energies Emin of CAs with different porosities at different thicknesses were calculated (Figure 3F and Figure S7). Overall, the Emin decreased as the porosity increased. With a porosity of 0.99 and a thickness of 5 μm, the Emin was as high as 1 J/cm2, which is nearly 2 times larger than that of the other thickness CAs with the same porosity. For CA thicknesses below 17 μm, the carbon bed is too thin to absorb all the incident light energy, so the Tmax is


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relatively low and the Emin is large. For CA thicknesses above 17 μm, the additional carbon bed thickness serves as a heat sink, resulting in a decrease in the maximum temperature. Although the calculated Emin of CAs with porosities lower than 0.95 is more than 3 J/cm2, these CAs cannot be flash ignited at such an energy fluence. The low heat capacity and thermal conductivity of low-density and high-porosity CAs are one of the primary factors to high temperature and structural reconstruction. However, these factors are not enough to explain the local heat confinement effect in the nanostructures. The relationship between the subwavelength pore structures and thermal conductivity properties in low-density CAs will be discussed later. The Metal Catalyst Effect. To investigate the effect of metal catalysts during the possible flash ignition process of high density CAs, a porous, 3D-like Fe/carbon foam with a high density (100 mg/cm3) and low surface area (255 m2/g) was created by an easy, green, one-step method from the precursors FeNO3 and sodium alginate. The morphology, elemental distribution, and porous attributes of the Fe/carbon foam were characterized by SEM/EDS (Figure 4A), TEM/HRTEM (Figure 4B, C) and nitrogen sorption tests (Figure S8). Based on the HRTEM image, the lattice fringe with a


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distance of 2.1 A, depicting a highly crystalline structure, corresponds to the (110) plane of α-Fe crystal. The lattice fringes of carbon cannot be observed, which matches well with the XRD results, as shown in Figure 4F. The as-synthesized Fe/C foams are composed of amorphous carbon, α-Fe (PDF#87-0721), and tetragonal Fe3O4 phase (PDF#88-0315). The Fe3O4 is originated from incomplete reduction of Fe(NO3)3 in the preparation process. Based on the phase content analysis by XRD, the content of Fe3O4 is approximately 15 wt% of Fe. The resultant Fe/C foam shows an enhanced light absorption capacity in the visible-light range due to the surface plasmon resonance (SPR) absorption of Fe nanoparticles (Figure 4D). The flash ignition experiments show that the Fe/carbon foam with about 39 wt% Fe loading (Figure S9) can be ignited by the camera flash with an energy fluence of approximately 1.89 J/cm2 (Figure 4E). The Fe nanoparticles along with the oxygen were used to grow and support the combustion process which causes heat to accumulate more quickly than it dissipates. After flash ignition, the residues were carefully collected, and its powder diffraction pattern was measured, as shown in Figure 4F. The Fe nanoparticles embedded in Fe/C foam were almost oxidized to polycrystalline Fe3O4, which is likely formed as a kinetic product due


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to the fast oxidation reaction. The maximum temperature calculated by HSC Chemistry 6 during the fast oxidation process is 2150.3 ℃. The Flash Ignition Mechanism. Although we attribute the enhanced photothermal effect and ignition to the low density and high porosity of CAs, ignition did not occur for other lowdensity carbon materials such as graphene, graphite powder, fluffy carbon soot, and C60. As we mentioned above, the microstructure in CAs is an essential factor. Here, we proposed a local heat confinement effect caused by the micropore structures in low-density CAs to explain the enhanced photothermal effect and structural reconstruction (Figure 5A). Generally, the total thermal conductivity λt of CAs include three parts:33 the gaseous thermal conductivity λg, the radiative thermal conductivity λr, and the solid thermal conductivity λs. The total thermal conductivity λt is the sum of three thermal conductivities:32-34

t  g  r  s

(2)

The gaseous thermal conductivity for porous material can be written as:35

0g 0g g   1  2Kn 1  2  g D

(3)

where λ0g and Kn are the thermal conductivity of gas in free space and the Knudsen number, respectively, β represents a coefficient dependent on the energy accommodation coefficient and the adiabatic coefficient of gas. The Knudsen number Kn can be expressed as the ratio of the mean free path (Λg) of gas molecule to the mean pore diameter of porous material (D). The pore size distributions of CAs are usually random and non-uniform. Based on Eq. 3, the gaseous thermal conductivity in smaller pore is lower than that in larger pore. CAs were heated when


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they absorb light flash, and the heat wave is locally confined in the carbon skeleton near the microporous structure because the gaseous thermal conductivity in micropore is extremely low. For CAs, the contribution of radiative heat transfer and electronic transport to the solid thermal conductivity is negligible.32 Therefore, the solid thermal conductivity in CAs is mainly dominated by the phonon diffusion:

s 

1 c  ph Lph 3

(4)

where cv represents the specific heat at constant volume, vph and lph are the mean velocity and mean free path of the phonons, respectively, and ρ is the sample density. The mean free path of the phonons lph is controlled by the molecular order in the solid phase of the sample and is affected by the pyrolysis temperature. In this work, all the CA samples with different densities were prepared at the same pyrolysis temperature 1050 ℃, so the mean velocity and the mean free path of the phonons are almost the same. The solid thermal conductivity λs in CAs is mainly controlled by density. The low solid thermal conductivity in low-density carbon aerogels will lead to local heat confinement effect as well. In addition, as super black materials, the CAs with the lowest density can absorb the flash light almost perfectly at all angles (absorption≈ 99.8 %).36 CAs composed of more micropores (subwavelength scale) could obviously increase the absorption of flash light (Figure S11).30 In classical electrodynamics, the vibration of electrons in the free-state is determined by the intensity of incident light and conductivity of the conductor, and the vibration amplitude Afree can be written as:30


19


Af r ee 

eE0 m   i  Ne2 m



2

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

(5) where e and m are the electric quantity and mass of the electron, respectively, E0 is the electricfield intensity, ω is the angular frequency of E0, N is the number density of the electrons, and σ is the electrical conductivity. Normally, the vibration amplitude Afree is approximately several nanometers and the vibration is not affected by the microstructure. However, there are many micropores existed in the low-density CAs. For the electrons near the interface of the micropores, the vibration will be greatly affected by the pore structure, and the vibration amplitude Afree will decrease by an order of magnitude (Figure 5B). The decrease in Afree results in a decrease of the refractive index and thus an increase in optical absorption.30 Normally the vibration amplitude is about several nanometers, so the limiting effect of micropore (0-2 nm) is far higher than that of mesopore (2-50 nm). On the other hand, the mesopore as nanopore can limit gaseous heat conduction, but its limiting effect is obviously not as large as that of micropore. So compared with micropores, the contribution of mesopores to the enhanced optical absorption and local heat confinement effect of CAs is negligible. The microporous carbon nanoparticles are composed of micropores and graphite crystallite. The local heat confinement effect will lead to a temperature far beyond the calculated maximum temperature of 1782 ℃. At such a high temperature, carbon bonds will be broken, and carbon atoms in graphite crystallite will rearrange.


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As for higher density CAs, the micropores gradually disappear and the temperature rise △T in carbon nanoparticles can be estimated as:6

T  A

I CV

(6) where A is the cross-sectional area of the CAs, η is the conversion efficiency of absorbed light energy into heat, I is the absorbed light energy, and ρ, C, and V are the density, specific heat, and the volume of the CA, respectively. The heat generated in carbon nanoparticles quickly dissipates into the air present in macropores, and the temperature rise is not sufficient to ignite CAs (Figure 5C). Under this condition, the addition of Fe nanoparticle catalyst is helpful to the ignition (Figure 5D). In a typical high density Fe/C foam, Fe nanoparticles can absorb Xe flash energy and convert it to heat. Normally, Fe nanoparticles are extremely reactive in oxidizing environments, and the ignition temperature is greatly influenced by the particle size. For a well dispersed 100 nm Fe nanoparticle bed, the temperature rise is less than 200 ℃, which only causes slow oxidation of the Fe nanoparticles. For the carbon in the Fe/C foam, although the


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temperature rise in the carbon nanoparticles (>400 ℃) is not sufficient to ignition porous carbon, the rise is much greater than that in the Fe nanoparticles due to their lower density and higher absorbance of flash energy. The temperature difference between the C and Fe nanoparticles causes heat to flow into the pyrophoric Fe nanoparticle catalyst, and the temperature of the Fe nanoparticles rapidly increases by more than 300 ℃ to cause violent combustion. The vigorous combustion of Fe is the direct factor that ignites the porous carbon and supports combustion with oxygen. To date, photoignition with enhanced photothermal effect has been reported in three carbon rich materials, including single-wall carbon nanotubes (SWNTs)/metal catalyst,2 graphene oxide (GO) nanoplatelets,5, 19 and microporous CAs,30 which share the same graphite-like structure with sp2 hybridized carbon atoms. Based on our study, the flash ignition can be achieved in pure carbon materials if they have low-density, high surface area, unique porous structure, and almost perfect absorption of flash light. The pure SWNTs can also be flash ignited if they have the same low-density, high surface area, and unique porous structures as the low-density CAs. The purified SWNTs aerogels are fabricated by subjecting a SWNTs (metal catalyst and SDBS are removed) wet-gel to


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critical-point-drying. We found that the purified SWNTs cannot be flash ignited while the SWNTs aerogels can. The enhanced photothermal effect, local high temperature, and structural change caused by flash exposure are derived from the nature of carbon which is relevant to the sp2 bonding structure of carbon atoms in graphite crystallite structure. For GO, we believe that a similar heat confinement effect caused by the nanostructure, dimensions, and electronic structure results in the enhanced photothermal effect and flash ignition, but more studies are needed to better understand the detailed mechanism.

Conclusions In summary, we have demonstrated that low-density and high-surface-area CAs can be flash ignited in ambient air by a conventional camera flash. The photothermal effect and heat confinement in nanostructures can thus lead to local structural reformations: the destruction of the graphitized carbon structure with sp2 bonding in CAs. The calculated maximum temperature (Tmax) of CAs with an ultra-low density of approximately 20 mg/cm3 is close to 1800 ℃. The unique porous structure, low density,


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and high surface area of CAs are the primary factors to successful flash ignition, and intimate contact with metal catalyst is not necessary. For low-density CAs, the low gaseous thermal conductivity in micropores and low solid thermal conductivity result in local thermal confinement effects. The restriction of the vibration amplitude of free-state electrons induced by the microporous structure under flash exposure results in an increase in optical absorption. For high-density CAs, the addition of metal catalyst increases both the light absorption capacity in the visible-light range and the heat accumulation capacity. All these factors lead to enhanced photothermal effect and ignition in CAs. We believed that the strong photothermal behavior in the as-presented low-density CAs suggests extensive applications in light-induced ignition, nanowelding, and nanoreactors, and is helpful for understanding the large optical enhancement in lower dimensional carbon materials.

Experimental Section Synthesis of Carbon Aerogels (CAs). CAs were prepared by pyrolysis of organic gels process. Resorcinol (R) and formaldehyde (F) with a molar ratio of 1:2 were added to


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distilled water and completely dissolved. A certain amount of catalyst (C) sodium carbonate (molar ratio R: C=200) was added to the RF solution. To obtain different density CAs, the concentration of the RF solution was controlled at 1, 3, 4, 5, and 10 wt%. The resulting solutions were transferred into sealed glass vials in an 80 ℃ water bath for 5 days to obtain the wet gels. The gel precursors were aged in 3 vol% trifluoroacetic acid for 3 days. After aging, the wet gels were exchanged with pure acetone to ensure the water content was less than 5000 ppm and subsequently dried using supercritical CO2. The RF aerogels were slowly heated at 1050 ℃ under Ar flow to obtain the CA monoliths. To investigate the photothermal effect of the CAs, CA monoliths were made into powders with an agate mortar and pestle by hand. To ensure uniform particle size distribution, the micro-sized particles were screened out by a classifying screen. The particle size distribution of CA powder after milling and screening is shown in Figure S12. The as-synthesized CAs were dried under vacuum at 200 ℃ for 4 h before performing any flash ignition experiments. Synthesis of Fe/carbon Foam. First, 50 mL of a 1 wt% aqueous solution of sodium alginate was first added dropwise to 50 mL of a 0.24 mol/L Fe(NO3)3 solution under 300


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r/min stirring, followed by 24 h standing prior to the following distilled water washing process. Next, the washed biopolymer microspheres were exchanged with ethanol for 2 days to obtain dehydrated alcogel beads. Then, the resultant products were dried under supercritical CO2. Finally, the Fe/alginate foam beads were pyrolyzed at 1400 ℃ for 4 h in an argon atmosphere to obtain a porous, 3D-like Fe/carbon foam. A schematic of the synthesis process of the Fe/carbon foam and the reactions of the Fe/alginate foam in the pyrolysis process are shown in Figure S10. Characterization. SEM and EDS characterizations were performed by an FEI Quanta 450 instrument at an accelerating voltage of 15 kV. TEM characterizations were performed using a JEM-200CM microscopy with an acceleration voltage of 200 kV. XRD measurements were performed on a PANalytical X’Pert Pro X-ray diffractometer with nickel-filtered Cu Kα radiation as the X-ray source. The chemical compositions of CAs were analyzed by an ESCALAB250Xi X-ray photoelectron spectroscopy. The absorbance was measured using a UV-vis-NIR Shimadzu UV-3700 spectrometer. Thermogravimetric analysis and differential scanning calorimetry (TG-DSC) were carried out by an SDT Q600 thermal analyzer. Raman spectroscopy was performed


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using a Renishaw Raman microscope with a 514.5 nm laser. Total pore volume, BET surface area, and pore-size distributions by density functional theory (DFT) and the Barret−Joyner−Halenda (BJH) method were determined from nitrogen adsorptiondesorption isotherms measured by a Quantachrome Autosorb-1 instrument. The Flash Ignition Experiments. The flash lamp used in the experiments was a conventional Canon Speedlite camera flash. All the flash experiments under Ar atmosphere (99.999%) were performed inside a glovebox (MIKROUNA Upuer): water content < 0.1 ppm, oxygen content < 0.1 ppm. The flash impulse as a function of time was recorded by a Tektronix DPO4054B digital phosphor oscilloscope using a Thorlabs DET36A high-speed photodetector. The spectrum of the Xe flash was analyzed by an Ocean Avantes2048 optical fiber spectrometer. The experimental setup to study the Xe flash is shown in Figure S4. The combustion process videos were recorded by a Phantom MIRO R310 high-speed camera with a frame rate of 5 kHz. Numerical Simulation. The dynamic temperature profiles inside the CA particles were calculated using COMSOL Multiphysics software and the the adiabatic flame


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temperatures were calculated by HSC Chemistry 6 software (details in supplementary materials).

A

B

C

D

Graphite dspace

E

F G

Figure 1. Materials synthesis, characterization and schematic of flash ignition experiment. (A) Synthesis process of different density CAs with different pore structures. (B) SEM images of CAs with a density of 20 mg/cm3, inset: EDS map, scale bar: 400 nm. (C) TEM images of CAs with a density of 20 mg/cm3. (D) High-resolution TEM image of a single carbon nanoparticle, inset scale


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bar: 2 nm. (E) Pore size distributions of CAs with densities of 20 and 200 mg/cm3. (F) Schematic of the experimental setup for the flash ignition of CA powders by a conventional camera flash. (G) Optical images of CAs before flash ignition.

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Figure 2. Facial optical ignition and structural changes after exposure to the Xe flash. (A) Optical images of flash ignition CAs with a density of 20 mg/cm3. (B) High-speed camera images capturing the ignition and combustion processes of CA powders exposed to the Xe flash. (C) XRD pattern of as-prepared CAs, flashed CAs in Ar, and the residues after the flash. (D) Raman spectrums of assynthesized 20 mg/cm3 CAs and flashed CAs in air.

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Figure 3. Numerical simulation of unsteady temperature fields. (A) The computational domain and corresponding boundary conditions of the heat transfer model. (B) Calculated time dependent temperature profile for a density of 100 mg/cm3 and porosity of 0.95. (C) Calculated time dependent temperature profile for a density of 50 mg/cm3 and porosity of 0.975. (D) Calculated time dependent temperature profile for a density of 20 mg/cm3 and porosity of 0.99. (E) Calculated maximum temperature rises as functions of the porosity and thickness. (F) Calculated minimum flash ignition energies of CAs as functions of the porosity and thickness.

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F

Figure 4. Morphology and microstructure evolution after light absorption. (A) SEM image of the asprepared Fe/C foam, inset: elemental distribution map of Fe/C foam. (B) TEM image of Fe/C foam. (C) HRTEM image taken at the Fe nanoparticle edge. (D) UV-vis-NIR spectrums of PEG-Fe/C, SWNTs, and CAs. (E) Optical images of Fe/C foam during flash ignition. (F) XRD patterns of Fe/C foam before and after flash exposure in ambient air.


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Figure 5. Mechanism of the enhanced photothermal effect and ignition. (A) Schematic illustration of local heat confinement mechanism of low-density CAs with mircropore structures. (B) The restriction of the vibration amplitude of free-state electrons induced by the microporous structure under flash exposure results in an increase in optical absorption. (C) The thermal dissipation mechanism in high-


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density CAs. (D) The enhanced photothermal effect and ignition in Fe/C foam caused by heat flow into the pyrophoric Fe nanoparticle catalyst from the carbon bed.

ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge on the ACS Publications website at DOI: Supplementary Figures: SEM and high-resolution TEM images of low-density CAs, Pore size distributions and XPS spectra of low-density CAs, Characterization of Xe flash, Numerical simulations, Thermal analyses of low-density CAs and Fe/C foam, Schematic of the synthesis process of Fe/carbon foam, Optical absorption and thermal deexcitation of CAs. Supplementary Tables: BET surface areas and pore structures of CAs, Ignition energy at different pulse durations of camera flash, Parameters and values of CAs used in the COMSOL simulation . Supplementary Movies: Real-time and high-speed camera videos of the ignition and burning of low-density CAs.


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AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected], *E-mail: [email protected]

ORCID

Fan Yang: 0000-0002-9395-5310

Yongjian Tang: 0000-0001-5631-3426

Zhibing Fu: 0000-0002-6764-4244

Author Contributions F.Y., X.K., Z.F., and Y.T. designed and supervised the project; F.Y., X.Y., M.Z., X.L., and J.L. performed synthesis experiments and characterization; Y. Z. and F.Y. performed the numerical simulations; F.Y., Z.Y., C.W., H.Z., and Z. F. contributed to analysis the experimental data; F.Y., X.L.K.,J.S.L, Y.T. performed the mechanism analysis; F.Y. and X.Y. wrote the manuscript.


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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (No. 51306165, 51606158), Open Foundation of Joint Laboratory for Extreme Conditions Matter Properties, Southwest University of Science and Technology and Research Center of Laser Fusion, CAEP (No. 12zxjk01).

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Enhanced Photothermal Effect in Ultra-Low-Density Carbon Aerogels

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