Lightweight, Mesoporous, and Highly Absorptive All-Nanofiber

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 1104−1112

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Lightweight, Mesoporous, and Highly Absorptive All-Nanofiber Aerogel for Efficient Solar Steam Generation Feng Jiang,†,‡,# He Liu,‡,# Yiju Li,‡ Yudi Kuang,‡ Xu Xu,‡ Chaoji Chen,‡ Hao Huang,§ Chao Jia,‡ Xinpeng Zhao,∥ Emily Hitz,‡ Yubing Zhou,‡ Ronggui Yang,∥ Lifeng Cui,*,† and Liangbing Hu*,‡ †

School of Environment and Civil Engineering, Dongguan University of Technology, Guangdong 523808, China Department of Materials Science and Engineering and §Department of Mechanical Engineering, University of Maryland, College Park, Maryland 20742, United States ∥ Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309, United States ACS Appl. Mater. Interfaces 2018.10:1104-1112. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/07/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: The global fresh water shortage has driven enormous endeavors in seawater desalination and wastewater purification; among these, solar steam generation is effective in extracting fresh water by efficient utilization of naturally abundant solar energy. For solar steam generation, the primary focus is to design new materials that are biodegradable, sustainable, of low cost, and have high solar steam generation efficiency. Here, we designed a bilayer aerogel structure employing naturally abundant cellulose nanofibrils (CNFs) as basic building blocks to achieve sustainability and biodegradability as well as employing a carbon nanotube (CNT) layer for efficient solar utilization with over 97.5% of light absorbance from 300 to 1200 nm wavelength. The ultralow density (0.0096 g/cm3) of the aerogel ensures that minimal material is required, reducing the production cost while at the same time satisfying the water transport and thermal-insulation requirements due to its highly porous structure (99.4% porosity). Owing to its rationally designed structure and thermal-regulation performance, the bilayer CNF−CNT aerogel exhibits a high solar-energy conversion efficiency of 76.3% and 1.11 kg m−2 h−1 at 1 kW m−2 (1 Sun) solar irradiation, comparable or even higher than most of the reported solar steam generation devices. Therefore, the all-nanofiber aerogel presents a new route for designing biodegradable, sustainable, and scalable solar steam generation devices with superb performance. KEYWORDS: cellulose nanofibrils, carbon nanotube, aerogel, solar steam generation, water treatment

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INTRODUCTION Solar energy, as a clean and inexhaustible energy source, if harvested efficiently, could potentially relieve human beings from the global energy crisis and environmental deterioration caused by our reliance on fossil fuels. In fact, the hourly solar irradiation on earth generates more than enough energy to sustain human consumption for a year.1 Aiming at efficient utilization, persistent endeavors have been devoted toward harvesting and converting solar energy into varied forms of energy, including electrical energy via solar photovoltaics2 and concentrated solar thermoelectrics,3,4 chemical energy via photocatalysis5 and photoelectrolysis,6 as well as thermal energy via the most recent developed solar steam process.7−16 16 Among these solar-energy utilization approaches, light-toheat conversion by a heat localizing layer with excellent visible and near-infrared light absorbing capacity has been demon© 2017 American Chemical Society

strated to be extremely efficient, showing solar thermalconversion efficiency as high as 90%.7,17 This solar-driven steam generation process, without relying on additional energy input, could be effectively applied in water purification and desalination, to address the global water shortage crisis. Although water evaporation is a phenomenon occurring naturally within the ecosystem, it is a sluggish process due to the low efficiency in heating a bulk water body. Therefore, for efficient solar steam generation, several design criteria must be met, including efficient broadband light absorption with thermal management to confine the absorbed light in a local area and excellent water absorbing properties to transport water Received: October 11, 2017 Accepted: November 28, 2017 Published: November 28, 2017 1104

DOI: 10.1021/acsami.7b15125 ACS Appl. Mater. Interfaces 2018, 10, 1104−1112

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Graphical illustration of all-nanofiber CNF−CNT aerogel used for solar steam generation. The solar steam generation device is composed of a bilayer structure of a bulk CNF aerogel with a thin CNT coating layer. (b) Photograph of bilayer CNF−CNT aerogel standing on top of a dandelion, demonstrating lightweight performance.

upward to the heating zone.18 Most current strategies in solar steam generation generally rely on either expensive metallic nanoparticles for plasmonic heating7,17,19−21 or complicated fabrication processes to design porous structures required for water transport, such as chemical vapor deposition growth of porous graphene22 and nanoporous anodic aluminum oxide.7,17 Aerogel, due to its extraordinary properties such as super absorbency, ultralow density, and ultrahigh porosity, has attracted much research interest in water treatment applications such as oil/water separation.23,24 Recently, a graphene oxide based aerogel that incorporated multiwalled carbon nanotubes for broadband solar absorption and sodium alginate for water absorption was developed, demonstrating 83% efficiency in solar steam generation, which confirms the benefits of a porous network structure in solar steam generation.25 As a natural nanomaterial, cellulose nanofibrils (CNFs) have attracted abundant research interest due to their nanoscale dimensions, excellent mechanical properties, sustainability, and biodegradability.26−31 Recently, cellulose has been widely used as green material for solar-energy conversion and storage application, including as photoanodes and polymer electrodes for dye-sensitized solar cells,32−34 as a light management layer for a thin film solar cell,35,36 as well as a binder and current collector in next-generation lithium-ion batteries.37 CNF can be facilely assembled into a 3D porous aerogel structure with excellent structural integrity and resilience, which has been extensively investigated in areas including wastewater treatment, thermal insulation, and electrochemical energy storage.23,38−42 However, application of this super solar-absorbing and thermal-insulating CNF aerogel in solar steam generation has not been reported, which drives us to explore its potential in this emerging area.

which function as conduits to pump water upward owing to the extremely hydrophilic nature of CNF and the capillary forces exerted by these macrochannels. The CNF aerogel also works as thermal insulation due to its high porosity,43 effectively reducing the parasitic thermal loss to the bulk water. The light absorbance of CNT closely resembles the blackbody behavior, with an extremely high absorptivity of 0.98−0.99 over 250− 1200 nm wavelength,44,45 which allows it to function as an effective solar-absorption material in varied energy and environmental applications.46−49 To improve both interfacial bonding between CNT and CNF and the structural integrity of the bilayer aerogel, CNT containing 3−6% carboxylic was used as the heat absorption layer,50 which can strongly interact with the (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) oxidized CNF through hydrogen bonding. With this rationally designed bilayer structure, the bilayer aerogel demonstrated broadband solar-absorption efficiency at the CNT layer for efficient lightto-heat conversion. The highly porous CNF aerogel structure and the confined thin CNT coating are effective in local heat management, guaranteeing minimum heat loss to the bulk water body or surrounding environment. As water evaporates from the top CNT layer, continuous water flow supply is well maintained due to the highly absorptive nature of CNFs and the all-nanofiber structure to ensure interpore connectivity for sustained water transport. Besides these advantages, the bilayer CNF−CNT aerogel is also ultralightweight with a low material packing density, as demonstrated by a piece of aerogel standing on top of a dandelion, without bending or deforming the seed hair (Figure 1b), which can ensure its portability and reduce the production cost. In essence, this bilayer aerogel design satisfies all the critical design criteria required for high efficiency solar steam generation, including high porosity of the CNF aerogel for reducing heat loss, macrochannel structures for water absorption and transport, an all-nanofiber design to promote water and vapor escape, a CNT coating for solar-energy concentration, and seamless binding between the carboxylic groups of CNF and CNT to ensure structural integrity. To fabricate the bilayer aerogel structure, TEMPO oxidized CNF, with a width of approximately 3−5 nm and a length of several hundred nm to 1 μm (Figure S1), was facilely

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RESULTS AND DISCUSSION Herein, we report a bilayer-structured aerogel composed of two types of nanofibers (i.e., CNF and carbon nanotube (CNT)) seamlessly integrated with each other to form a compressible and efficient solar steam generator, as shown in Figure 1a. This bilayer aerogel contains a bulk CNF skeleton with anisotropic vertically elongated macrochannels of 200−300 μm width, 1105


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Figure 2. (a) Schematic illustration of the fabrication of the CNF−CNT bilayer aerogel via ice-templating, freeze-drying, and CNT coating; SEM images of (b,c) CNF and (e−g) CNF−CNT aerogel: (b,e) top-view showing the large pores; (c,f) longitudinal view showing the elongated channel structures; (d) photograph of the bilayer aerogel showing white CNF aerogel with a black CNT coating layer; (g) high magnification SEM image showing fibrous CNT network structure on top of the bilayer aerogel.

Figure 3. Characteristics and mechanical stability of the CNF−CNT bilayer aerogel used for solar steam generation. (a) Density and porosity of the CNF and CNF−CNT bilayer aerogel, both showing extremely low densities of less than 0.01 g/cm3 and high porosities of over 99%; (b) FTIR spectra of CNF and CNF−CNT bilayer aerogel showing the presence of polar groups contributing to water absorption; (c) compressive stress− strain curves of the CNF−CNT bilayer aerogel in both dry and wet states; (d) cyclic compressive stress−strain curves of CNF−CNT bilayer aerogel in water.

around the formed ice crystals, which act as porogens after sublimation, leaving a honeycomb-like structure with hexagonal pores at a scale of 200−300 μm wide at the cross section of the aerogel (Figures 2b and S2). The pores are elongated in the

assembled into a 3D porous aerogel structure by freezinginduced ice crystal templating (−20 °C) and subsequent freezedrying (Figure 2a). Freezing the CNF aqueous suspension leads to the growth of ice crystals that segregate CNF to wrap 1106


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Figure 4. Thermal properties of the CNF−CNT bilayer aerogel that benefit the solar-energy concentration and thermal management for efficient solar steam generation. (a) Light absorbance spectra within the visible and near-infrared region for both CNF and CNF−CNT bilayer aerogels insets are photographs of the respective top surface (upper-right, CNF aerogel; lower-left, CNF−CNT bilayer aerogel); (b) infrared photos showing the surface temperature of the CNF−CNT bilayer aerogel under 1 kW m−2 over timethe aerogel was circled for clarity; (c) surface temperature increase as a function of irradiation time under solar irradiation of 1 kW m−2; (d) the equilibrium maximum surface temperature under varied levels of solar irradiation ranging from 1−10 kW m−2.

transport between adjacent pores, critical for sustained water supply during the solar steam generation. Material characteristics including the density and porosity are two important factors that potentially affect the portability and water transport of solar steam generation (Figure 3a). Owing to the nanoscale dimensions and the prevalent polar groups at the surface, CNFs tend to assemble into aerogels at an extremely low concentration of 0.1 wt %, rendering ultralow density.23 The density of the CNF aerogel used in this study is 0.0078 g/ cm3, which slightly increased to 0.0098 g/cm3 after CNT coating, but is still extremely low to offer easy portability. The areal density of CNT coating on the aerogel was estimated to be as low as 0.0015 g/cm2, resulting in a cost-effective solar steam generation device from the reduced CNT packing density. The porosity of the CNT−CNF bilayer aerogel is as high as 99.4%, which, combined with the low density, offers the feasibility of this device to be floating at the air−water interface for solar steam generation (Figure S6). Fourier transform infrared spectroscopy (FTIR) spectra of the CNF aerogel showed a typical cellulose structure with a broad peak located at 3000−3500 cm−1 assigned to OH groups as well as a peak at 1605 cm−1 due to the carboxylate groups introduced by TEMPO oxidation (Figure 3b), proving its hydrophilic nature for water absorption and transport. These functional groups are still present after CNT coating, albeit with much lower intensities due to large quantity of CNTs. The mechanical robustness of the CNF−CNT bilayer aerogel was investigated by the compressive stress−strain test under both dry and wet states (Figure 3c). The dry aerogel demonstrated three stages of compressive behaviors of an initial linear response before the yield point at 0.15 strain, plastic deformation with a slower stress increment after the yield point,

longitudinal direction, suggesting preferentially vertical growth of the ice crystals along the longitudinal direction, which can benefit the upward water transport (Figure 2c). To introduce the light absorbance layer, CNT dispersed in isopropanol at 2.5 mg/mL was deposited on the top of the CNF aerogel under vacuum, which is applied to infiltrate the CNT inside the macropores, covering the white CNF surface with a black CNT layer (Figure 2d). The morphology of the as-fabricated CNF−CNT bilayer aerogel was closely examined by SEM, which helps to reveal the bilayer structure as well as highlight the top CNT coating layer. The top surface remained porous after CNT coating but apparently smaller in pore sizes and less in quantities as compared to the original CNF aerogel, suggesting some pores were covered by CNTs (Figures 2e and S3). Closer inspection of the coated CNT layers showed randomly oriented CNTs overlaid with each other, forming numerous nanopores that benefit water and vapor penetration (Figure 2g). The longitudinal direction of the aerogel showed elongated macropores similar to the CNF aerogel, which is expected since the CNT coating was only applied to the top surface (Figure 2f). The gradual transition of the CNT coating inside the aerogel is disclosed by the high magnification SEM images, showing isolated CNT deposited on the surface of pore walls at 0.5 mm below the top surface (Figure S4) and essentially clean pore wall surfaces without any signs of CNTs at the bottom of the aerogel (Figure S5). Individual CNFs are hardly discerned on the pore walls due to the close associations between these CNFs through interfibril hydrogen bonding. However, numerous mesopores of several tens of nm wide are present on the surface of the pore walls, which can facilitate water 1107


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Figure 5. Solar steam generation performance of the CNF−CNT bilayer aerogel. (a) Photographs of steam generated under varied levels of solar irradiation energy of 1, 3, 5, 7, and 10 kW m−2 (1−10 Suns); (b) the dynamics of solar steam generation under different levels of solar irradiation; (c) the performance of solar steam generation by the CNF and CNF−CNT bilayer aerogels under different levels of solar irradiation, as expressed by the equilibrium water evaporation rate (E.R.); (d) cumulative mass change due to water evaporation over time for water in dark field and under 1 kW m−2 as well as CNF and CNF−CNT aerogel under 1 kW m−2; (e) comparison of the equilibrium evaporation rate and solar steam generation efficiency of water alone and with CNF and CNF−CNT bilayer aerogels; (f) the solar steam generation efficiency of the CNF−CNT aerogel under varied levels of solar irradiation; (g) stability of solar steam generation under 1 Sun of irradiation for 50 cycles.

and the last sharp densification stage up to 0.8 strain. This large strain deformation indicates that the aerogel is very flexible and does not break apart under compression, which further confirms its portability together with the low density. However, the dry aerogel showed almost no shape recovery from the compression state, as demonstrated by the fast drop of stress to 0 after releasing the compressive force. When wetted, the aerogel became significantly weaker due to the plasticization effect of water, with the stress at 0.38 strain and Young’s modulus being reduced to 2.7 and 4.6 kPa from 25.7 and 141 kPa for the dry aerogel, respectively. However, the wet aerogel demonstrated better shape recovery as revealed by the small hysteresis between the compression and release curves (Figure 3d). This wet resilience could be triggered by the interaction between water and CNFs that breaks the interfibril hydrogen bonding. Most importantly, the wet aerogel is strong enough to withstand repetitive compression−recovery for up to 100 cycles without breaking apart. This robustness, especially the longterm stability of CNF−CNT bilayer aerogel under water, warrants its application for solar steam generation.

The efficiency of solar steam generation depends highly on the efficiency of the device surface in absorbing solar energy; therefore, materials with high light absorbance are more desirable. The absorbance of both CNF and CNF−CNT aerogels in the range of 300−1200 nm was recorded and presented in Figure 4a. CNF aerogel is white with both high transmittance (21.7%) and reflectance (68.6%), leading to very low light absorbance of less than 10% across the UV−vis−nearIR region. As CNF aerogel contains large several micron wide pores and smooth pore walls, the incident light can be directed into the pores, with some of it penetrating through the pore walls while other light is reflected by the smooth cellulose surface, leaving very small portions of incident light to be absorbed by the aerogel. After CNT coating, the top aerogel surface turned into black, showing almost zero transmittance and 2.5% reflectance, and therefore a very high absorbance of 97.5%. As revealed from the SEM images, CNT coating introduces a porous network structure on the CNF aerogel, thus effectively blocking the reflectance and elongating the light transport path into the CNT network. Therefore, both transmittance and reflectance can be minimized to ensure the 1108


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(i.e., pure water with and without solar irradiation as well as CNF aerogel and CNF−CNT aerogel under 1 kW m−2) was recorded in Figure 5d. It shows that without solar irradiation, the water evaporates very slowly at 0.05 kg m−2 h−1 into the environment, which has been subtracted from all evaporation rate calculations throughout the experiments. Under 1 kW m−2, water can absorb solar irradiation energy that heats the entire water body, leading to an increased evaporation rate of 0.24 kg m−2 h−1 (Figure 5e). With the presence of the pure CNF aerogel, the water evaporation rate further increases to 0.61 kg m−2 h−1, possibly due to the better thermal-insulation effect of the CNF aerogel that could reduce the parasitic thermal loss to the bulk water body. Coating of CNT solar-absorption layer further enhanced the solar-evaporation rate to 1.11 kg m−2 h−1 by taking advantage of better thermal-localization and management capabilities using this bilayer structure. The solar steam generation efficiency η of the bilayer CNF−CNT bilayer aerogel can be calculated using the following equation

broadband light absorbance required by efficient solar steam generation. The heat localizing effect was investigated by measuring the surface temperature of aerogel at the air−water interfaces using an infrared (IR) camera. Surface heating of the CNF−CNT bilayer aerogel was visualized using an IR camera (Figure 4b), which showed that the surface temperature increased gradually over time, and the absorbed solar energy is confined on the surface of aerogel without dissipating to the surrounding water body (dark blue area in the IR pictures). Under 1 kW m−2, the surface temperature of CNF−CNT bilayer aerogel sharply increased from 20 to 29.6 °C within 300 s and then gradually increased and stabilized at 32.7 °C (Figure 4c). In contrast, the surface temperature of CNF aerogel only slightly increased from 20 to 23.6 °C over the same irradiation period, indicating a better heat concentrating effect with this bilayer structure. The equilibrium surface temperature after 1800 s of irradiation under varied solar-energy intensities for both the CNF and CNT−CNT aerogel was recorded (Figure 4d). With solar irradiation intensities of 1, 3, 5, 7, and 10 kW m−2, the equilibrium surface temperature increased almost linearly from 32.7, 43.7, 57.4, 72.3, and 95.3 °C, all significantly higher than the respective 23.6, 28.2, 31.1, 34.2, and 37.9 °C for that of the CNF aerogel, highlighting the more effective solar concentrating effects of the coated CNT layer. Due to the highly porous aerogel structure, the aerogel presents a low thermal conductivity of 0.06 W/mK, which is critical for thermal management, to reduce the thermal loss into the water body by confining the heating zone on the top air−water interface layer. All previous morphological, chemical, structural, and thermal characterizations suggest that the CNF−CNT bilayer aerogel meets all the desired criteria for solar steam generation, including solar absorption and heat localization as well as water absorption and transport. In order to investigate the solar steam generation performance, the CNF−CNT bilayer aerogel saturated in water was irradiated with a solar simulator under varied irradiation intensities. No clear steam column could be observed under 1 kW m−2, possibly due to the relative lower surface temperature that is insufficient to generate enough steam to form a continuous escaping column (Figure 5a). A small steam column starts to appear at 3 kW m−2 and continues to expand with the gradual increase in the solar irradiation, suggesting that the evaporation was significantly enhanced at higher solar irradiation energy. The evaporation rates under varied solar irradiation were recorded over irradiation time (Figure 5b). Similar to the surface temperature increase curve, the evaporation rates under all irradiation conditions increased rapidly over the first few minutes, corresponding to the temperature increase at the early stage of irradiation. The evaporation rates reached a nearly constant value once the temperature stabilized, indicating that equilibrium was established between the evaporation and upward water transport by the highly absorptive aerogel. As expected, the evaporation rate increased almost linearly with increasing solar radiation energy, showing respective values of 1.11, 3.52, 5.89, 8.06, and 11.37 kg m−2 h−1 under 1, 3, 5, 7, and 10 kW m−2 (Figure 5c), significantly higher than the respective evaporation rates of 0.71, 1.2, 1.83, 2.33, and 2.70 kg m−2 h−1 for the CNF aerogel. The enhancement factor between the evaporation rates of the CNF−CNT bilayer aerogel and the CNF aerogel were determined as 1.6, 2.9, 3.2, 3.4, and 4.2 under 1, 3, 5, 7, and 10 kW m−2 (Figure S7). The mass change due to water evaporation under varied experimental conditions

η = mh ̇ LV /CoptP0

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where ṁ (kg m−2 h−1) denotes the evaporation rate, hLV is the total enthalpy of the liquid−vapor phase transition including the sensible heat, Copt is the optical concentration, and P0 represents the nominal solar irradiation value of 1 kW m−2. The efficiencies of solar steam generation under 1 Sun of irradiation are calculated as 16.4, 57.8, and 76.3% for water only, with CNF and CNF−CNT aerogels, respectively (Figure 5e). The efficiency slightly increased to 81.4% with an increase in solar energy from 1 to 3 kW m−2, and then, the efficiency kept constant with a further increase in the solar energy up to 10 kW m−2 (Figure 5f). Although a high efficiency (81.4% at 3 kW m−2) and evaporation rate (11.37 kg m−2 h−1 at 10 kW m−2) can be achieved at higher levels of solar irradiation, the strong solar irradiation can be difficult to achieve in normal environment. It is therefore more meaningful to compare the performance at 1 kW m−2. The 76.3% efficiency at 1 kW m−2 is significantly higher than most of the reported solar steam generation devices, including bilayer wood (57.3%),51 a plasmonic gold-deposited nanoporous template (65%),7 double-layer structured carbon foam with an exfoliated graphite layer (70%),52 and flame treated wood (72%).10 This indicates that the bilayer CNF−CNT aerogel can serve as efficient solar steam generation device under 1 Sun of irradiation. Stability tests showed that this CNF−CNT bilayer aerogel can function stably under 1 kW m−2 for up to 50 cycles, with an almost constant evaporation rate ranging from 1.09 to 1.11 kg m−2 h−1 (Figure 5g).

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CONCLUSIONS A bilayer ultralightweight (density of 0.0098 g/cm3) and porous (porosity of 99.4%) aerogel structure was successfully assembled from CNFs and CNTs and has been demonstrated as an efficient solar steam generation device. The aerogel was fabricated through systematic design strategies that satisfy the criteria for solar steam generation, including hydrophilic CNFs for water absorption, a highly porous aerogel structure for water transport, an all-nanofiber network structure for water supply and vapor escape, low thermal conductivity for heat localization to prevent parasitic thermal loss to the bulk water body, a surface coating of black CNTs to absorb 97.5% of incident light from 300 to 1200 nm for heat concentration, and structure flexibility and good mechanical performance for long-term 1109


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carbon tape is evaluated and eliminated by measuring the sample with different thicknesses. Solar Steam Generation. The solar steam generation was tested using an optical measurement system composed of a multifunctional solar simulator (Newport Oriel 69907) and optical components (Newport Oriel 67005). The CNF−CNT aerogel was placed in Petri dishes filled with water as the solar steam generation setup. The weight loss during solar steam generation (at ambient conditions) under different irradiation intensities (1−10 kW m−2) was measured using a properly calibrated electronic balance (Citizen CX301, accuracy: 0.1 mg). The infrared (IR) picture and surface temperature increment were recorded using an IR camera.

under water durability. Owing to these structure and performance advantages, the bilayer aerogel demonstrated a high solar thermal-conversion efficiency of 81.4% at 3 kW m−2 of solar irradiation, as well as a high evaporation rate of 11.37 kg m−2 h−1 at 10 kW m−2. Also, cyclic tests suggest that the bilayer aerogel can be reused for over 50 cycles without losing its steam generation performance. This high-performance bilayer solar steam generation device offers a new designing strategy that utilizes nature-based material as the primary building blocks, which ensures a sustainable way for the generation of fresh water by means of desalination and purification.

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EXPERIMENTAL SECTION

Synthesis of CNF−CNT Bilayer Aerogel. TEMPO ((2,2,6,6tetramethylpiperidin-1-yl)oxyl) oxidized (5 mmol of NaClO loading per g of cellulose) softwood CNFs were used for the CNF aerogel assembly. A portion of the 10 mL CNF aqueous suspension (0.7 wt %) was frozen in a 10 mL small beaker at −20 °C for 6 h. The beaker was wrapped with an insulation foam layer to ensure unidirectional freezing from the bottom of the beaker. The frozen CNF suspension was immediately transferred to the chamber of a freeze-drier (FreeZone 1.0 L Benchtop Freeze-Dry System, Labconco, Kansas City, MO) to sublimate the ice (−50 °C, 20 Pa, 48 h), resulting in a white porous CNF aerogel. The aerogel was cut to 5 mm in thickness, and 2 mL of CNT in isopropanol suspension (2.5 mg/mL) was deposited on top of the CNF aerogel repeatedly to ensure complete coverage and then dried under vacuum. Characterization. The morphology of both CNF and CNF−CNT aerogels were characterized by scanning electron microscopy (SEM, Hitachi SU-70). The density of the aerogel (ρa) was calculated based on the ratio of weight (wa) and volume (va) using the equation

ρa =

wa va

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b15125. TEM image of CNF; SEM image of the cross section of CNF and CNF−CNT aerogel; cross section SEM image of the CNF−CNT aerogel at 0.5 mm below the top CNT coating layer and at the bottom; photograph of the CNT−CNT aerogel floating on the air−water interface; the enhancement factor (E.F.) of solar steam generation by the CNF−CNT bilayer aerogel as compared to the pure CNF aerogel (PDF)

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*E-mail: [email protected] (L.C.) *E-mail: [email protected] (L.H.) ORCID

Yiju Li: 0000-0001-9240-5686 Ronggui Yang: 0000-0002-3602-6945 Liangbing Hu: 0000-0002-9456-9315

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Author Contributions #

F.J. and H.L. contributed equally to this work. F.J., L.H., and L.C. conceived and designed the experiment. F.J., H.L., Y.L., and X.X. conducted the experiments. Y.K. contributed in graphitic illustration. C.C. and H.H. contributed in mechanical testing. C.J. contributed in light absorbance analysis. X.Z. and R.Y. contributed in the thermal-insulation measurement. E.H. contributed in the writing of the paper. F.J., L.H., and L.C. wrote the paper. Y.Z. performed TEM analysis. All authors commented on the paper.

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where ρf indicates the density of nanofibers taken as 1.6 g/cm3. The FTIR-ATR spectrum was carried out using a Thermo Nicolet NEXUS 670 FTIR-ATR. The mechanical properties of the aerogel in air and water were determined using a fixed strain rate of 1 mm/min using a RSA III dynamic mechanical analyzer (TA Instruments). For testing in water, the CNF−CNT aerogel was completely immersed in deionized water during the measurement. The light absorbance measurement of the aerogels was carried out on a UV−vis Spectrometer Lambda 35 from 300 to 1200 nm (PerkinElmer, U.S.A.) with an integrated sphere for reflected light collection. Thermal-Conductivity Measurement. The thermal conductivity of the CNF−CNT aerogel was measured using a Netzsch laser flash apparatus (LFA 457), a noncontact transient method to measure thermal diffusivity of materials. During the measurement, an instantaneous laser pulse is used to heat up one side of the aerogel, and the response of temperature on the other side is recorded by a detector. The thermal conductivity k (W/mK) of the samples can then be calculated by the following equation

k = αρCp

AUTHOR INFORMATION

Corresponding Authors

The porosity was calculated from the equation ⎛ ρ⎞ P = ⎜⎜1 − a ⎟⎟ × 100% ρf ⎠ ⎝

ASSOCIATED CONTENT

S Supporting Information *

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (Grant No. 51528202). REFERENCES

(1) Lewis, N. S. Toward cost-effective solar energy use. Science 2007, 315 (5813), 798−801. (2) Gratzel, M. Photoelectrochemical cells. Nature 2001, 414 (6861), 338−344. (3) Tritt, T. M.; Boettner, H.; Chen, L. Thermoelectrics: Direct solar thermal energy conversion. MRS Bull. 2008, 33 (4), 366−368. (4) Kraemer, D.; Poudel, B.; Feng, H. P.; Caylor, J. C.; Yu, B.; Yan, X.; Ma, Y.; Wang, X. W.; Wang, D. Z.; Muto, A.; McEnaney, K.; Chiesa, M.; Ren, Z. F.; Chen, G. High-performance flat-panel solar thermoelectric generators with high thermal concentration. Nat. Mater. 2011, 10 (7), 532−538.

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where α (mm2/s) is the measured thermal diffusivity along a particular direction, Cp (J/g K) is the heat capacity, and ρ (g/cm3) is the density. The heat capacity, Cp, is measured with scanning calorimetry (DSC 204 F1 Phoenix). However, as the aerogel in this work is highly porous, the laser may penetrate the sample directly rather than generate heat at the surface. Thus, a sandwich structure was designed by covering the CNF−CNT aerogel with a highly conductive carbon tape on both sides to block the laser penetration. The influence of the 1110


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