Accessible Graphene Aerogel for Efficiently ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Accessible Graphene Aerogel for Efficiently Harvesting Solar Energy Yang Fu, Gang Wang, Tao Mei, Jinhua Li, Jianying Wang, and Xianbao Wang* Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry-of-Education Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei Key Laboratory of Polymer Materials (Hubei University), School of Materials Science and Engineering, Hubei University, Wuhan 430062, P. R. China S Supporting Information *

ABSTRACT: Solar steam generation through heat localization is a new approach to efficiently utilize solar energy. Nanocomposites with noble metals and other porous materials have been employed to generate solar vapor at a high light intensity. However, large-scale applications of the nanocomposites based on noble metals are restricted due to their high cost, complex preparation, and low recycling stability. Herein, we report a simple method toward fabricating graphene aerogel (GA) from graphene oxides only by photoreduction, which is for the first time used to harvest solar energy. GA can not only convert almost the entire incident solar light to heat energy but can also self-float on the surface of water and pump the interface water forming a constant water steam. Solar steam generation efficiencies of 53.6 ± 2.5% and 82.7 ± 2.5% are achieved at light intensities of 1 and 10 kW m−2, respectively. Furthermore, this efficiency is still kept at a high value, and the morphology of GA is hardly broken after 10 cycles of testing. This technology of steam generation through efficiently harvesting solar energy is highly promising for sterilization of waste and seawater desalination. KEYWORDS: Graphene aerogel, Photothermal conversion, Solar steam, Photoreduction, Heating

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layer of the solar spectrum such as the plasmonic film of gold nanoparticles at the water−air interface,6,21−23 bifunctional membrane,24 carbon-based membrane,25−28 and other absorption materials.19,29−31 The second one is a low thermal conductivity of material to prevent heat from transferring away from the hot region.26 The third one is a hydrophilic surface to promote fluid flow to the surface.21 Last one is a porous structure, in which open pores enhance the capillary effect and closed pores enable itself to float on the water−air interface.27,32 The efficiencies of solar thermal conversion from 57% to 85% are obtained by those structures in a certain light intensity. Most of those works have been focusing on doublelayer structure materials to realize performances of high absorption, heat insulation, and hydrophilicity, while singlelayer materials have not received due attention yet. Gold nanoparticles with collective plasmonic properties are widely used as an absorption layer, but they may fuse together after long-time illumination, which weakens the electronic oscillations at the particles surface.33 In contrast, carbon-based composites with higher operation stability, better broadband solar absorptance, and lower cost than metal nanoparticles could be proper alternatives.34,35 Graphene, as an advanced material in carbon materials, is one of the promising candidates

INTRODUCTION As a natural energy, solar radiation has been recognized as one of the most competitive energy sources in the future, with its abundant reserves and nonpollution.1−3 Solar thermal conversion owns the highest energy efficiency compared with other solar utilizations like photovoltaics and photocatalysis.4 Recently, intense interest has been paid in solar steam generation for its potential applications such as sterilization, water purification, and hygiene systems.5−12 Plasmonics has quite extensive applications in the area of solar energy conversion as the localized plasmon resonance at the surface of metallic nanoparticle leads to enormous enhancements of near field intensity and consequent heat localization around nanoparticles.5,13−17 Under a high light intensity, the temperatures of noble metal particles,5,13,18 semiconductor nanoparticles,19 or carbon-based nanoparticles20 can reach well above the boiling point of water, and the converting heat is apt to evaporate a surrounding water, which results in solar steam generation. All those methods of generating steam in nanofluids, relaying on surface- or cavityabsorbing solar radiation, require high light intensities (usually above 10 suns, one sun ≈1 kW m−2), which gets a relative low vapor generation efficiency (usually below 70%) and increases the surface heat loss. In order to reduce heat loss, more and more researches have been devoted to local heat and heatinsulation structures. Those structures have four main characteristics in common. The first one is a high absorption © 2017 American Chemical Society

Received: December 31, 2016 Revised: April 25, 2017 Published: April 26, 2017 4665

DOI: 10.1021/acssuschemeng.6b03207 ACS Sustainable Chem. Eng. 2017, 5, 4665−4671

Research Article

ACS Sustainable Chemistry & Engineering

chemical bonds were determined by Fourier transform infrared (FTIR) (NICOLET iS50, Thermofisher, U.S.A.) analysis and X-ray photoelectron spectroscopy (XPS) (PHOIBOS150, Specs, Germany). The water contact angle (WCAs) of the sample was measured by using a PT-705 CA meter (DongGuan Precise Test Equipment Co., Ltd.). The samples were degassed at 90 °C for 6 h before measurements by a surface area and porosity analyzer (ASAP 2020HD88, Micromertitics), and specific surface area and pore-size distribution were determined using the Brunauer−Emmett−Teller (BET) and Barrett−Joiner− Halenda (BJH) models, respectively. Solar Steam Generation Test. An experimental setup was used to measure the weight loss of water, as shown in Figure S1 in the Supporting Information, which includes (1) an illumination source simulating solar light by the xenon lamp with a solar filter (AM 1.5), (2) test chamber (beaker coated by PE foam) with an aperture upside to make sure light can only be irradiated to the surface of sample, (3) an analytical balance, (4) a computer to record the weight data in real time, and (5) an infrared camera (FLIR E4 Pro, America). Light intensity was measured by a full-spectrum optical power meter (CELNP2000-2, Beijing Education Au-light Co., Ltd.). In each test, 100 mL of water at room temperature was poured in the PP beaker with a diameter of 44 mm and a height of 68 mm. During the test, the room temperature was maintained at 25−27 °C, and the humidity was balanced between 50% and 55%.

for use in high light intensity. So three-dimensional (3D) structures of graphene-based materials with high absorption and heat insulation will be the best substitute for double-layer structures to be solar vapor generators. Although 3D porous graphene sheets synthesized by a chemical vapor deposition method have been used to efficiently generate solar steam,32 the harsh synthesis conditions may hinder its broad-use in practice. With the hydrothermal method36−40 for preparing graphene aerogel (GA), it is hard to obtain large area samples in the usual hydrothermal reaction vessel, and the hydrophobic surface of GA with this approach is unfavorable for a capillary effect. Thus, hydrophilic GA with low cost, effective light absorption, high thermal insulation, and mesoscopic porosity for capillarity should be an optimal choice to be used in application of solar vapor generation. Here, an accessible, large-area, and porous GA is prepared simply by light reduction of a graphene oxide (GO) aerogel membrane (GOAM), which is obtained from freeze-drying GO dispersion. GA with an area of 8.6 cm2 converts almost all the incident light to heat, and the local heat effect makes the efficiency of solar steam generation reach up to 82.7 ± 2.5% at a light intensity of 10 kW m−2. Compared with those reported carbon-based structures, our large-area GA prepared by freezedrying and photoreduction methods is for the first time used as a solar steam generator, and this reduction method of sunlight is for the first time developed to reduce GO to graphene without any reductants. Furthermore, the designed structure and properties of a single-layered insulation material (high absorption, self-floating, and hydrophilicity) are superior to those of the reported double-layered structures, consisting of a supporting layer and an absorption layer. In addition, four factors (light absorption, hydrophily, thermal conductivity, and porosity) affecting the efficiency of solar steam generation were detailedly analyzed in this work. The simple synthesis of GA from common and inexpensive raw material provides potential applications in seawater desalination and sterilization of waste.

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RESULTS AND DISCUSSION As shown in Figure 1a, GA self-floating on the surface of water absorbs the light irradiation, forming a localized superheat

EXPERIMENTAL SECTION

Chemicals and Materials. Graphite was purchased from Qingdao Duratight Seal Product Co., Ltd., Qingdao, China. Hydrochloric acid, ammonia, nitric acid, sulfuric acid, sodium hydroxide, hydrogen peroxide, potassium permanganate, ethanol, and phosphoric acid were obtained from Sinopharm Chemical Reagent Co. All chemicals were of analytical grade and used as received. Deionized (DI) water used in the experiment was freshly prepared from a Kertone Ultrapure Water System P60-CY (Kertone Water Treatment Co., Ltd., resistivity = 18.25 MΩ cm) and outgassed by heating and pumping. Preparation of Graphene Oxide (GO), Graphene Oxide Aerogel (GOA), GOAM, and GA. GO was prepared by oxidation of the powdered flake graphite according to the method of Marcano.41 The prepared GO was dispersed in DI water with ultrasound. Then 1.2 μL of NH3·H2O (HCl) per milliliter of GO dispersion was added to this GO dispersion, and GO dispersion was frozen in a refrigerator. GOA was obtained simply by freeze-drying GO dispersion. After GOA was rolled by a rolling machine (MSK-HRP-MR 100A, Hefei Kejing Materials Technolgy Co., Ltd.), GOAM was fabricated. GA was prepared directly by exposing GOAM under a xenon lamp for 5 s at 10 suns (CEL-HXF 300, Beijing Education Au-light Co., Ltd.). Characterization. Scanning electron microscopy (SEM) images were obtained on FE-SEM (ZEISS, Germany). A Shimadze UV−visNIR UV-3600 double beam spectrophotometer was used to record the absorption, reflectance, and transparency spectra of the membranes. The Raman spectroscopic analysis was carried out on a LabRAM HR 800UV (HORIBA JobinYvon, France) with an excitation laser of 532 nm. The X-ray diffraction (XRD) analysis was conducted with a D8Advance diffractometer (Bruker, Germany). Functional groups and

Figure 1. (a) Diagrammatic cross section of solar steam generation: temperature distribution (left) and GA floating on the water−air interface (right). (b) Photo and SEM image of GA. (c) Schematic illustration of fabrication processes of black GA.

region where water is evaporated to generate solar vapor (Movie S1), and the capillary effect in porous GA provides a steady stream of water. Macroscopic structures of porous GA are shown in Figure 1b, and the microscopic porous structures can be reflected by its BET surface area of 44.75 m2 g−1 and BJH desorption cumulative volume of pores between 1.7000 and 300.0000 nm width of 0.326423 cm3 g−1 with an average pore size of 37.90 nm (Figure S2). The fabrication strategy of accessing GA for harvesting and converting solar energy to interfacial water is schematically shown in Figure 1c. In brief, claybank GO is dispersed in water, forming the GO dispersion with adding trace ammonia (Note S1, Figures S3 and S4). After freeze-drying the GO dispersion, graphene oxide aerogel (GOA) is obtained. Finally, to improve its optical absorption property and structure stability, GOA is rolled to GOAM and then reduced to GA by irradiation under a simulating solar light 4666


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Figure 2. (a) Photos of different materials used in steam generation test. Mass loss (b) and evaporation rate (c) of different materials versus irradiation time under 3 suns. (d) Bulk water temperature distribution monitored by IR camera after 30 min of irradiation.

Figure 3. Optical properties of different materials. (a) Experimental absorption spectra measured in the wavelength ranges of 250−2500 nm. (b) Transmittance in the wavelength range of 250−2500 nm. (c, d) Reflectance of materials in the wavelength range of 250−2500 nm including both diffusive and specular reflectance.

Figure 2b and c show mass changes and water evaporation rates of pure water, rGO dispersion, GO dispersion, GOAM, and GA floating on the water−air interface. During irradiation, steam generation results in mass loss of bulk water. GA floating on the surface of water achieves the most water evaporation in 30 min, while mass loss in pure water is the lowest. At the beginning, the evaporation rates of all samples increase with irradiation

at 10 suns. The solar light is simulated by a xenon lamp in the following experiments. An experimental device is set up to quantify solar steam generation of the materials (Figure S1). The weight changes of bulk water are recorded in real-time by a system with 100 mL of water at room temperature for each sample, and optical photos of the different samples used are presented in Figure 2a. 4667


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that reduced GA appears to be the optimum graphene sheets for full absorption of solar illumination in a wide range of spectrum. During the irradiation process, GA could be also obtained from GOAM within 5 s at a light intensity of 10 suns. Some oxygen-containing groups are removed after GOAM absorbed the intensive light. Characterizations of FT-IR spectra, XPS, XRD data, and Raman spectra of GOAM and GA are used to prove this reduction process (Note S3, Figures S5 and S6). Compared with those traditional chemical and thermal reduction methods, this rapid irradiation reduction is the most suitable method in this work because the structure of GOAM would be destroyed if it was reduced in solvent or in thermal method. Meanwhile, it is observed that the GOAM volume expands and the roughness of its surface increases due to the reaction gas escaping from the inside of GOAM such as carbon dioxide and water, which has a positive effect on improvement of light absorption and heat insulation. After oxygen-containing groups escaped from GOAM in the form of gases, the quality is reduced by about a half (Table S1) and the apparent density is also reduced from 8.08 to 3.21 mg mL−1(Table S2). Though some oxygen-containing groups are removed in GOAM, those remaining ones would exert positive effects on water self-pumping through those channels in GA. The irradiation time has an influence on the structural stability and steam generation of GA. Figure 4 shows the pictures of

time, but the increasing trend of the evaporation rate slows with prolonging irradiation time (∼30 min). The evaporation rate will reach a stable value when there is a balance between the source energy from solar radiation and dissipated energy for steam generation and heating the bulk water and heat loss to the environment. The evaporation rate of GA is 1.4 times, 5 times, and 13 times higher than those of GOAM, rGO dispersion, and pure water, respectively, showing its outstanding harvesting and converting capacities of solar energy. Infrared thermal imaging is an efficient and convenient method to study the temperature of a material. Figure 2d gives temperature distributions of five different samples. A uniform temperature profile is obtained in water and rGO dispersion because of low absorption and large reflectance of solar radiation. This indicates an undesirable matter of bulk water heat behavior in solar-driven water evaporation. In contrast, a higher temperature region exists in GO dispersion, GOAM, and GA. All bulk water of GO dispersion is heated, and the high temperature distribution (red area) is wider than that of GOAM and GA, indicating that more absorption energy is transferred to heat the bulk water in GO dispersion. In contrast, the temperature of the bottom water of GOAM and GA is still low (blue area). The narrower distribution (red area) of high temperature in GA, compared with GOAM, indicated that the heat localization effect in GA is more sensible. The unexpected heat conduction leads to more heat loss, which reduces the utilization efficiency of solar energy. Therefore, GA floating on the water−air interface has the lowest thermal energy loss (for heating bulk water) and the highest evaporation rate of water, which is consistent with results of the mass loss test. Reasons for the high water evaporation rate of GA are systemically explored as follows. The optical properties (reflectance and transmittance) of materials have a direct effect on the absorption efficiency of materials for solar energy. The optical properties of GA, GO dispersion (Note S2), water, and GOAMs with different reduction degrees are studied in Figure 3. Porous GA, appearing black in color, absorbs all visible light, as the color of an opaque material is determined by the wavelength of reflected light. The absorption, transparency, reflectance of samples are studied by a UV−vis-NIR spectrophotometer. Water is used as a contrast material, so its absorption, transmittance, and reflectance is 0%, 100%, and 100%, respectively. The spectral data beyond 1400 nm for water and GO dispersion could not be stable since a water molecule is the main absorber of infrared radiation; so, optical data of water and GO dispersion are shown between 250 and 1400 nm. GA demonstrates the highest absorption and the lowest transparency (ca. 3%) compared with water, GO dispersion, and GOAM in a wide wavelength range from 250 to 2500 nm (Figure 3a and b). There is a sharp increase in absorption and a plunge in transmittance in GO dispersion around 290 nm, which is due to the n → π* transition of CO bonds.42 Those GOAM samples are labeled as GOAM, GOAM-20s, GOAM40s, GOAM-60s, and GA, representing irradiation times of 0 s, 20 s, 40 s, 60 s, and 15 min under a light intensity of 5 suns. In Figure 3c and d, reflectivity of GOAM is affected by its surface structure, which is involved in different irradiation times. It is shown that the reflectance of GOAM is sensitive to irradiation time and decreases with the increase in irradiation time (Figure 3d). It is obvious that GA owns the broadband antireflective property with the lowest specular and diffuse reflection of less than 4% in the range of 250−2500 nm. These results indicate

Figure 4. Morphologies of GOAMs in water with different irradiation times after photothermal test.

samples with varying irradiation time after a photothermal test. Obviously, those first three GOAMs become loose and are dispersed into water after the test, while GA still keeps completely in the water−air interface. The shorter the irradiation time of GOAM is, the worse the stability of GOAM is since the unreduced portion could be apt to be dispersed into the bulk water. Interestingly, it is obvious that the mass loss of water increases with the irradiation time (Figure S7) of GOAM. In contrast, those changes in evaporation rate for GOAMs are insensitive to the irradiation time of GOAM, and GA floating on the water−air interface still presents the highest mass loss and evaporation rate. Other than the high absorption, low transmittance, and reflectance, the high stability of GA makes it an ideal material for harvesting solar energy, and the hydrophilic property and porous structures of GA play important roles in its solar steam generation. Though some oxygen-containing groups have been removed by irradiation, it is the rough surface and remaining hydrophilic groups that lead to its excellent wetting properties,41 which is shown in Movie S2. Porous structures with open or closed pores are furnished during the stacking process of GO sheets, of which the interactions are from hydrogen bonds, van der Waals’ forces, and electrostatic forces. Open pores could improve the capillary effect, while closed ones improve the 4668


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localization and prevents the heat from transferring to heat the nonevaporation portion of water. In addition, the higher surface temperature shows that the local heat effect can be enhanced by increasing the thickness of GA (Figure 5b). There may be two reasons contributing to those higher temperatures in thicker GA. On the one hand, thick GA could increase the optical absorption. On the other hand, the closed pores in GA contributed to a higher insulation,7 and the enhancement of the local heat can improve the mass loss of bulk water and the solar steam generation rate (Figure S12). Thus, GA is expected to obtain effective light absorption, porosity for capillarity, and high thermal insulation. In order to evaluate the solar thermal efficiency of the evaporation process (η evap), an equation is defined as

thermal insulation property of GA. The micromorphology and the distance between sheets of GA could be altered though adjusting the pH of GO dispersion. The distance between graphene sheets in GA, fabricated from the alkaline GO dispersion, is 10.51 Å compared with 8.3 Å from acid GO dispersion (Figure S8), and the looser structure could be found in the alkaline GO compared with the compact structure in acid GO (Figure S9). Compact GA may be associated with the suppressed viscosity of water in the nanopore channels,43 which can cumber the delivery of bulk water to a hot area. So, the upper surface of this porous GA could remain wetted of its selfpumping water through those channels even when the light intensity is 10 kW m−2 (Note S4). Since the thickness of materials has an influence on their light transparency and heat conductivity,32 the increment of the thickness may reduce the transmission of light and improve the heat insulation capacity, leading to a higher local heat effect. We fabricated GAs with different thicknesses from 2 to 15 mm by changing the volume of GO dispersion with a concentration of 3 mg mL−1, and it is impossible to form complete GA when the volume is below 3 mL in this experimental condition. Figure 5a

ηevap =

mfluhvap Qs

× 100% (1)

Figure 5. (a) Optical photos of GAs with different thicknesses. (b) Surface IR images of GAs with different thicknesses floating on the water−air interface after 30 min irradiation (3 suns).

where mflu is the mass loss rate (evaporation rate) of water, hvap is the total enthalpy of liquid−vapor phase change (sensible heat and potential heat), and Qs is the illumination intensity. We neglect the change in thermal physical properties of water with temperature increment such as heat capacity. The evaporation rate η evap of GA is an increasing function of light intensity, and the evaporation efficiency goes from 47.2 ± 2.5% at 800 W m−2 to 82.7 ± 2.5% at 10 kW m−2 (Figure 6a). The same GA is used to perform 10 cycles of photothermal experiments to explore their reusable performances (Figure 6b). The evaporation rate is relatively stable. The evaporation efficiency is still kept at 66.3 ± 2.5% under 3 suns for 10 recycling tests, and the morphology of GA is hardly broken. The intrinsic mechanical property of GA is also beneficial to recycle this robust material, and this porous structure, high absorption, and heat insulation material contributes to the high solar vapor generation efficiency.

shows optical photos of GA with different heights of 2, 5, and 10 mm. Low thermal conductivities of materials are also significant for heat insulation. So, we measured the conductivity of GA in air by an IR camera (Note S5, Figure S11), which is only 0.1868 W m−1 K−1 due to the porous structure of GA. This low thermal conductivity contributes to both high heat

CONCLUSION In summary, a simple, efficient, and low cost process is utilized to fabricate superlight GA, which is for the first time used to generate solar steam. High photothermal conversion efficiency up to 80% of GA in the water evaporation process is attributed to its high sunlight absorption, excellent local heat effect,

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Figure 6. (a) Solar evaporation rate (left axis) and corresponding evaporation efficiency (right axis) of the photothermal conversion process for GA under a range of light intensities. (b) Solar evaporation rate (left axis) and corresponding evaporation efficiency (right axis) of 10 cycles for the same GA under 3 suns irradiation. 4669


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superhydrophilicity, and porous structures. It is found that several factors, such as the reduction degree of GOAM and the thickness of GA, could affect its photothermal performances. Self-floating robust GA has high evaporation efficiency even when it is recycled 10 times. Such a GA can not only be used in the field of heat localization like desalination, wastewater treatment, and sterilization, but can also open a high efficiency way of using solar energy.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b03207. Schematics of experimental setup, nitrogen absorption and desorption measurements, concentration of GO dispersion to fabricate GOAM, choice of GO dispersion and rGO dispersion when operating the optical test, proof of GA reduced from GOAM though sunlight irradiation, mass loss of different GOAMs floating on the water, XRD data and SEM images of GOAMs with different pH, dynamics of wetting, thermal conductivity measurement, mass loss of GAs with different thickness floating on water, and quality and density comparison table about GOAM and GA. (PDF) Movie S1 of solar team generation of GA. (AVI) Movie S2 of contact angle test of GA. (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yang Fu: 0000-0002-1839-6053 Jinhua Li: 0000-0002-5226-0272 Xianbao Wang: 0000-0001-7765-4027 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the Ministry of Science and Technology of China (Grant 2016YFA0200200) and National Natural Science Foundation of China (Grants 51272071, 51203045, and 21401049) and Hubei Provincial Department of Science & Technology (Grant 2014CFA096).

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REFERENCES

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