Functionalized Graphene Enables Highly Efficient Solar Thermal

May 16, 2017 - This will lead to thinner water films close to the three-phase contact line, where the water surface temperature is higher since the re...
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Functionalized Graphene Enables Highly Efficient Solar Thermal Steam Generation Junlong Yang,†,‡,# Yunsong Pang,†,# Weixin Huang,§,∥ Scott K. Shaw,∥ Jarrod Schiffbauer,† Michelle Anne Pillers,∥ Xin Mu,† Shirui Luo,† Teng Zhang,† Yajiang Huang,‡ Guangxian Li,‡ Sylwia Ptasinska,§ Marya Lieberman,∥ and Tengfei Luo*,†,⊥ †

Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering of China, Sichuan University, Chengdu 610065, P. R. China § Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556, United States ∥ Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States ⊥ Center for Sustainable Energy at Notre Dame, Notre Dame, Indiana 46556, United States ‡

S Supporting Information *

ABSTRACT: The ability to efficiently utilize solar thermal energy to enable liquid-to-vapor phase transition has great technological implications for a wide variety of applications, such as water treatment and chemical fractionation. Here, we demonstrate that functionalizing graphene using hydrophilic groups can greatly enhance the solar thermal steam generation efficiency. Our results show that specially functionalized graphene can improve the overall solar-tovapor efficiency from 38% to 48% at one sun conditions compared to chemically reduced graphene oxide. Our experiments show that such an improvement is a surface effect mainly attributed to the more hydrophilic feature of functionalized graphene, which influences the water meniscus profile at the vapor−liquid interface due to capillary effect. This will lead to thinner water films close to the three-phase contact line, where the water surface temperature is higher since the resistance of thinner water film is smaller, leading to more efficient evaporation. This strategy of functionalizing graphene to make it more hydrophilic can be potentially integrated with the existing macroscopic heat isolation strategies to further improve the overall solar-to-vapor conversion efficiency. KEYWORDS: functionalized graphene, hydrophilic groups, solar steam generation, high efficiency evaporation, vapor−liquid interface

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dwindles, the ability to use renewable solar energy to expand the drinking water resources by tapping into the seawater or brackish water can be highly impactful.8−10 Solar-heatingenabled phase transition can also be potentially applied to chemical fractionation.11 Plasmonic gold nanoparticles have shown a promising capability to enable local phase transition of liquid at their vicinities due to highly efficient localized surface plasmon resonance effect.11,12 In such demonstrations, the water vapor was generated despite a bulk temperature lower than the boiling point. Lots of interesting fundamental studies around this topic have been performed.13−15 Neumann et al.11 found that the estimated thermal-to-vapor conversion efficiency of

olar irradiation is the most abundant renewable energy resource in our planet. To make use of solar energy in large scale, it needs to be converted into other forms (e.g., thermal or electrical energy) through efficient and affordable technologies so as to meet the ever growing global energy demand.1,2 Solar thermal technologies, including solar thermal water heater and concentrating solar power, are among the most common forms of solar energy applications. Solar water heaters can be installed on rooftops, but they usually have lower efficiency.3,4 Concentrating solar power can potentially have high efficiency and can be combined with thermodynamic cycles to produce electricity, but they are only economical at large centralized scale.5,6 The ability to use solar heat for efficient liquid-to-vapor phase transition can potentially enable a wide variety of applications. For example, solar heatinginduced water evaporation can be used for distillation water desalination and sanitization.7 As water and fossil energy both © 2017 American Chemical Society

Received: January 17, 2017 Accepted: May 16, 2017 Published: May 16, 2017 5510

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Figure 1. (a), (b), and (c) XPS C 1s spectra of GO, crGO, and f-crGO and the contact angles (inset) of each sample. Based on the analysis of the XPS peaks, 4% carboxyl group was featured in f-crGO. (d) SEM images of f-crGO from cross-section view showing highly porous structures, which enable good water wicking ability.

In most of the above-mentioned studies, hydrophilic features of the solar absorbers were emphasized mainly to wick water into the solar-heated structure to enable a passive flow for continuous evaporation. However, what has not been systematically investigated is that hydrophilicity might have also helped water−solid interaction to alter the liquid−vapor interface to impact the evaporation process. For example, it is known that thin liquid films close to the three-phase contact line can lead to improved evaporation efficiency.30,31 Hydrophilic surfaces can elongate the liquid thin-film regime and thus may enhance evaporation. Meanwhile, functionalized hydrophilic surfaces have also been shown to enhance heat transfer from the solid to contacting liquids,32−36 which may also influence the temperature distribution of the liquid phase and thus evaporation. In parallel to the macroscale thermal insulation, tuning the hydrophilicity may create new opportunities to further improve the overall efficiency of solar thermal phase change applications. In this work, we demonstrate that by functionalizing graphene using hydrophilic groups, such as hydroxyl (−OH) and carboxyl (−COOH) groups, we are able to improve the overall solar-to-vapor efficiency. Even with a relatively small increase in hydrophilic functional groups, obvious improvements in efficiency are observed, with the largest found to be 26% at 1 sun (i.e., overall efficiency increases from 38% to 48%). We explore the mechanism of such improved efficiency using a combination of experiments and theoretical analyses. We find that hydrophilic functionalization may lead to thin-film water evaporation near the three-phase contact line at the functionalized graphene surface, which enhances the overall evaporation performance. These results may provide insights to

such plasmonic nanoparticle-induced thermal effect was 82% when ignoring heat losses. However, the overall solar-to-vapor efficiency was around 24%, largely due to the optical spectra mismatch between the gold nanoparticles and the solar irradiation. Graphite has long been known as one of the best solar absorbers16,17 and explored for solar thermal energy storage applications.18−21 This is mainly because they are black (i.e., low reflection loss) and absorb light well over the whole solar spectrum. Taking advantage of such a feature, it was recently demonstrated that by thermally insulating a solar-heated expanded graphite powder, the overall solar-to-vapor conversion efficiency can be as high as 85% at 10 suns.22 Besides the good solar absorption of expanded graphite, such a success is also largely due to the special macroscopic thermal insulation, which minimizes heat loss to the environment. Further studies around the thermal insulation have been performed,23,24 and an overall efficiency of ∼64% at 1 sun was recently achieved when using thermal insulating materials to localize heat within a spectrally selective absorber.23 The good optical absorption of graphite has also benefited volumetric solar heating of nanofluids for direct vapor generation,25 although the efficiency appears to be lower due to the lack of thermal insulation. More recent works revealed that an 83% efficiency can be achieved under 1 sun illumination (1 kW m−2) using a free-floating graphene oxide-based aerogel with carefully tailored properties.26 By developing spectrum-selective plasmonic absorbers27,28 or designing confined two-dimensional water paths,29 the solar-to-vapor conversion efficiency can be further improved to above 90%. 5511

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content after reduction and functionalization compared to GO (Figure S2) most likely due to the use of hydrazine hydrate and nitric acid when preparing crGO and f-crGO, respectively. However, the FTIR spectrum (Figure S1) shows that the C− OH population has also been increased after functionalization (i.e., from crGO to f-crGO). As a result, the higher C−OH/C− N peak in the C 1s spectrum after functionalization should be due to the increase of both C−OH and C−N densities. While it is difficult to quantitate their ratios, both of these two functional groups are more polarized than the normal C−C bonds in the graphene,40,41 and they should both promote stronger hydrophilic interactions between the graphene derivatives and water molecules. We have also performed atomic force microscopy (AFM) of a f-crGO layer separated from the powder (Figure S3). The thickness of the f-crGO layer is approximately 1.0 nm, which is thicker than that of the defectfree monolayer graphene (∼0.34 nm). This is likely due to the grafted functional groups after the functionalization process, which agrees with that previously found in ref 42. In addition, the f-crGO layer shows a relatively uniform thickness without any obvious signs of small molecular attachments, indicating that byproducts have been removed by the washing step in the f-crGO preparation process. To further evaluate the role of functionalization on water wettability, we conducted contact angle measurements on coldpressed pellets from the corresponding powders (see SI Section 1 for details). As shown in the insets of Figure 1a−c, the contact angles correspond well with the densities of the total oxygen/nitrogen-related functional groups in these samples (Table 1). GO shows the smallest contact angle (85 ± 2°), which means that it is the most hydrophilic among all three samples, has the largest oxygen functional group density of 61.05%. crGO shows the least hydrophilic feature (106 ± 2°) of the three samples, suggesting that the removal of oxygen groups can significantly change the wettability of graphene, as indicated by the total oxygen functional group density reducing to 23.09%. f-crGO, which features an increased total oxygen functional group density of 32.82% compared to that of crGO, becomes more hydrophilic (87 ± 2°). Although the overall oxygen-containing functional groups in f-crGO have much smaller density compared to that of GO (Table 1), their contact angles are close. Carefully comparing the oxygen groups in f-crGO and GO (Figure 1 and Table 1), it indicates that although the epoxy (C−O−C) and carbonyl (CO) groups are decreased, C−OH/C−N and carboxyl (O−CO) groups are slightly increased, implying the significant roles of these groups in enhancing the interaction between graphene and water. This is likely because the carboxyl and hydroxyl groups are overall more polarized than carbonyl and epoxy groups and thus can form stronger hydrogen bonds with water molecules.40,41 Notably, it has been shown that one carboxyl group can form two hydrogen bonds with two water molecules.43 It is noted that although the three graphene derivatives show different intrinsic hydrophilicities, their powders can still have very good wicking ability due to the porous structure of solar absorber layer (see Figure S4 and Supplementary Videos S1, S2, S3, and S4 in SI). As an example, the cross-sectional SEM of a compacted f-crGO absorber layer shows a porous structure (Figure 1d). Figure 4, shown later, includes the SEM images of the surfaces of the three graphene samples (uncompacted), which also illustrates the porous feature of the three types of graphene-based solar absorbers.

the rational design of solar absorbing materials, which can be readily combined with the existing macroscopic thermal insulation strategies, to advance the solar thermal phase transition technologies.

RESULTS AND DISCUSSION Material Characterization. Three types of graphene derivatives are used as the solar absorber to understand the effect of functional groups on the solar thermal evaporation. These include graphene oxide (GO), chemically reduced graphene oxide (crGO), and functionalized crGO (f-crGO). Details of synthesis are included in the Methods Section. Different techniques are used to characterize the synthesized graphene derivatives (see Supporting Information (SI) Section 1 for details). X-ray photoelectron spectroscopy (XPS) is employed to study the changes in functional groups of all three samples (Figure 1). The C 1s XPS spectrum for GO shows different peaks centered at 284.5, 285.6, 286.7, 288.1, and 289.1 eV (Figure 1a), corresponding to the C−C, C−OH/C−N, C− O−C, CO, and O−CO bonds, respectively.37 After reduction from GO to crGO by hydrazine, the intensities of all C 1s peaks of the carbons bound to oxygen decreased, accompanied by the significant increase in the C−C peak (Figure 1b). This reveals that most of the oxygen-based functional groups are removed after reduction and the integrity of graphene is largely restored. Further analysis by characterizing the specific percentages of different types of carbon bonds (percentiles shown in Figure 1 and Table 1) shows that the Table 1. Percentage of Different Types of Carbon Bonds Calculated from the Areas of the Fitted XPS Peaks bond type

GO (%)

crGO (%)

f-crGO (%)

C−C C−O−C CO C−OH/C−N O−CO total functional group

38.95 46.47 10.12 2.47 1.99 61.05

76.91 6.86 2.06 14.17 0.00 23.09

67.18 7.83 5.40 15.51 4.08 32.82

most reduced oxygen functional groups are the epoxy (C−O− C) and the carbonyl (CO) groups, and some of them might have been converted into other hydrophilic groups as evident by the increased C−OH/C−N populations. Carboxyl groups have also been largely reduced as indicated by the eliminated O−CO peak (Figure 1b and Table 1). After functionalization, it is clear from Figure 1c and Table 1 that the peaks corresponding to the oxygen/nitrogen functional groups increase, and specifically a peak for the O−CO bond has emerged in the f-crGO. Based on the peak area ratios in the C 1s peak, the concentrations of all the oxygen/nitrogen groups improved in f-crGO featured by a ∼ 4% increase in carboxyl groups (Table 1). The functionalization is also confirmed by the Fourier-transform infrared (FTIR) spectrum of f-crGO, which compared to that of crGO shows more pronounced peaks corresponding to the relevant carbon oxygen bonds (see Figure S1 in SI).38 It is worth noting that the C−N peak (285.9 eV) and the C− OH (285.6 eV) peak are very close to each other in the C 1s XPS spectrum,39 and thus they are fitted using a single peak. It was also reported that hydrazine hydrate does not reduce the C−OH groups effectively.38 We also examined the N 1s XPS spectra and found that there is indeed increase in the nitrogen 5512

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Figure 2. (a) A schematic illustration of the solar water generator system to convert solar light into water steam. (b) A schematic of the solar steam generation experimental setup and the solar-vapor generator floating on water in a beaker wrapped with thermal insulating materials (insets). (c) Optical image of steam generation using f-crGO as solar absorber under the solar irradiation intensity of 3.6 kW/m2 (steaming video is also available in Supplementary Video S5).

Figure 3. (a), (b), and (c) The mass change of water due to solar thermal evaporation using different graphene derivatives under the solar intensity of (a) 1.0 kW m−2, (b) 2.0 kW m−2, and (c) 3.6 kW m−2 under room temperature of 22 °C and humidity of 60%. The crude curves in that figure are probably ascribed to the high resolution of the balance used to track the weight of the sample, which leads to a small fluctuation in mass change under low solar intensity. (d) The calculated overall solar-to-vapor conversion efficiency with the dashed lines drawn to guide the eye.

Solar-to-Vapor Efficiency. The graphene derivatives, as solar absorbers, are responsible for converting solar irradiation

into heat, and thus their optical properties are also important. The optical transmittance of the GO, crGO, and f-crGO is 5513

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Figure 4. (a) The temperature of graphene derivatives and water under the 3.6 kW m−2 solar illumination intensity. SEM images of GO (b), crGO (c), and f-crGO (d) surface measured with dried samples. The stacked GO sheets remain a flat surface while both crGO and f-crGO show porous structures.

solar energy, convert it into heat and evaporate the water wicked into the structure (Figure 2c). While we did not focus on special macroscopic thermal insulation materials (e.g., carbon foam floater,22 aerogel matrix,44 and air pack cover23) to minimize heat loss to the bulk water, we wrapped the beaker wall with a simple thermal insulating glass fiber to prevent excessive heat loss to the environment (Figure 2b inset). However, since our focus of this study is the functionalization of the solar absorbing material, we did not try to perfect the thermal insulation with knowing that our functionalization strategies can be easily combined with existing macroscopic thermal insulation methods to benefit the solar thermal evaporation process. The whole steam generator is then placed on a precision balance and exposed to a solar simulator with an illumination intensity up to 3.6 kW m−2 (Figure 2b and Sections 3 in SI). The water evaporation rates are measured by recording the mass change as a function of time under the solar intensities of 1.0, 2.0, and 3.6 kW m−2. Each experiment lasts for more than 4000 s. Every experiment is repeated 3−4 times, and the results are averaged for each experimental condition (i.e., different samples and solar intensities). The energy conversion efficiency (ηth) is calculated to evaluate the overall solar-to-vapor efficiency of all three samples, which is defined in the same way as those in recent studies:22,24,26,45

measured using a UV−vis spectroscope (Figure S5a in SI), and the total reflectance including both diffusive and specular reflectance of each sample is also measured using a UV−visNIR spectroscope (Figure S5b in SI). Both these two characterizations show that GO has higher reflectance and transmittance, suggesting inferior solar absorption than that of crGO and f-crGO, which have virtually no difference in the optical properties. Actually, GO shows a dark brownish color while crGO and f-crGO are black (insets in Figure S5a in SI). These results indicate that the hydrophilic functionalization (fcrGO) does not impair the solar absorption ability of crGO, allowing us to directly characterize the effect of functionalization on the heat-to-vapor conversion. When working as solar absorbers for water evaporation, the graphene-based samples are soaked with water. As a result, we further measured their total reflectance in the wetted state and found that all reflectance is reduced (Figure S5b), which may be attributed to water absorbing light. However, wet crGO and f-crGO still show almost identical reflectance, which is still smaller than that of the wet GO. To perform the solar thermal phase transition experiments, graphene derivatives (∼100 mg) are loaded into a boat made of a thin hydrophilic membrane (0.1 mm) attached to a polystyrene foam ring so that the whole structure floats on water due to buoyancy in a snuggly fitted beaker (Figures 2b and S6). The hydrophilic polymer membrane enables water to be wicked into the graphene solar absorbers through capillary action to provide continuous water supply during steam generation. Under solar irradiation, graphene derivatives absorb

ηth = 5514

mh ̇ LV Coptqi

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ACS Nano where ṁ is the mass loss rate per unit area calculated from the slop of mass loss curves at the steady state, hLV is the total enthalpy of sensible heat and liquid−vapor phase change, Copt is the optical concentration, and qi is the nominal direct solar irradiation of 1 kW m−2. More details on the description of estimation of conversion efficiency can be found in Section 4 of the SI. At 1 kW m−2 solar intensity, the evaporation rates for GO, crGO, and f-crGO are measured to be 0.34, 0.37, and 0.47 kg m−2 h−1, respectively, while it is 0.10 kg m−2 h−1 for pure water without any graphene solar absorber (Figure 3a). As solar illumination intensity increases, the evaporation rates of each sample increase accordingly (Figure 3b,c). Using eq 1, we have calculated the solar-to-vapor efficiencies of the three graphenebased solar steam generators and plotted in Figure 3d. Among the three graphene derivatives, GO shows the lowest solar thermal evaporation rates and lower overall efficiencies under all illumination intensities. This can be attributed to the lower absorption of solar light compared to the other two graphene derivatives (Figure S5 in SI), which can limit energy harvesting from solar irradiation. crGO and f-crGO, both of which have good solar absorption over the whole spectrum range (Figure S5 in SI), show larger evaporation rates and higher efficiencies than those of GO. At 1 kW m−2 solar intensity, f-crGO achieved the highest water evaporation rate of 0.47 kg m−2 h−1 among all three samples. It is instructional to compare this value to that of crGO (0.37 kg m−2 h−1), which indicates a 26% improvement in the overall efficiency (from 38% to 48%, Figure 3d). The observed overall efficiency improvements from crGO to f-crGO are 10% and 8% for solar illumination intensities of 2.0 kW m−2 and 3.6 kW m−2, respectively. Since optical properties and thus solar-toheat efficiency are virtually the same for crGO and f-crGO (Figure S5 in SI), we can attribute the improvements in the overall solar-to-vapor conversion efficiency to the enhancement in the thermal-to-vapor conversion efficiency. On the other hand, compared to GO, the increases in efficiency are 36, 29, and 11% for the three solar intensities for f-crGO, and such larger improvements can be attributed to the better solar absorption performance. It is worth noting that at 3.6 kW m−2, the overall solar-to-vapor efficiency using f-crGO can reach 81%, which is even higher than that achieved using expanded graphite powder with dedicated thermal insulations (∼67%) at the same solar intensity.22 Mechanism Understanding. To help understand the mechanism of the enhancement in solar thermal evaporation achieved through graphene functionalization, the temperature evolution of the graphene samples and that of water in the beaker at the solar intensity of 3.6 kW m−2 are monitored in real-time during the experiments. One thermocouple is placed on top of the graphene sample surfaces and the other is fixed in water 2 mm underneath the membrane (inset in Figure 4a). The top thermocouple is placed just under the surface of the graphene samples with a very thin layer of graphene covering the thermocouple, which largely eliminates the effect of direct solar irradiation on the temperature measurement. The bottom thermocouple is fixed using a 3D-printed frame to keep it in the same position for all three experiments. As shown in Figure 4a, the temperatures of the crGO and f-crGO samples quickly reach a steady state of 80 °C within 200 s, whereas that of the GO sample is around 70 °C at the same time and slowly increases to 75 °C after running the experiment for 4000 s. The temperature history of water 2 mm underneath the solar-vapor

generator indicates a continuous temperature increases of bulk water, suggesting that heat leaked into the bulk water instead of being localized within the graphene region. This is likely because no special thermal insulation layers are placed inbetween the solar-vapor generator and the bulk water as those used in some other recent studies.22,23 Moreover, it is apparent that when crGO and f-crGO are used as the solar absorbers, the water temperatures are lower than that of the GO case, which is likely due to the fact that part of the solar energy in the nearinfrared region (>700 nm) can transmit through the GO layer (Figure S5 in SI) and thus heat water directly. It is noted that the thermocouple placed at the surface of the soaked porous graphene samples is largely in contact with liquid water, and thus the measured steady state temperature is different from the actual vapor temperature (100 °C) as found in refs 22, 24. Liquid-to-vapor phase transition can happen both in nonequilibrium boiling or equilibrium evaporation. Given the low solar intensity (up to 3.6 kW/m2), nonequilibrium boiling at the vicinity of graphene flakes is not possible, which is consistent to the conclusion from refs 22, 25 (see Section 5 and Figure S7 in SI). Here, we focus on the difference in equilibrium evaporation efficiency between crGO and f-crGO, which have similar optical properties but differ in hydrophilicity. Since the equilibrium evaporation is a surface effect, we expect that even without solar irradiation, the natural evaporation of water from the three graphene samples should already show difference in efficiency. To verify this point, we performed natural evaporation experiments, where the whole beaker with different graphene samples were maintained in the same dark environment (25 °C and 60% relative humidity) for 1 h, and the natural evaporation performances are evaluated by tracking the mass loss rate (Figure S8 in SI). Comparing the mass loss rates in Figure S8, it is clear that fcrGO enables more efficiency evaporation than crGO, which is consistent with their respective hydrophilicity as indicated from the contact angles (Figure 1). This is also consistent with our solar thermal evaporation experimental results (Figure 3). Moreover, it is understandable that crGO has slightly smaller evaporation efficiency than GO considering the differences in hydrophilicity (Figure 1). However, the difference in evaporation rates between GO and crGO is not as big as the difference between f-crGO and crGO. Besides the intrinsic hydrophilicity of the materials, the surface morphology can also play a role in evaporation.46 To understand the reason, we took SEM images of the sample surfaces and found that crGO and fcrGO show very close surface morphology visually while that of GO is evidently different (Figure 4b−d). It appears that the surfaces of crGO and f-crGO are much more porous than that of GO. As a result, it is very possible that the effective surface area of GO is less than those of crGO and f-crGO, which limits the evaporation. To further quantify the porous feature of the samples, we have determined the specific surface area of the three samples using the BET (Brunauer−Emmett−Teller) method. The data in Figure S9 show that GO has a much smaller specific surface area of ∼200 m2/g than those of crGO and f-crGO, which are 515 m2/g and 400 m2/g, respectively. The much larger specific surface areas of crGO and f-crGO than GO are likely due to the exfoliation of graphene stacks during the reduction processes, where ultrasonication was used. The slightly reduced specific surface area of f-crGO compared to crGO is probably caused by the presence of hydrophilic functional groups which may have led to graphene sheets to restack in the liquid-phase process due to interlayer attractions. 5515

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Figure 5. (a) Profile of the liquid meniscus between two graphene flakes separated by 50 μm predicted from the Young−Dupree equation. Predicted (b) liquid surface temperature and (c) mass loss rate as a function of y. Note: due to symmetry, only half of the temperature and mass loss profiles are shown.

additional liquid above the flat liquid line will further lower the liquid surface temperature due to added resistance and thus further reduce the mass flux (Figure 5a). The predicted liquid surface temperature profiles for the hydrophilic meniscus corresponding to a contact angle of 87° (f-crGO) and the flat meniscus are shown in Figure 5b, and the corresponding mass loss rates are shown in Figure 5c. The values for the areaaveraged mass loss rate for f-crGO and the flat meniscus are 0.0252 kg/m2-s and 0.0249 kg/m2-s, respectively. The mass loss rate of a hydrophobic meniscus (crGO) is expected to be further smaller. As a result, we believe that the hydrophilic feature of the f-crGO is the key to explain the trend we observed in our solar-thermal evaporation experiments. Beside surface morphology and hydrophilicity, there was another mechanism recently proposed that can potentially lead to improved evaporation at heated solid surfaces.48 From a recent experimental study, it was hypothesized that enhanced escape rate of molecules at the very tip of the three phase contact lines can help evaporation. In our case, there are two competing effects when the graphene is more hydrophilic. On one hand, the improved thermal conductance can improve the temperature of the molecules. One the other hand, the more hydrophilic graphene will also lead to stronger binding between molecules and graphene. To estimate the competing effects of these factors, we compared the Boltzmann factors (e−ΔE/kBT) of the hydrophobic and hydrophilic cases, which roughly indicates the relative stability of a water molecule attached to the heated graphene surfaces. When considering a heat flux of 3.6 kW/m2 and assuming the temperature of graphene to be the same, the temperature difference between water molecules attached to crGO and f-crGO is expected to be on the order of 10−3 K, which is negligible (see Section 6 in the SI). Since the interfacial binding energy of a hydrophilic surface is a few times larger than that of a hydrophobic surface,32,34 the hydrophilic graphene will make the water molecules attached to it much more difficult to escape, i.e., lower evaporation rate. As a result,

However, despite the decreased specific surface area, which should also mean a decreased effective area of the top surface, the increased evaporation performance of f-crGO further highlights the importance of hydrophilicity in evaporation. To further verify the hydrophilic effect, we used the firstorder asymptotic solution to the Young−Laplace equation to study the liquid surface profile between two parallel graphene flakes separated by 50 μm. The solution obtained by expanding in δ = w/2r, where w is the thickness of graphene and 2r is the separation distance between graphene flakes, and demanding y2 − r 2

symmetry about y = 0 is z(y) ≅ (1 − sinθ ) rcosθ + O(δ 3) for the liquid−vapor profile (Figure 5a). Since we believe that the hydrophilicity is the major difference maker between crGO and f-crGO, we used their respective contact angles (106° and 87° shown in Figure 1) for the calculation. The calculated liquid meniscus profile is shown in Figure 5a. The calculation of the evaporative mass loss follows the method outlined in ref 30, which considers the Hertz-Knudsen equation for mass loss, Fourier’s Law of heat conduction and liquid−vapor interface energy balance. We use 0.072 N/m2 for the surface tension, 0.6 W/m2 for water thermal conductivity, 40.65 × 103 J/mol-K for enthalpy of vaporization of water,47 and 0.3 for the accommodation coefficient. 30 The temperatures of the graphene flakes are set to be 80 °C according to that measured in Figure 4a. We should note that this method, mainly the spatial heat conduction decomposition, is not directly applicable to the hydrophobic case where a convex meniscus profile is present, where a directly two-dimensional numerical simulation is needed to iteratively solve the above-mentioned three intercorrelated equations. Such a dedicated numerical simulation falls out of the scope of this study, especially since the purpose of this analysis is to identify possible mechanisms for our experimental observation. Thus, instead, we studied the trend of temperature profile and mass flux for the concave meniscus up to a flat one, while understanding that the trend will persist into the convex regime considering that the 5516

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AUTHOR INFORMATION

this mechanism can be excluded from the reason for the observed improved evaporation of f-crGO.

Corresponding Author

*E-mail: [email protected].

CONCLUSIONS We have shown that by functionalizing graphene, the solar thermal water evaporation efficiency can be enhanced at all solar illumination intensities studied (up to 3.6 W m−2). With small increases in hydrophilic functional groups, the overall solar-to-vapor efficiency is improved from 38% to 48% at 1 sun. Our experiments combined with theoretical analysis show that such an improvement is a surface effect mainly attributed to the more hydrophilic feature of functionalized graphene, which influences the water meniscus profile at the vapor−liquid interface due to capillary effect. This will lead to thinner water films close to the three-phase contact line, where the water surface temperature is higher since the resistance of thinner water film is smaller, leading to more efficient evaporation. This strategy of functionalizing graphene to make it more hydrophilic can be potentially integrated with the existing macroscopic heat isolation strategies to further improve the overall solar-to-vapor conversion efficiency.

ORCID

Junlong Yang: 0000-0002-8987-3769 Yajiang Huang: 0000-0002-1803-1580 Sylwia Ptasinska: 0000-0002-7550-8189 Marya Lieberman: 0000-0003-3968-8044 Tengfei Luo: 0000-0003-3940-8786 Author Contributions #

J.Y. and Y.P. contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge the financial support from Army Office of Research (W911NF-16-1-0267) managed by Dr. Chakrapani Venanasi. The experimental part was supported by the Notre Dame Materials Characterization Facility and the Integrated Imaging Facility. The computation work was supported in part by the University of Notre Dame, Center for Research Computing, and NSF through XSEDE resources provided by SDSC Comet and TACC Stampede under grant number TG-CTS100078. J.S. thanks the support from the ND Energy postdoctoral fellowship. J.Y., Y.H., and G.L. thank the support from the National Natural Science Foundation of China (51421061) and the Programme of Introducing Talents of Discipline to Universities (B13040). J.Y. also thanks the support from the Chinese Scholarship Council. Y.P. thanks Xingfei Wei for his help in hydrogen bond identification. We also would like to thank Prof. Paul McGinn and Robert Jonson for their help in preparing the cold pressed graphene tablets for the contact angle measurements.

METHODS Material Synthesis. GO was first prepared by an improved Hummers’ method49 and then reduced by hydrazine with the assistance of an ammonia solution.50 The f-crGO was synthesized by a mild acid treatment, which decorates the crGO with more hydrophilic functional groups, such as hydroxyl and carboxyl groups.51 To prepare GO using the improved Hummers’ method,49 a 9:1 mixture of concentrated H2SO4/H3PO4 (360:40 mL) was gradually added to a mixture of graphite powder (3.0 g) and KMnO4 (18.0 g). The reaction was maintained at a preheated oil bath (50 °C) and stirred for 12 h. The reaction was cooled to room temperature and then poured into ice (400 mL). Superfluous H2O2 aqueous solution (30%) was added to reduce the residual KMnO4 until the bubbling disappeared. Finally, the obtained mixture was washed by a hydrochloric acid solution (2 wt%) until the sulfate ion could not be detected by barium chloride. A homogeneous suspension was collected after washing with deionized water which removes acid. GO powder was obtained by freeze-drying the suspension. crGO was prepared by reducing GO using hydrazine with the assistance of an ammonia solution.50 Typically, 0.5 g of GO was suspended into 500 mL water and exfoliated by ultrasonication at 200 w for 10 min to obtain a yellow-brown dispersion. Then, 0.35 g of hydrazine hydrate and 2.45 g of ammonia solution were added into the solution, which is subsequently placed into an oil bath at 95 °C for 3 h. After washing with deionized water for several times, crGO was achieved by filtering the dispersion and then dispersed in water again for further use. fcrGO was obtained by refluxing crGO with dilute nitric acid (2 M) for 12 h following the procedure described in ref 51. The produced fcrGO was then washed with distilled water and centrifuged repeatedly to remove trace amount of acid. f-crGO was then dispersed in water for further use.

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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/acsnano.7b00367. Information on details of materials synthesis, characterization methods, solar-vapor generator, experimental setup, estimation of energy conversion, AFM pictures, FITR spectra, XPS N 1s spectra, wettability study, optical transmittance of dry and wet samples, possibility of nonequilibrium evaporation, and estimation of molecular temperature attached to graphene (PDF) 5517

DOI: 10.1021/acsnano.7b00367 ACS Nano 2017, 11, 5510−5518

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