WoodâGraphene Oxide Composite for Highly Efficient Solar Steam

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Wood−Graphene Oxide Composite for Highly Efficient Solar Steam Generation and Desalination Keng-Ku Liu,† Qisheng Jiang,† Sirimuvva Tadepalli,† Ramesh Raliya,‡ Pratim Biswas,‡ Rajesh R. Naik,*,§ and Srikanth Singamaneni*,† †

Department of Mechanical Engineering and Materials Science, Institute of Materials Science and Engineering, Washington University in St. Louis, St. Louis, Missouri 63130, United States ‡ Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, Missouri 63130, United States § 711 Human Performance Wing, Air Force Research Laboratory, Wright-Patterson Air Force Base, Dayton, Ohio 45433, United States S Supporting Information *

ABSTRACT: Solar steam generation is a highly promising technology for harvesting solar energy, desalination and water purification. We introduce a novel bilayered structure composed of wood and graphene oxide (GO) for highly efficient solar steam generation. The GO layer deposited on the microporous wood provides broad optical absorption and high photothermal conversion resulting in rapid increase in the temperature at the liquid surface. On the other hand, wood serves as a thermal insulator to confine the photothermal heat to the evaporative surface and to facilitate the efficient transport of water from the bulk to the photothermally active space. Owing to the tailored bilayer structure and the optimal thermo-optical properties of the individual components, the wood−GO composite structure exhibited a solar thermal efficiency of ∼83% under simulated solar excitation at a power density of 12 kW/m2. The novel composite structure demonstrated here is highly scalable and cost-efficient, making it an attractive material for various applications involving large light absorption, photothermal conversion and heat localization. KEYWORDS: graphene oxide, wood, photothermal, solar steam, desalination

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INTRODUCTION

promising for water purification using sustainable energy source.2,5,6 However, low efficiency due to the heat loss associated with heating the bulk water and the requirement for high optical concentration limit the utilization of solar desalination in stand-alone solar power applications.3,7 In the last 3 years, significant research efforts have been dedicated to minimize the heat losses and improve the efficiency of water desalination by using a bilayered structure for solar steam generation.8−10 In a typical bilayered structure, the top layer is comprised of a photothermal material that efficiently absorbs light and converts it into heat. The bottom layer, typically with

Meeting the ever increasing fresh water needs of the growing world population is one of the most serious global challenges of the 21st century.1 Apart from the improved use of existing fresh water resources, desalination and water reuse are considered to be critical to overcome water scarcity that is affecting roughly half of the world’s population. Two methods, namely thermal desalination and reverse osmosis technology, have been widely employed for desalination of seawater, which represents a virtually unlimited source.2 Solar water desalination, which relies on a sustainable and renewable energy source, is a promising method to alleviate fresh water scarcity in parts of the world with ample sunlight with low environmental impact.3,4 Steam generation using solar energy has been proven to be technically feasible and considered to be a highly © 2017 American Chemical Society

Received: January 25, 2017 Accepted: February 2, 2017 Published: February 2, 2017 7675

DOI: 10.1021/acsami.7b01307 ACS Appl. Mater. Interfaces 2017, 9, 7675−7681

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ACS Applied Materials & Interfaces

Figure 1. Schematic illustration depicting the fabrication of wood−GO composite and setup for solar steam generation using the bilayered composite structure.

Figure 2. (A,B) Low- and high-magnification SEM images of wood cross section showing the microchannel structures of wood. (C) SEM image showing the long microchannels in the wood. (D) Absorption spectrum of radially cut wood. (E−F) Thermal conductivity of wood in dry- and wetstates (inset of each panel showing the temperature gradient along the thickness of wood).

candidate for steam generation applications.16−19 The unique optical properties of GO have been extensively investigated for various optoelectronic and biomedical applications.18,20 More recently, we have demonstrated a bilayered hybrid biofoam composed of reduced GO/bacterial nanocellulose as a photothermal material for a solar steam generation, which exhibited excellent steam generation efficiency and stability.21 Apart from the materials employed in the photothermal layers, support materials on which the photothermal layers are deposited are equally important for a high-efficiency solar steam generation. The key considerations for such support materials are hydrophilicity and porosity for efficient transport of water from the bulk to the evaporative surface, low thermal conductivity to impede the flow of heat from the evaporative surface to the bulk water, lightweight to ensure that the

low thermal conductivity, serves as a thermal insulation layer to minimize the heat loss to the bulk water, thus improving the overall efficiency of the solar steam generation. A number of efforts have been dedicated to the use of novel nanomaterials as light absorbing and heat generating materials. For example, plasmonic nanostructures, which exhibit large absorption and scattering of light in the visible and nearinfrared (NIR) regions of the electromagnetic spectrum, have been demonstrated to be excellent candidates for the steam generation.6,9,11−13 Owing to their photothermal properties, carbon-based materials such as graphene,14 carbon black nanoparticles15 and carbon foam8 have also been employed for solar steam generation. Yet another promising photothermal material, graphene oxide (GO), exhibits a broadband light absorption from visible to NIR range, making it an excellent 7676


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Figure 3. (A) AFM image of GO flakes deposited on a silicon substrate. (B) Absorption spectrum of GO flakes dispersed in water.

photothermally generated heat to the evaporative surface. To investigate the ability of wood to confine heat at the evaporative surface, we have measured the thermal conductivity of wood in both wet and dry states. The thermal conductivity of wood was obtained using infrared images of wood sandwiched between two glass slides held at two different temperatures (see the Experimental Section for details). The IR images of the wood reveal a gradient in the temperature along the thickness (inset of Figure 2E). The thermal conductivity of wood in the dry state was found to be 0.120 W m−1 K−1, which is higher than that of air (0.024 W m−1 K−1 at room temperature) and significantly lower than that of water (0.600 W m−1 K−1) (Figure 2E). The thermal conductivity of wood obtained here is in agreement with previously reported value.24 Wood, as a support material with a low thermal conductivity, can efficiently suppress photothermal heat transfer to the bulk water and improve the solar steam generation efficiency. Because the wood piece is in a hydrated state during solar steam generation, it is important to investigate the thermal conductivity in the wet state. For the wet wood, the thermal conductivity was found to be 0.525 W m−1 K−1 (Figure 2F), which is larger than that in the dry state but is still lower than the thermal conductivity of water and seawater.25 The thermal conductivity of wood in the wet state is comparable to the heat-insulating materials reported in recent literature for solar steam generation.8,14 It is worth pointing out that the thermal conductivity of wood in the wet state is lower than that of the exfoliated graphite layer with water (0.959 W m−1 K−1), which has been employed for solar steam generation.8 GO flakes were synthesized using a method reported by Tour and co-workers.26 Atomic force microscopy (AFM) image revealed the thickness of GO flakes deposited on a silicon substrate to be ∼1.0 nm (Figure 3A). The thickness of GO flakes corresponds to monolayer and bilayer of GO. The Raman spectrum of GO flakes revealed the characteristic graphite band (G-band) at ∼1580−1600 cm−1 and defect band (D-band) at ∼1330−1350 cm−1 (Figure S1A). GO flakes dispersed in water exhibited a broad optical absorption in the visible and NIR parts of the electromagnetic spectrum (Figure 3B). As noted above, the high absorption of GO within the solar spectrum makes GO a highly promising material for photothermal heating under solar illumination and solar steam generation. Wood−GO composites have been prepared by depositing the aqueous GO solution on the surface of the wood followed by natural drying (see the Experimental Section for details). Following the deposition GO on the wood surface, the

materials remain afloat on water surface, and cost-efficiency and scalability for real-world application. A number of materials such as anodic aluminum oxide (AAO) membrane,13 gauze,15 paper10 and biofoam based on bacterial nanocellulose21 have been employed as the supporting materials for steam generation in the past three years. Here, we demonstrate that a number of inherent physical and chemical properties of wood such as high porosity, lightweight, low thermal conductivity and hydrophilicity make it an excellent material for a solar steam generation. We introduce wood−GO composite for a solar steam generation that enables heat localization at the evaporative surface and provides efficient transport of water to the evaporative surface through the microchannels of the wood. The unique properties of wood, as well as GO, are well-suited for high optical absorption, photothermal conversion, heat localization, water transport and rapid evaporation resulting in a highly efficient solar steam generation system. The wide availability of wood combined with the simple coating process makes the wood−GO composite demonstrated highly attractive for steam generation and water distillation in resource-limited settings with ample sunlight. Owing to its abundance, biocompatibility and natural vessel structure, wood has attracted significant attention in various advanced applications including green electronics, biological devices, bioenergy and energy storage.22,23 The fabrication of wood−GO composite involves the deposition of GO flakes on the surface of a radially cut piece of wood (Figure 1). SEM image of the top surface of the wood depicts the highly porous microstructure of wood (Figure 2A,B). Cross-sectional SEM image reveals long cylindrical microchannels with a diameter of a few tens of micrometers (Figure 2C). It is known that wood cells (axial tracheids) exhibit cylindrical structure with a high aspect ratio and primarily run parallel to the trunk of the tree.22 These high aspect ratio microchannels combined with ray cells that run radially from the heartwood to the bark, form a continuous porous network that enables the transport of water and nutrients. We exploit the microchannel network in the wood to transport water from the bulk to the photothermally active layer at the evaporative surface. The extinction spectrum of the wood depicts the broad optical absorption in the visible part of the electromagnetic spectrum (Figure 2D). The broad optical absorption of wood has an appreciable overlap with the solar spectrum causing a significant temperature under solar illumination (discussed below). As mentioned earlier, low thermal conductivity of the support layer is important to ensure confinement of the 7677


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Figure 4. (A) Optical image of wood without (left) and with (right) GO flakes coating on the surface. SEM images of wood cross section without (B) and with (C) GO on the surface of the microporous structure. (D) Raman spectrum of wood with (blue) and without (red) GO flakes coating on the surface confirming the GO coating. (E,F) XPS spectra of wood and wood−GO composite.

Figure 5. (A) IR images showing the temperature of the wood−GO (top panel) and wood (bottom panel) under 808 nm laser illumination at various time points in the dry state. (B) Plot showing the surface temperature of the wood−GO and wood under 808 nm laser illumination at various time points. (C) IR images showing the temperature of the wood−GO (top panel) and wood (middle panel) floated at the air/water interface and water only (bottom panel) under 808 nm laser illumination at various time points. (D) Plot showing the surface temperature of the wood−GO and wood floating at the air/water interface and water only under laser illumination as a function of laser irradiation time. (E) Plot showing the cumulative mass change through water evaporation for the wood−GO and wood floated at the air/water interface and water only under laser illumination as a function of laser irradiation time.

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Figure 6. (A) IR images showing the temperature of the wood−GO floated at the air/saline water interface under solar illumination of 12 kW/m2 at various time points. (B) Optical images of wood−GO floated at the air/saline water interface and steam generation under solar illumination. (C) Plot showing the surface temperature of the wood−GO and wood floated at the air/saline water interface and saline water only under solar illumination of 12 kW/m2 as a function of solar illumination time. (D) Plot showing the cumulative mass change through water evaporation for wood−GO and wood floated at the air/saline water interface and saline water only under solar illumination of 12 kW/m2 as a function of solar illumination time.

bilayered structure is evident from dark layer on the light colored wood (Figure 4A). SEM images revealed the complete and conformal coverage of the microporous structure of wood with the GO layer (Figure 4B,C). The thickness of the GO layer was determined to be around 1.1 μm (Figure S1B). It has been recently demonstrated that even for thick GO layers, an unimpeded permeation of water occurs through nanoscale pores, which has been employed for water filtration.27−31 This large permeation of the water is highly advantageous for solar steam generation when the GO layers are suspended on porous and thermally insulating support layers such as wood. Raman spectra of wood−GO composite revealed the characteristic Gband (∼1580−1600 cm−1) and D-band (∼1330−1350 cm−1) corresponding to GO (Figure 4D). X-ray photoelectron spectroscopy (XPS) was employed to investigate the surface chemical composition of wood and wood−GO composite (Figure 4E,F). The 1s spectra of carbon can be deconvoluted into three peaks corresponding to the sp2 domains (CC with a binding energy of 284.5−285 eV and the sp3 domains (CO with a binding energy of 286 eV and CO with a binding energy of 288 eV).32,33 The C/O ratio obtained from the area under the peaks corresponding to sp2 domains and oxidized sp3 domains, revealed a decrease in the C/O ratio from wood sample (C/O = 1.96) to wood−GO sample (C/O = 1.90). The increase in the oxygen bearing groups at the surface confirmed the successful deposition of GO on the surface of the wood. To verify the robustness of the wood−GO composite, we subjected the wood−GO composite to water and saline water for extended duration (Figure S2−S4). The wood−GO composite subjected to water for 6 h remained intact (Figure S2) and the microchannels of wood are still clearly visible after incubating them to 10% NaCl (Figure S4). These results

demonstrate the stability of the wood−GO composite for practical desalination application. Now we turn our attention to the photothermal activity of wood−GO composite. First, we investigated the temperature rise associated with wood and wood−GO under near-infrared (NIR) laser illumination (808 nm, power density of 5 kW/m2) in a dry state (Figure 5A and Figure S5). Upon laser irradiation, the temperature of wood−GO rapidly increased from room temperature (27 °C) to around 70 °C, whereas the temperature of wood reached around 37 °C under identical irradiation conditions (Figure 5B). The large temperature rise of wood− GO (ΔT = 43 °C) compared to the relatively small increase in the temperature of wood (ΔT = 12 °C) upon laser irradiation demonstrates the high optical absorption and effective photothermal conversion efficiency of GO under NIR illumination. We have also investigated the photothermal activity of wood−GO in the wet state because the wood−GO piece is in a hydrated state during solar steam generation (Figure 5C,D and Figure S6). IR images reveal the temperature of the wood and wood−GO floated on the surface of water under 808 nm laser illumination (power density of 5 kW/m2) at various time points (Figure 5C). Upon laser irradiation, the temperature of wood−GO floating on water rapidly increased from room temperature (27 °C) to around 60 °C (Figure 5D). On the other hand, the temperature of wood and water did not exhibit a significant increase within an irradiation time of 1000 s (Figure 5D). The mass change of water as a function of irradiation time was employed to quantify the steam generation efficiency of the wood−GO composite and wood. Over 1000 s laser irradiation, the mass change of water from wood−GO floated on water was found to be around 1 kg/m2, which is nearly 7 times higher compared to that observed for wood on water (0.14 kg/m2) and water (0.12 kg/m2) (Figure 5E). This 7679


ACS Applied Materials & Interfaces significantly higher steam generation efficiency of wood−GO composite compared to wood stems from the higher NIR light absorption of GO compared to wood, with the latter predominantly absorbing in the visible part of the electromagnetic spectrum (as shown in Figure 3B). To evaluate the steam-generation efficiency and the desalination ability of wood−GO composite under simulated solar illumination (power density of 12 kW/m2), the weight loss of saline water due to the water evaporation (3% salinity) was measured (Figure 6). As discussed above, GO flakes exhibit a broad optical absorption over visible and NIR parts of the electromagnetic spectrum. Combined with the absorption of wood in the visible region, the large temperature rise of the wood−GO composite under simulated solar illumination resulted in the appearance of steam above the cuvette, which signifies the rapid evaporation of water (Figure 6A,B and Figure S7, S8). The temperature of the wood−GO composite rapidly increased from room temperature to around 67 °C within tens of seconds after the onset of simulated solar irradiation and remained constant over the remaining irradiation time (Figure 6C). In the case of pristine wood (i.e., in the absence of GO layer), the temperature raised from 27 to 54 °C. Compared to 808 nm laser, the larger temperature rise for pristine wood under simulated solar illumination can be attributed to the higher optical absorption of wood in the visible part of the electromagnetic spectrum that exhibits a large significant overlap with the solar spectrum. On the other hand, the temperature rise of saline water itself was found to be significantly smaller (ΔT = ∼10 °C). Under solar illumination, the cumulative weight loss was found to increase linearly with the irradiation time (Figure 6D). The weight loss over a duration of 1000 s was found to be 5.2 kg/m2 for wood−GO composite. Over 200 s of solar irradiation, the steady-state evaporation rate was calculated to be 14.02 kg m−2 h−1 for wood−GO composite. In the case of pristine wood, the steady-state evaporation rate was calculated to be 10.08 kg m−2 h−1. Without considering the optical concentration losses in the analysis, such as surface radiation and reflection, the evaporation efficiency of the wood−GO composite was calculated to be 82.8% at a power density of 12 kW/m2 (see Experimental Section for details). In the case of pristine wood, the evaporation efficiency was found to be around 59.5%. These results demonstrate the excellent photothermal capabilities of wood−GO composite and its application in solar steam generation. Furthermore, the wood− GO composite was found to be highly stable and could be reused multiple times without any noticeable degradation of the solar steam generation efficiency (Figure S9). In conclusion, we have introduced a simple and scalable material, wood−GO composite, for highly efficient solar steam generation. Owing to its natural vessel structure, hydrophilicity, low thermal conductivity, optical absorption with a significant overlap with solar spectrum, wide availability, and biocompatibility, wood forms an excellent candidate as a support layer in bilayered solar steam generators. The broad optical absorption of wood−GO composite results in a large temperature rise under solar illumination and generates steam by heat localization. The steam generation efficiency was found to be 82.8% at a power density of 12 kW/m2. The novel composite, owing to its high efficiency, facile fabrication and wide availability of the materials, is highly attractive for solar steam generation and desalination in resource-limited settings.

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

Materials and Chemicals. Graphite flakes (cat # 332461), potassium permanganate (cat # 223468), hydrogen peroxide (cat # 216763) and hydrochloric acid (cat # 258148) were purchased from Sigma-Aldrich. Sulfuric acid (cat #2876-46) was purchased from Macron. Phosphoric acid (cat # 0260-33) was purchased from J. T. Baker. Basswood from Walnut Hollow Company was used in this study. Preparation of Wood−GO Composite. Graphene oxide was synthesized using the method reported by Tour and co-workers.26 The wood−GO composite was prepared by drop casting aqueous GO solution (0.3 wt %) on the surface of the radially cut wood and set aside for the GO solution to dry naturally. Material Characterization. Scanning electron microscopy (SEM) images were obtained on a FEI Nova NanoSEM 2300 at an acceleration voltage of 10 kV. Atomic Force microscopy (AFM) image was obtained using Dimension 3000 (Bruker Inc.) in light tapping mode. The Raman spectra were obtained using a Renishaw inVia confocal Raman spectrometer mounted on a Leica microscope with a 50× objective and a 514 nm wavelength laser as an excitation source. Absorption spectra were collected using a Shimadzu UV-1800 UV−vis spectrophotometer. XPS spectra were obtained using a Physical Electronics 5000 VersaProbe II Scanning ESCA (XPS) Microprobe. Thermal Conductivity Measurements. The thermal conductivity of wood in the dry and wet state was measured by sandwiching the wood between two glass slides. The sandwich structure was placed on a hot plate with ice on the top side of glass. The temperature distribution along the cross-section of the sandwich structure was monitored using an IR camera (ICI 7320 P-Series). The Fourier equation was used to calculate the thermal conductivity using the ΔT formula: q′ = K ΔX . q′ is heat flux per unit area, K is thermal conductivity of glass (1.05 W m−1 K−1), ΔT is temperature difference, and ΔT is distance difference. The calculation of thermal conductivity was based on the assumptions that the sample and the glass slides were experiencing the same heat flux, and the emissivity coefficient of sample and glass slide was 0.9. Steam Generation Measurements. The temperature change and weight loss from evaporation of water for wood−GO, wood and water only were measured under the irradiation of 808 nm laser at a power density of 5 kW/m2 or simulated solar illumination (Newport AM1.5) at a power density of 12 kW/m2. In the case of solar illumination, the solar beam was concentrated using a magnifying lens and illuminated onto the surface of floating sample. The temperature was measured using an IR camera and the weight loss from evaporation was measurement using an electronic microbalance with an accuracy of 0.0001 g. A 1 cm × 1 cm GO-coated wood with a thickness of 3 mm was floated on the surface of water in a plastic cuvette with dimensions of 12.5 mm (W) × 12.5 mm (D) × 45 mm (H). The evaporation mh ̇ efficiency (η) is given by8 η = ILV , where η is evaporation efficiency, ṁ is the evaporation rate, hLV is the total enthalpy of sensible heat (294 J g−1, from 30 to 100 °C with specific heat of 4.2 J g−1 K−1) and vaporization of water (2257 J g−1) and I is the incident power density of solar illumination.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b01307. AFM scratch test to determine the thickness of GO layer; temperature associated with GO layers formed using varying concentrations of GO solutions; steam generation experiments using GO−wood composite under 808 nm wavelength excitation; optical images of solar steam generation using pristine wood (PDF) 7680


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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.S.). *E-mail: [email protected] (R.R.N.). ORCID

Pratim Biswas: 0000-0003-1104-3738 Rajesh R. Naik: 0000-0002-7677-928X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the Air Force Office of Scientific Research (S.S., Award # FA9550-151-0228; R.R.N., Award # 12RX11COR), AFRL/RH and National Science Foundation (CBET-1604542). The authors thank Prof. Lihong Wang from Department of Biomedical Engineering at Washington University in St. Louis for providing access to IR camera and the Nano Research Facility (NRF) at Washington University in St. Louis for providing access to electron microscopy facilities.

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WoodâGraphene Oxide Composite for Highly Efficient Solar Steam

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WoodâGraphene Oxide Composite for Highly Efficient Solar Steam