Synthetic Graphene Oxide Leaf for Solar Desalination with Zero Liquid

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Synthetic Graphene Oxide Leaf for Solar Desalination with Zero Liquid Discharge Casey Finnerty, Lei Zhang, David L. Sedlak, Kara L. Nelson, and Baoxia Mi Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03040 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 11, 2017

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Environmental Science & Technology

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Synthetic Graphene Oxide Leaf for Solar Desalination with Zero Liquid Discharge

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Revision submitted to

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August 29, 2017

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Casey Finnerty, Lei Zhang, David L. Sedlak, Kara L. Nelson, Baoxia Mi*

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Department of Civil and Environmental Engineering

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University of California, Berkeley, California 94720, United States

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The author to whom correspondence should be addressed. fax: +1-510-643-5264

e-mail: [email protected]; tel.: +1-510-664-7446,

ACS Paragon Plus Environment


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ABSTRACT

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Water vapor generation through sunlight harvesting and heat localization by plasmonic and

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carbon-based porous thin film materials holds great promise for sustainable, energy-efficient

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desalination and water treatment. However, the applicability of such materials in high-salinity

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environment emphasizing zero liquid discharge brine disposal has not been studied. This paper

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reports the characterization and performance of a nature-inspired synthetic leaf made of graphene

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oxide (GO) thin film material, which exhibited broadband light absorption and excellent stability

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in high-salinity water. Under 0.82-sun illumination (825 W/m2), a GO leaf floating on water

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generated steam at a rate of 1.1 liters per m2 per hour (LMH) with a light-to-vapor energy

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conversion efficiency of 54%, while a GO leaf lifted above water in a tree-like configuration

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generated steam at a rate of 2.0 LMH with an energy efficiency of 78%. The evaporation rate

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increased with increasing light intensity and decreased with increasing salinity. During a long-

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term evaporation experiment with a 15 wt.% NaCl solution, the GO leaf demonstrated stable

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performance despite gradual and eventually severe accumulation of salt crystals on the leaf

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surface. Furthermore, the GO leaf can be easily restored to its pristine condition by simply

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scraping off salt crystals from its surface and rinsing with water. Therefore, the robust high

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performance and relatively low fabrication cost of the synthetic GO leaf potentially unlock a new

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generation of desalination technology that can be entirely solar-powered and achieve zero liquid

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discharge.

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INTRODUCTION

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As water resources dwindle while demands soar, desalination is becoming a viable water

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treatment option to augment water supplies. Major advances have been made in recent years to

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reduce the energy consumption of desalination technologies. For example, state-of-the-art

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reverse osmosis (RO) membrane systems desalinate seawater with a recovery rate of 50% while

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consuming as little as 2 kWh/m3, which is only 25% higher than the practical minimum energy

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requirement.1 However, as feed water salinity increases, the energy consumption, operating

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pressure, and fouling/scaling potential of the membrane system increase. This issue makes it very

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difficult and costly to further enhance the recovery of an RO-based desalination system or

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operate it under high salinity conditions.2 In the meantime, existing brine treatment technologies

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such as zero-liquid discharge (ZLD) systems are particularly energy-intensive. For example, the

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most widely used technology, which relies on mechanical vapor compression (MVC)-based brine

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concentrators and crystallizers, consumes 20-25 kWh/m3 and 52-66 kWh/m3, respectively.2

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Recently, brine concentrators based on membrane processes such as forward osmosis3 and

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membrane distillation

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low-grade heat as an energy source, but it is unclear if they offer substantial advantages over

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existing MVC-based concentrators. Because high energy consumption remains a major barrier to

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ZLD, innovative approaches that are more sustainable and energy-efficient than current

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technologies are desirable.

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have been the subject of increasing interest due to their ability to use

Recent developments in photothermal steam generation have greatly enhanced the solar 5

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light-to-heat conversion efficiency

and hold immense promise for increasing the overall

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efficiency of desalination and water treatment. In this process, materials exhibiting mesoscopic

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porosity and efficient light absorption6-12 are used for localized heat transfer from the light-

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absorbing material to water in pores. However, the absorption bandwidth of the plasmonic

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nanoparticles frequently used for absorbing light and converting it to heat under surface plasmon

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resonance depends on their size and shape,13 which increase the complexity and cost of the

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materials.14, 15 Additionally, plasmonic nanoparticle films tend to be fragile and thus limit their

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use under realistic operation conditions.

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In contrast, the robust and inexpensive carbon-based materials such as graphene,5, 16

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graphene oxide (GO),17-19 and graphite20 are natural broadband light absorbers that are capable of

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absorbing about 97% of incident sunlight, making them an ideal material for sustainable solar-

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powered desalination.

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prior to their effective use for photothermal steam generation. This is because hydrophilic

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materials like GO readily swell or dissolve in water21 and are thus unsuitable for direct water

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treatment without necessary modification,22-24 while hydrophobic materials like graphene and

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black carbon cannot maintain good contact with water and thus have limited effectiveness in

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transferring heat to water.

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However, further development of carbon-based materials is required

Looking to nature, mangrove trees rely on a complex salt filtration process involving 25

to thrive in waters with a salinity higher than that of seawater.26

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passive water transport

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Evaporation of water from leaf stomata under solar irradiation, together with capillary action,

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creates a negative pressure that transports water from the growing medium into the roots,

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through the xylem, and eventually to the leaves. Coupled with such water transpiration,

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mangrove trees have developed two salt-regulating mechanisms. One is to partially exclude salt

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from water at their roots to minimize the accumulation of salt in their xylem and leaves, and the

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other is to excrete salt from glands, which are ~30 µm in diameter and mostly distributed over

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the upper surface of mangrove leaves.27 By mimicking the water transpiration and salt-excreting

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capability of mangroves, we may innovate an artificial tree system that simultaneously acts as a

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water purifier and a salt crystallizer, leading to a solar-powered desalination technology with

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ZLD capability and eliminating the two energy-intensive steps — brine concentration and

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crystallization — required by the existing technologies.

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This paper reports the first-phase study of a nature-inspired technology for solar-

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powered desalination with ZLD by using synthetic leaves to enable photothermal water

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evaporation. We used GO to fabricate a synthetic leaf that is stable in water and capable of

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generating steam with a high energy conversion efficiency. The evaporation performance of the

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synthetic GO leaf was evaluated in two configurations of the solar desalination system: (1) a

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floating configuration, in which the GO leaf floated on and was in direct contact with bulk water,

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and (2) a tree configuration, in which the GO leaf was lifted above and connected via a water-

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absorbing sheet to bulk water. Infrared thermal imaging was used to visualize the evaporation

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mechanism of each configuration. The effects of light intensity and water salinity on the

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performance of the GO leaf in the tree configuration were systematically investigated. Lastly, the

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durability and reusability of the GO leaf operated in high-salinity conditions were studied to

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assess its potential for long-term effective treatment of highly saline water.

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MATERIALS AND METHODS

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Chemicals. Chemicals needed for GO synthesis included graphite flakes with 99% purity

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(Alfa Aesar, Ward Hill, MA), sodium nitrate (Sigma-Aldrich, St. Louis, MO), sulfuric acid

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(Sigma-Aldrich, St. Louis, MO), potassium permanganate (Fisher Scientific, Hampton, NH), and

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hydrogen peroxide (30% solution, Fisher Scientific, Hampton, NH). Additionally, hydrochloric

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acid (37% solution, Fisher Scientific, Hampton, NH) and ethanol (Koptec, King of Prussia, PA)

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were used as washing solutions. Fabrication of the synthetic GO leaf required

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triethylenetetramine (TETA) and 1,4-butanediol diglycidyl ether (BDGE) (Sigma-Aldrich, St.

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Louis, MO) as crosslinking agents to stabilize GO nanosheets. Cellulosic filter paper (Whatman,

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Maidstone, United Kingdom) was used as a substrate. Deionized water was produced by a

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Barnstead Smart2Pure water purification system (Thermo Fisher Scientific, Waltham, MA).

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GO Nanosheet Preparation.

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procedure adapted from a previous study,24 which was based on the modified Hummers’

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method.28, 29 Briefly, 5 g of powdered graphite flakes was oxidized in a mixture of 2.5 g of

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sodium nitrate, 115 mL of sulfuric acid, and 15 g of potassium permanganate in a 35°C water

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bath for 30 min, then transferred to a 98°C water bath and diluted by 230 mL of deionized water,

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where it remained for 15 min. After cooling for 10 min, the mixture was diluted to 700 mL and

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then added with 5 mL of hydrogen peroxide. The suspension was filtered using a polyethylene

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terephthalate fiber, and the filtrate was centrifuged at 8,000 x g for 2 h with an Avanti J-30I

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centrifuge (Beckman Coulter, Indianapolis, IN). The supernatant was decanted and the

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precipitate was washed in successive cycles of deionized water, hydrochloric acid, and ethanol

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using the centrifuge until the supernatant reached pH 7. Next, the GO precipitate was freeze-

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dried at -50°C and under a pressure of less than 0.2 mbar in a freeze dryer (FreeZone 1,

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Labconco, Kansas City, MO) to obtain GO powder. The GO was then resuspended in a 20 g/L

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solution by sonication with a Q500 Sonicator (Qsonica, Newton, CT) in an ice bath at an

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amplitude of 80% for 2 h. The resulting GO suspension was purified by placing it in a dialysis

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bag and soaking the bag in 2000 mL deionized water, which was replaced daily for 6 days or

GO nanosheets were synthesized from graphite flakes using a

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until the water conductivity stabilized.

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GO Leaf Fabrication.

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nanosheets with TETA and BDGE.30 As schematically illustrated in Figure 1, 0.5 mL of 20 g/L

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GO suspension was first mixed with 0.1 mL of 0.01 M sodium hydroxide in a 2 mL tube on ice.

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The solution was then mixed with an Analog Mini Vortexer (Fisher Scientific, Hampton, NH) for

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1 min and placed back on ice. Next, 24 µL of TETA was added and the solution was vortex-

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mixed for 1 min and placed back on ice. Then, 65 µL of BDGE was added and the solution was

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vortex-mixed before placing the tube in an ice bath that was sonicated at 40% amplitude for 4

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min. Two circular coupons, each with a diameter of approximately 4.5 cm, were cut from the

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filter paper with a circle cutter (Fiskars, Finland). The coupons were dip-coated in the GO-

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TETA-BDGE solution and left undisturbed at room temperature for 1 min, leading to a GO mass

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loading density of about 0.29 mg/cm2 on the coupon. The coupons were then frozen by placing

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them on a plastic petri dish wrapped in aluminum foil and suspending it in liquid nitrogen for 2

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min. The petri dish with the coupons was placed in a freeze dryer (Fisher Scientific, Hampton,

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NH) overnight at -50°C and under a pressure of less than 0.2 mbar, and then the coupons were

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transferred to a 100°C oven and stored for 24 h to facilitate the crosslinking reaction. After that,

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the coupons were soaked in deionized water for 24 h, during which the water was changed at

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least 5 times. Finally, the resulting GO-TETA-BDGE coupons were stored at 60°C in an Isotemp

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General-Purpose Heating and Drying Oven (Fisher Scientific, Hampton, NH) to remove any

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residual water.

The synthetic GO leaf was prepared by crosslinking the GO

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FIGURE 1

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GO Leaf Characterization. To investigate the stability of the synthetic GO leaf, its elemental

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compositions before and after solar irradiation were analyzed by X-ray photoelectron

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spectroscopy (XPS). To verify the light absorption capability of the GO leaf, its absorption

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spectrum was measured using an ASD QualitySpec Pro UV-Vis spectrophotometer with a

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Muglight attachment (ASD Inc., Boulder, CO). The absorption spectrum of the filter paper

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substrate was also collected as a control to determine light absorption enhancement by GO. The

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deposition of GO-TETA-BDGE onto the filter paper substrate was verified by collecting the

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absorption spectra with an iS50 Fourier transform infrared (FTIR) spectrometer (Thermo Fisher

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Scientific, Waltham, MA). Lastly, the pore size and thickness of the GO leaf were determined by

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visualizing it with a Gemini Ultra-55 analytical field emission scanning electron microscope

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(SEM) (Zeiss, Jena, Germany).

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Solar Evaporation Experiments and Evaluation. Inspired by the water transpiration

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process of a natural tree, a bench-scale setup was built to evaluate the solar evaporation

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performance of the synthetic GO leaf. As shown in Figure 2, the synthetic GO leaf was placed on

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top of a water reservoir (100 mL beaker with an inner diameter of 4.5 cm) by positioning it on a

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piece of polystyrene sheet. Water was transported from the bulk water to the GO leaf by capillary

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action through a water-absorbing sheet (Nalgene Versi-Dry Surface Protectors, Thermo Fisher

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Scientific, Waltham, MA). To promote good contact between the GO leaf and the water absorber,

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the two materials were sandwiched together using two sheets of polystyrene plastic. As a control,

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a floating configuration was evaluated, in which the beaker was filled with deionized water and

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the GO leaf was placed directly on the water surface. Furthermore, baseline tests on water

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evaporation in the absence of a GO leaf were conducted for the tree and floating configurations,

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respectively.

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FIGURE 2

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The evaporation experiment was started by irradiating the water in the reservoir (with or

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without a GO leaf installed) with artificial sunlight, which was produced by a 1,000-watt large

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area light source solar simulator (Spectra-Physics, Stratford, CT) and had a solar spectrum

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comparable to that of natural sunlight over the visible light region (Figure S1). A P-8001 mass

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balance (Denver Instrument, Bohemia, NY) was placed beneath the light source of the solar

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simulator to record the mass of water remaining in the reservoir at 1-min intervals during

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irradiation cycles. The evaporation rate was then calculated as the change in the recorded mass of

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water normalized by the leaf area and the corresponding time interval, that is, the slope of the

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curve that depicted the relationship between the mass of water evaporated per unit leaf area and

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the irradiation time. The same analysis was also performed for all control experiments (where

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light irradiation was not applied) to obtain baseline evaporation rates in dark conditions. The

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light intensity and solar spectrum were obtained by an LX1330B lux meter (Precision Vision,

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Woodstock, IL) and an EPP 2000 radiometer (StellarNet Inc., Tampa, FL), respectively. Unless

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otherwise specified, each solar experiment was performed with a light intensity of 825 W/m2,

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which corresponds to 0.82-sun illumination since 1,000 W/m2 is often considered as the default

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value for one sun condition in related literature.17

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taking infrared images with a Ti100 infrared camera (Fluke Corporation, Everett, WA) before

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irradiation and at regular time intervals during irradiation.

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Solar Desalination Performance Evaluation. The stabilized evaporation rates for the tree

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configuration were determined by running the evaporation experiments for at least 5 h using bulk

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NaCl solutions of different initial salinities (1.7, 3.5, 5.25, and 7 wt.%), representing 0.5, 1.0, 1.5,

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The temperature evolution was monitored by



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and 2.0 times the concentration (3.5 wt.%) of NaCl in seawater. The crystal formation on the GO

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leaf surface was examined by SEM when the salinity of the solution remaining in the reservoir

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reached a certain level (~8.8 wt.% NaCl). The long-term evaporation performance of the GO leaf

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was examined using 15 wt.% NaCl solution (~4 times seawater salinity), and the effects of salt

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crystals formed over the leaf surface on temperature distribution and evaporation performance

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were investigated. The reusability of the material was assessed by scraping off the accumulated

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salt crystals from the GO leaf using a metal scoopula and briefly soaking it in deionized water

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for 5 min before reusing the cleaned GO leaf in another evaporation cycle.

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RESULTS AND DISCUSSION

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Properties of GO Leaf.

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photothermal capability, hydrophilicity, and low cost. The TETA-BDGE-crosslinked GO

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nanosheets were deposited across the whole depth of the filter paper, as indicated by the change

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of color from white to black on both top and bottom surfaces of the filter paper after GO

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deposition (Figure S2).

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dissolving in water) and durability (to resist mechanical damages), as the GO leaf did not exhibit

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signs of cracking or tearing after repeated folding and unfolding in either dry or wet/humid

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condition (Figure S3).

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elemental composition of the GO leaf due to sunlight irradiation were small.

GO was selected as the solar absorber in this study because of its

Such a fully interwoven structure led to excellent stability (to avoid

Furthermore, XPS analysis (Figure S4) revealed that the changes in

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A major advantage of the GO leaf is its broadband light absorption capability, which is

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critical for enhancing the overall energy efficiency of solar evaporation. As shown in Figure 3A,

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the light absorption of the GO leaf was consistently higher than that of the filter paper substrate.

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Absorption throughout the visible and near infrared regions of the electromagnetic spectrum is

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attributable to both the broad absorption spectrum and surface roughness (which enhances

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internal light scattering) of GO.17 Such mechanisms for photothermal conversion are

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advantageous over plasmonic heating, which requires noble metals (e.g., gold and silver) and

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complex material processing to convert narrowband to broadband light absorption.

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The composition of the GO leaf was evaluated by FTIR. As shown in Figure 3B, the

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FTIR spectrum of the GO leaf exhibits peaks at 1204, 1566, 1645, and 1724 cm-1 that are not

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present in the spectrum of the filter paper, indicating the existence of epoxy, carboxylate, amide,

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and ester groups, respectively, in the GO leaf. It is believed that the carboxylate and, to a lesser

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extent, epoxy and ester peaks were likely contributed by GO, the amide peak was solely due to

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TETA, and the epoxy and ester peaks were primarily associated with BDGE. The surface (Figure

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3C-D) and cross-section (Figure 3E-F) SEM images reveal that the fabricated GO leaf was

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around 170-µm-thick and had an extremely porous structure, both external and internal, created

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by the crosslinked GO nanosheets on the cellulosic fibers of the filter paper substrate.

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The pores of the 3D microporous network of the GO leaf had a planar diameter in the

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range of 10 to 30 µm, similar to the size of stomatal openings in natural tree leaves. However,

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unlike stomata, which mainly serve as gates to regulate the escape of water vapor,31, 32 the fixed-

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sized openings of the GO leaf contributed to water evaporation in three different ways. First, the

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presence of these pores increased the effective surface area and thus enhanced the overall rate of

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evaporation. Second, because water diffuses much faster near the edge of a pore than at its center,

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a phenomenon known as the perimeter effect,33 the rate of pore evaporation was higher than that

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of bulk water evaporation. Third, the porous GO structure enabled effective heat localization by

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directly transferring the heat generated by absorbed light to water within the pores.

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FIGURE 3

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Heat Localization Enabled by Tree Configuration.

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GO leaf in the floating configuration (where the GO leaf was in direct contact with bulk water)

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and the tree configuration (where the GO leaf was lifted above and insulated from bulk water, as

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assembled in Figure S5) were compared to understand how the GO leaf enhanced the water

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evaporation through heat localization. The GO leaf heated up much faster than its control in the

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floating configuration (Figure 4A-B) as well as in the tree configuration (Figure 4C-D). This is

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because the photothermal properties of the GO leaf facilitated more light to be converted into

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heat, increasing the temperature at the evaporation interface. The heat generated in the floating

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configuration was partly transferred into the bulk water, as evidenced by an increase in the

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temperature of bulk water, thereby reducing the amount of heat available for evaporation in the

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GO leaf. In contrast, the bulk water did not significantly heat up in the tree configuration,

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indicating that heat was mainly retained in the pores of the GO leaf. Note that compared with the

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floating configuration, the tree configuration only led to a small increase of temperature at the

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evaporation surface. This is because, although the tree configuration improved heat localization,

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the corresponding higher rate of evaporation required more energy to vaporize additional water,

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thus limiting the amount of heat available for increasing the temperature of the GO leaf.

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The temperature evolutions of the

FIGURE 4

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Solar Evaporation Performance.

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configurations were examined. As shown in Figure 5A, the water evaporation rate (calculated as

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the slope of the curve) of the GO leaf in the floating configuration was 1.1 LMH, marking a 59%

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increase from 0.70 LMH of its control experiment without GO leaf. Similarly, the evaporation

The evaporation performances of both floating and tree

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rate of the GO leaf in the tree configuration was 2.0 LMH, which more than doubled the 0.88

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LMH of its control without GO leaf.

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The evaporation efficiency (i.e., light-to-vapor energy conversion efficiency), as

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calculated in the Supporting Information with baseline dark-condition corrections (Figure S6),

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increased from 20% for bulk water to 54% for the GO leaf in the floating configuration and 78%

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for the GO leaf in the tree configuration. While both configurations relied on the photothermal

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properties of the GO leaf to convert sunlight into usable heat, the evaporation rate for the tree

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configuration was higher due to better heat localization at the evaporation surface. Figure 5B

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illustrates the variation in the relative proportions of the total solar energy having different

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ultimate fates — loss by light reflection and transmission, loss to the surrounding environment

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through heat transport (i.e., conduction, radiation, and convection), and latent heat for phase

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change (evaporation) — among different configurations under 0.82-sun illumination. Detailed

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calculation of the energy distribution is explained in the Supporting Information. For both bulk

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water and tree configurations without a GO leaf, most (72-83%) of the energy from the incident

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light was lost due to light reflection and transmission. In contrast, adding the GO leaf in both

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floating and tree configurations made it possible to capture significantly more solar energy while

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reducing the amount of energy lost as reflected/transmitted light to 10%. Besides, the tree

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configuration with a GO leaf was very energy-efficient viewing that only 12% of the total energy

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was lost as heat into the environment. In contrast, the floating GO leaf configuration lost 36% of

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the total energy to the bulk water that had a direct contact with the GO leaf. Therefore, a key

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advantage of the tree configuration over the floating configuration is that more heat can be

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locally trapped by the lifted GO leaf due to the air insulation between the GO leaf and bulk water,

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thus significantly minimizing the amount of heat energy lost into the aqueous environment.

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Our data agreed well with literature data in revealing the importance of having an

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insulation layer between the bulk water and evaporation surface to enhance the overall light-to-

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vapor energy conversion efficiency.

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78% under 0.82 sun, which is comparable to the efficiency (80-85% under one sun) of other

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recently reported carbon-based evaporators with good insulation.5, 16-20 Some other biological

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and engineered materials (e.g., carbonized mushroom, two-dimensional MXene) with good

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photothermal properties and proper insulation also led to a relatively high efficiency (~80%).34, 35

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However, when an effective insulation layer was absent, a much lower efficiency (50-64%) was

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reported for GO,17 plasmonic material,12,

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conversion efficiency in general increases with increasing light intensity. For example,

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efficiencies as high as 90-94% were achieved by GO 16 and plasmonic material-based systems 12,

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15

Our tree configuration with a GO leaf had an efficiency of

15

and other materials.36

Note that the energy

when subjected to a much higher light intensity (> 4 suns).

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Additional characterization of the GO leaf in a tree configuration showed a positive

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correlation between the evaporation rate and light intensity. As plotted in Figure 5C, the

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evaporation rate increased approximately linearly with light intensity and nearly doubled as the

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light intensity increased from 450 to 900 W/m2. Note that other environmental factors such as

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humidity, temperature, and wind speed also likely affect the evaporation rate.37 Research on the

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effects of these factors is warranted in the future but is beyond the scope of the present study.

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FIGURE 5

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Solar Desalination Performance. The GO leaf was tested in a tree configuration

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under the 0.82-sun condition (825 W/m2) and at different salinity levels. It is observed in Figure

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6A that the stabilized evaporation rate of the GO leaf was inversely proportional to water salinity.

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Specifically, the evaporation rate decreased from 2.0 LMH for fresh (i.e., zero-salinity) water to

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0.71 LMH for water with an initial salinity of 7.0% NaCl, twice that of seawater. As salinity

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further increased, the evaporation rate appeared to decrease much more slowly and reached ~0.5

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LMH at a salinity level of 15 wt.% NaCl. The evaporation rate at each salinity typically

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decreased initially and stabilized within the first hour, as demonstrated in Figure 6B using the

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salinity level of 15% as an example. Therefore, compared with the thick accumulation of salt

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crystals on the GO leaf, the initial formation of salt crystals, which took place within the pores

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and grow heterogeneously at random locations on the surface, likely played an important role in

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reducing the evaporation rate. In order to study the structure of salt crystals initially formed on

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GO surface before it was fully covered by a thick crystal layer, we used SEM to observe the

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crystal-free regions of the GO leaf with naked eyes after crystals started to accumulate in other

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regions, using NaCl solutions with initial salinity levels of 1.75% (Figure 6C), 3.5% (Figure S7),

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5.25% (Figure S7), and 7.0% (Figure 6D), respectively. As expected, an increase in salinity led

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to more severe crystal formation and thus narrower open pores of the GO leaf, decreasing the

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rates of water transport and evaporation (Figure 6A).

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It is observed in Figure 6D that open pores still existed in the GO leaf even after it had

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experienced significant crystal build-up at high salinity. According to Kelvin equation,38

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evaporation would occur more rapidly on the convex/flat surface of salt crystals formed over the

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GO leaf surface than on the concave water surface formed within the pores. We hypothesize that

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such preferential evaporation helped the pores of the GO leaf to remain open as crystals kept

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growing on the already-formed ones, thereby enabling water evaporation to continue despite salt

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crystal accumulation on the leaf surface. Similar preferential sites for salt accumulation was also

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observed in the process of water evaporation from porous media.39

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It is interesting to note that after the initial pore blockage the additional salt

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accumulation on the GO leaf surface did not appear to further reduce the evaporation rate. As

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shown in Figure 6B, the water evaporation vs. time curve has an almost constant slope during a

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5-day test (except the first hour) at a very high salinity of 15 wt.% NaCl. The time-lapse

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snapshots in Figure 6E demonstrate that crystals gradually grew on the GO leaf surface during

333

the 5-day test, with the overall thickness reaching more than 1 cm by day 5. A time-lapse video

334

documenting the 5-day test is available in the Supporting Information. In addition, the formation

335

of a thick crystal layer did not significantly decrease the light absorption efficiency of the GO

336

leaf; on the contrary, thermal images (Figure 6F) show that the surface temperature of the GO

337

leaf during the evaporation experiment using high-salinity (15%) water was actually higher than

338

that using deionized water (Figure 4D), indicating that a large portion of light was still able to

339

transmit through the salt crystals accumulated on the GO leaf. Note that the evaporation rate was

340

likely affected by a number of factors, including the surface temperature, pore size, effective

341

evaporation area, and capillary action within the leaf structure, some of which may have

342

conflicting effects on evaporation on the GO leaf.

343

GO leaf might quickly (say, within hours) reach a steady state and become the most limiting

344

factor during subsequent evaporation.

It is suspected that capillary action within the

Future work to fully understand such effects is needed.

345

Although the salt accumulation did not noticeably affect the water evaporation rate in the

346

present study, the salt build-up on the GO leaf surface could eventually damage the overall solar

347

desalination system due to, for example, the added weight. To resolve this issue, the accumulated

348

salt should be cleaned regularly during the evaporation operation — salt crystals were scraped

349

off from the GO leaf using a metal scoopula and then the GO leaf was soaked in deionized water

350

for 5 min (Figure S8A). In this way, the pristine state of the GO leaf was restored without

15


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351

apparent signs of physical damage or performance deterioration (Figure S8B). Note that the

352

accumulation of inorganic (e.g., gypsum) and organic (e.g., natural organic matter, protein and

353

other macromolecules) species on the GO leaf could also affect the evaporation performance.

354

Evaluation of such effects is necessary in the future toward further development of the present

355

solar desalination technology.

356

FIGURE 6

357

Outlook of Artificial Tree for Solar Desalination and Wastewater Treatment with

358

ZLD. An artificial tree system consisting of multiple synthetic GO leaves will be the next-

359

phase experimental configuration toward scaling up the present single-leaf device. We expect

360

that the multi-leaf configuration will make the solar desalination system more compact so as to

361

better withstand real-world environmental stressors. Besides, the multi-leaf configuration will

362

increase the so-called leaf area index (LAI), which is defined as the ratio of total one-sided leaf

363

area to canopy footprint area.40

364

efficient use of solar light by absorbing scattered/transmitted light and secondary irradiation, but

365

also enhance convective evaporation by increasing the available leaf surface area. In the

366

meantime, future research is warranted to optimize the structure and properties of the GO leaf for

367

maximum utilization of convective evaporation (in addition to direct solar evaporation) along

368

with the capillary force to take up and transport water. Eventually, water vapor generated by

369

synthetic leaves will be collected and condensed to produce clean water. Note that the latent heat

370

released from vapor condensation could be recovered and reused for evaporation, further

371

increasing the overall energy efficiency.

372

An artificial tree with a larger LAI will not only enable more

The direct crystallization of salt on synthetic leaves offers an unprecedented opportunity

16



Page 18 of 29

373

for brine disposal with ZLD. The unique capability of natural mangrove trees to predominantly

374

excrete NaCl crystals on leaf surface, which results in a buildup of magnesium and potassium

375

ions in leaves,27 suggests the possibility of salt separation and purification during crystallization.

376

In addition to salt excretion on synthetic leaves via water evaporation, salt exclusion at synthetic

377

roots made of semipermeable membranes can be implemented to enhance the overall

378

desalination performance, leading to a complete tree configuration. We expect that the artificial

379

tree system will be capable of treating high-salinity waters and recovering salts using sunlight as

380

the sole energy source. Considering the scale-up feasibility and low fabrication cost of the

381

synthetic GO leaf, the artificial tree system offers a great potential for important applications

382

such as off-grid, robust, and economical wastewater treatment and seawater desalination with

383

minimum environmental impacts.

384

385

ASSOCIATED CONTENT

386

Supporting Information.

387

(Figure S1); Comparison of the color and morphology of the filter paper substrate before and

388

after GO leaf synthesis (Figure S2); Folding and unfolding of the GO leaf to demonstrate its

389

material durability (Figure S3); XPS analysis of the GO leaf before and after sunlight irradiation

390

(Figure S4); Step-by-step assembly of the experimental setup for the tree configuration with a

391

GO leaf (Figure S5); Methods for calculating the evaporation efficiencies of the GO leaf in

392

floating and tree configurations, respectively; Evaporation performance of the GO leaf and the

393

control bulk water under dark conditions in floating and tree configurations, respectively (Figure

394

S6); Method for calculating the fate of solar energy irradiated on the evaporation surface; SEM

Comparison of the solar spectra of natural and simulated sunlights

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395

images of early-stage formation of salt crystals on the GO leaf surface at salinities of 3.5 and

396

5.25 wt.% NaCl, respectively (Figure S7); Reusability of the GO leaf after facile physical

397

cleaning (Figure S8). (PDF).

398

A time-lapse video documenting the salt crystal formation on the surface of the GO leaf in a tree

399

configuration, which was tested at a high salinity of 15 wt.% NaCl during a 5-day evaporation

400

experiment. (.mpg)

401

402

ACKNOWLEDGEMENTS

403

The material is based upon work supported by the U.S. National Science Foundation under

404

award no. CBET-1565452 and the U.S. Department of Energy under award no. DE-IA0000018.

405

Besides, CF acknowledges the support of the U.S. National Science Foundation Graduate

406

Research Fellowship (DGE 1106400), and BM is grateful for a Hellman Fellows fund. Work at

407

the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences,

408

of the U.S. Department of Energy under award no. DE-AC02-05CH11231. However, the

409

opinions expressed herein are those of the authors and do not necessarily reflect those of the

410

sponsors. We also thank Dr. Zhongying Wang at the University of California, Berkeley for the

411

assistance in the GO leaf characterization.

412

REFERENCES

413 414 415 416

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37. Kuiper, P. J. C., The effects of environmental factors on the transpiration of leaves, with special reference to stomatal light response. H. Veenman & Zonen: 1961. 38. Thomson, W., On the equilibrium of vapour at a curved surface of liquid. Philosophical Magazine 1871, 4(42), (282), 448-452. 39. Rad, M. N.; Shokri, N.; Keshmiri, A.; Withers, P. J., Effects of grain and pore size on salt precipitation during evaporation from porous media. Transport in Porous Media 2015, 110, (2), 281-294. 40. Breda, N., Ground-based measurements of leaf area index: A review of methods, instruments and current controversies. Journal of Experimental Botany 2003, 54, 2403-2417.

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520

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Page 23 of 29

521 522


T.O.C.

523

22



524

525 526 527 528

Figure 1. Schematic illustration for the fabrication of synthetic GO leaf.

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Page 25 of 29


529

530 531 532 533

Figure 2. Solar evaporation through the synthetic GO leaf to mimic water transpiration in natural trees.

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534 A

B

100 GO Leaf

1204 1566

Absorption

Absorption (%)

80 60 40

1645

Filter Paper

20

GO Leaf

1724

Filter Paper

0 500

535

1000 1500 2000 Wavelength (nm)

2500

500

1000

1500 2000 2500 Wavenumber (cm-1)

3000

536 537 538 539 540

Figure 3. Characterization of the synthetic GO leaf. (A) Absorption spectra and (B) FTIR spectra of the filter paper substrate and GO leaf; SEM images with different magnifications of (C and D) the surface morphology and (E and F) cross section of the GO leaf.

541 542 543

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544 545 546 547


Figure 4. Temperature evolution during water evaporation in different configurations: (A) bulk water as a control, (B) GO leaf floating on water surface, (C) tree configuration without GO leaf as a control, and (D) tree configuration with GO leaf.

548

26



Page 28 of 29

549

A

Light Intensity = 825 W/m2 (0.82 sun)

B

4

3

Tree with GO leaf 2.0 LMH

3

Floating GO 1.1 LMH

2

No GO

0.88 LMH

1 Bulk evaporation 0.70 LMH

0 0

20

40

550

B

60 80 Time (min)

Floating GO

72%

1

500

600 700 800 900 Light Intensity (W/m2)

Tree without GO

36%

1000

Tree with GO Leaf

83% 12%

Evaporation Loss as heat Loss as light

551 552 553 554 555 556

2

0 400

100

Bulk Evaporation

Evaporation Rate (L/m2/h)

Water Evaporation (L/m2)

GO Leaf in Tree Configuration

78%

Light reflection 10% 8%

20%

54%

6%

10%

11%

Figure 5. Solar evaporation enhanced by the GO leaf: (A) the water evaporation, with the slope denoting the evaporation rate in L/m2/h (LMH), for each configuration; (B) the fate of solar energy for each configuration; (C) the effect of light intensity on the evaporation rate of the GO leaf in the tree configuration.

27



Evaporation Rate (L/m2/h)

A

3 Light Intensity = 825 W/m2 (0.82 sun)

2

Seawater

1

B

70

0.8

Water Evaporation (L/m2)

Page 29 of 29

60

0.6

50

0.4 0.2

40 0.0

30

0

10

20 30 40 50 Time (Minutes)

60

0.5 L/m2/h

20 10 Salinity = 15 wt.% NaCl 0

0 0

557

558 559 560 561 562 563 564 565

2

4 6 8 10 12 14 Water Salinity (wt.% NaCl)

0

16

1

2 3 Time (Days)

4

5

Figure 6. Solar desalination by the GO leaf in a tree configuration. (A) Effect of water salinity on the stabilized evaporation rate; (B) 5-day evaporation performance of the GO leaf using 15 wt.% NaCl solution; (C and D) SEM images of the GO leaf surface after salt crystallization occurred with 1.75 and 7 wt.% NaCl solutions respectively ; (E) Time-lapse snapshots of salt crystal formation on the GO leaf surface during a 5-day evaporation experiment using 15 wt.% NaCl solution; and (F) thermal images revealing the effect of salt accumulation over the GO leaf surface on temperature evolution, using 15 wt.% NaCl solution.

28


Synthetic Graphene Oxide Leaf for Solar Desalination with Zero Liquid

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