Tailoring Nanoscale Surface Topography of Hydrogel for Efficient

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Tailoring Nanoscale Surface Topography of Hydrogel for Efficient Solar Vapor Generation Youhong Guo, Fei Zhao, Xingyi Zhou, Zhichao Chen, and Guihua Yu Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b00252 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 6, 2019

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Tailoring Nanoscale Surface Topography of Hydrogel for

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Efficient Solar Vapor Generation

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Youhong Guo,1# Fei Zhao, 1# Xingyi Zhou, 1 Zhichao Chen2 and Guihua Yu1*

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The University of Texas at Austin, Austin, TX 78712, USA

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TX 78712, USA

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*Correspondence: [email protected] (G. Yu)

Materials Science and Engineering Program and Department of Mechanical Engineering,

McKetta Department of Chemical Engineering, The University of Texas at Austin, Austin,

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TOC

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ACS Paragon Plus Environment

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Abstract

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Solar vapor generation, which can separate the soluble or dispersing contaminants from water,

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is particularly desirable owing to its green energy utilization for water purification technology.

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Here we present a concept of enhancing solar vapor generation by tailoring surface topography

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of the hydrogel-based solar evaporator. Via nanotexture-enhanced heat flux at evaporation

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front, the obtained solar evaporator achieves water evaporation rate of ~2.6 kg m-2 h-1 at ~91 %

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energy efficiency under one sun (1 kW m-2). An easy-to-install solar still based on this solar

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evaporator consisting of cost-effective polyvinyl alcohol and activated carbon is deployed to

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demonstrate the potential for domestic or urgent water purification purpose. Such new design

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principle of hydrogel-based solar evaportors provides a useful means for surface-enhanced

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water evaporation to inspire scalable and processable solar evaporators from accessible raw

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materials.

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Keywords Solar vapor generation, Desalination, Hydrogel, Surface topography, Water purification

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Introduction

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The continuing growth of modern societies has led to an increasing population under

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different water-scarce circumstances.1 Due to the inadequate supply of easily accessible

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freshwater, desalination of seawater has been viewed as a potential solution to growing water

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shortage. With the evolution of solar energy harvesting technology, the development of next-

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generation water purification technologies is aimed at solar-powered distillation, which

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directly uses natural sunlight as a sustainable and eco-friendly energy source. However, the

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diffused sunlight can barely satisfy the energy requirement of water evaporation−distillation as

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an energy-intensive process. In this context, achievements in advanced solar vapor generation

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(SVG) have been made by designing new materials and functional structures. Materials were

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designed to concentrate the solar energy by localizing heat near the water evaporation surface

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to improve energy utilization efficiency.2-9 Nanomaterials with the plasmon-enhanced

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absorption of a broad solar spectrum are adopted to boost the efficiency of solar energy

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harvesting further.10-13 Functional structures in nature, such as mushroom, tree, and leaf, are

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also introduced to bioinspired materials to reduce the energy loss during SVG.14-19

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Hydrogels are polymeric networks that contain a significant amount of water molecules.20

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The molecular architecture and morphology of hydrogels can be precisely controlled by

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tailoring cross-linking point, changing basic building blocks and modifications, leading to

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various chemical and physical properties.21-23 Recently, by tuning the interaction between

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polymer networks and water molecules, hydrogel-based material as an independent porous

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structured solar vapor generator was demonstrated to be capable of lowering the energy

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demand for water evaporation process to achieve a high evaporation rate.24,

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polymer chains in hydrogels are encapsulated in water, the surface of hydrogel serves as the

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evaporation front during SVG. Despite the prior simulations that have predicted the enhanced

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water evaporation caused by the designed topography of evaporation front,26-28 the fluidity of

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water hinders the experimental surface regulation of liquid water at the micro- or nano-scale.

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The hydrogel materials could provide a platform to shape the evaporation front and thus to

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have strong impact on the water evaporation behavior. Therefore, it is both theoretically and

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practically significant to investigate the surface effect of hydrogels with unrevealed properties

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that may favor SVG.


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Since the

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Figure 1. Schematic of surface-modified hydrogel for solar vapor generation. Under one sun solar radiation, the designed surface-modified hydrogel-based evaporator floats on saline bodies of water to absorb and convert incident solar flux into thermal energy for vapor generation while avoiding excess salt fouling on itself. Comparison of COMSOL simulation of temperature distributions of F-SH (flat-) and D-SH (dimpled) upon identical energy input, showing the topography enabled heat concentration effect.

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Here, we present a hydrogel with tailored surface topographies (termed as a surface-

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modified hydrogel, SH) that functions as the solar evaporator to achieve a high water

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evaporation rate under one sun (1 kW m-2 h-1). Through the template-assisted gelation of

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polyvinyl alcohol (PVA) hydrogel, the surface of resultant hydrogels is guided by liquid-solid,

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liquid-gas, and liquid-liquid interfaces, ideally forming flat, grooved and sharply dimpled

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surfaces, which are denoted as F-SH, G-SH and D-SH, respectively (Figure 1). Our designed

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solar vapor generator consisting of transparent hydrogel with tailored surfaces as the top layer

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to facilitate water evaporation and activated carbon (AC) paper as the bottom layer to absorb

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solar flux. The sharply dimpled surface increases the heat flux by concentrating the harvested

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energy to the nano-branches and lower the energy loss compared to that of the flat surface. The

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obtained hydrogel-based solar evaporator yields 2.6 kg m-2 h-1 water evaporation rate at ~ 91%

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energy efficiency under one sun while avoiding excess salts fouling. A solar water purification

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prototype further reveals the potential of our design for technologically and economically

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challenged communities to alleviate water scarcity.


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Results and Discussion

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In situ template-assisted gelation method was used to prepare PVA hydrogels with

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designed surface morphologies. Typically, the PVA solution (with cross-linker and initiator) is

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evenly smeared on the carbon paper. The surface of solutions are exposed to the air, closely

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covered by glass and overlaid by specific solvent (e.g., pentanol, see Methods in Supporting

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Information) to fabricate G-SH, F-SH and D-SH samples. The physicochemical properties such

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as size, shape, water content, mechanical strength, etc., could be tuned by the ratio of PVA and

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cross-linker.24, 29

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Figure 2. Surface topography of SHs. SEM images of SHs. (A-C) G-SH, (D-F) F-SH and (G-I) DSH. (J) The oil contact angle of each SH. Inserts are photographs of oil drop (1,2-Dichloroethane) on each modified hydrogel surface. (K) Surface area and root mean square (RMS) roughness of each SH from optical profilometer test.

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Scanning electron microscopy (SEM) is employed to confirm the surface topography of

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obtained hydrogels. After freeze-drying, the G-SH presents a rough outmost surface consisting

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of shallow pores (Figure 2A, B), but shows a relatively flat sheet-like structure under higher

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magnification (Figure 2C). The F-SH shows a rather smooth outmost surface decorated by very

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shallow pores (Figure 2D, E) with similar sheet-like structure under higher magnification

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(Figure 2F). In contrast, clear “canyons” could be observed in D-SH samples leading to sharply

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dimpled surface topography (Figure 2G). The porous structures are distributed on groups of

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canyons (Figure 2H). Additionally, D-SH has nanoscale pores nested in its micro-sized porous

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structure which do not exist in both G-SH and F-SH (Figure 2I). Such hierarchical structure

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near the surface of D-SH is resulted from the interpenetration of pentanol and PVA, in which

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PVA chains coil up and collapse to reduce the Gibbs free energy when in contact with pentanol,

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a poor solvent of PVA.29-31 In addition, the cross-section of SHs samples is observed and found

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that their PVA hydrogel skeleton is the same, and several additional tests also show that the

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template-assisted gelation only influences the surface instead of changing the original

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polymeric network (Figure S1). Such observation indicates different exposure area of these

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hydrogels when being used in SVG.

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The underwater oil contact angles (OCAs) of G-SH, F-SH, and D-SH (fully swollen) were

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measured to be 143. 87 ± 0.69 °, 133. 51 ± 0.83 °, and 150.65 ± 0.78 °, respectively (Figure

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2J), showing a superoleophobic (OCA ≥ 150°) property of D-SH.32 Since the surface of PVA

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hydrogel was previously proved to be both hydrophilic and oleophobic,33 and its

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superwettability can be further tuned by introducing the surface roughness,34-36 the highest

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OCA of D-SH could be due to the roughness level of its surface. Under profilometer, where

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SHs are fully hydrated, the 2D color contour map of G-SH shows lightly grooved surface

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(Figure S2A), F-SH has nearly perfect flat surface profile (Figure S2B), but D-SH presents the

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sharply dimpled outmost surface profile (Figure S2C), in consistence with SEM results. The

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root mean squared (RMS) roughness and measured surface area of these hydrogels were

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estimated based on the results from optical profilometer (Figure 2K). Owing to the highest

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RMS roughness, ~ 405 nm, the surface area of D-SH is ~5 times of its shadowed area, offering

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a larger water-air interface, i.e. evaporating surface during SVG, than those of G-SH and F-

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SH.


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To evaluate the light absorption of hydrogel-based solar vapor generator, we collected the

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ultraviolet-vis (UV-vis) near-infrared (NIR) spectra. As shown in Figure 3A, SHs presented

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excellent broadband absorption, with 96% or above weight by a standard solar spectrum of air

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mass 1.5 global (AM 1.5G). Also, the D-SH had the lowest reflectance (~2%) compared to the

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reflectance of G-SH and F-SH over 250 to 2000 nm wavelength (Figure S3). Such low

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reflectance of D-SH could be attributed to the multiple reflection by nanotextures on the

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surface,37, 38 indicating D-SH can capture the incident solar light more efficiently. The full

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spectrum light absorption showed efficient solar harvesting capability of all SHs. Upon

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exposure to solar irradiation, the temperature of the evaporation surface of each SH and bulk

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water under one sun irradiation over time were traced to study the photothermal behavior of

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each SH (Figure 3B). All SHs presented an instant increase of surface temperature in the first

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10 min and reached steady state equilibrium temperature (~38 °C) after 30 min, while the

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temperature of bulk water remained ~25 °C. These results indicate that the heat localization

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effect was achieved by SHs. D-SH showed slightly lower steady state equilibrium temperature

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because of its faster evaporation rate with cooling effect. In addition, infrared imaging was

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used to monitor the temperature distribution during the vapor generation process under one sun

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using D-SH. The photograph and infrared images of the solar vapor generation setup under the

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exposure time of 0 min, 10 min, and 60 min are shown in Figure 3C, confirming the thermal

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energy is confined at the evaporation surface instead of losing to the bulk water.


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Figure 3. Solar evaporation based on SHs. (A) UV-vis NIR spectra of each SH ~1.5mm thick. The normalized spectral solar irradiance density of air mass 1.5 global (AM 1.5 G) tile solar spectrum is shown by the black line. (B) The temperatures measured at both evaporation surface and bulk water under one sun irradiation. (C) The setups for solar vapor generation test using D-SH and corresponding infrared images showing the temperature distribution with an irradiation time of 0 min, 10 min, and 60 min. Scale bar: 1 cm (D) The mass change of water during one sun SVG. (E) The evaporation rate of SHs and corresponding energy efficiency under one sun.

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The solar-vapor generation performance of SHs was investigated by the one sun solar

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evaporation (see Methods in Supporting Information). With optimized gelation density (Figure

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S4) and selected solar absorber (Table S1, Figure S5), the mass change of water was tracked

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(Figure 3D). The D-SH showed the highest water evaporation rate, ~ 2.6 kg m-2h-1, compared

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to ~2.0 kg m-2h-1 for G-SH and 1.8 kg m-2h-1 for F-SH, which are significantly higher than the

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blank control sample (i.e. water without any solar absorber). Such results further confirm that


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the SVG benefited from the rugged surface of the hydrogel. The corresponding energy

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efficiency (η) for solar-vapor conversion can be calculated by:2

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η = mEequ CoptP0

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where m is the mass flux while reaching steady state condition, Eequ is the equivalent

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vaporization enthalpy of the water in SHs (Figure S6),24 P0 is the solar irradiation power of one

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sun (1 kW m−2), and Copt refers to the optical concentration on the absorber surface. The D-SH

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achieved a high energy efficiency of ~91% under one sun (Figure 3E), indicating its potential

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as an effective solar evaporator under natural sunlight. Such high performance resulted from

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topographical nanotextures which lead to an increase of heat flux and more effective spread of

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small amount of water toward the evaporation front, favoring fast water evaporation.39 In

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addition, the increased surface roughness as well as the vapor germination enhanced by the

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cavity structure can speed the heat removal during SVG.40

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Figure 4. Solar desalination performance of the D-SH. (A) Measured concentrations of four primary ions in seawater and the as-purified water after desalination. (B) The photographs of uncoated carbon paper D-SH after one-sun desalination for 180 minutes. The scale bars are 1 cm. (C) Evaluated concentrations of four primary ions accumulated in D-SH over time. (D) The duration test of D-SH for continuous 48 hours solar desalination.


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To further demonstrate the potential of D-SH for solar desalination, a real seawater sample

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(with a salinity of 3.5 wt%, from the Gulf of Mexico) was used here, and the quality of collected

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water was evaluated by inductively coupled plasma spectroscopy (ICP-OES, Figure 4A). The

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concentrations of four primary ions (Na+, Mg2+, K+, and Ca2+) were significantly reduced by

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~2 to 3 orders, indicating effective desalination. Anti-fouling property of D-SH was also

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observed after 180 minutes under one sun irradiation. In contrast to bare carbon paper, the D-

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SH generator showed no salt crystal on its surface (Figure 4B). A series of long-term

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desalination tests were carried out and the salt accumulation in D-SH was revealed by ICP-

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OES. As shown in Figure 4C, the ions accumulated in D-SH remained in the same order over

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entire 48 hours solar desalination, verifying that the low equilibrium ion concentrations

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accumulated in D-SH would not be able to trigger the crystallization of salt. In addition, the

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water evaporation rate was nearly constant over a 48 hours continuous test (Figure 4D),

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revealing that D-SH was capable of performing a practical long-term solar desalination without

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cycling treatment to clean the fouls.


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Figure 5. Outdoor testing of D-SH-based solar still under natural sunlight. (A) Schematic of a simple-to-use solar water purification prototype using D-SH. (B) Photograph of the solar still in operation at UT-Austin ETC building rooftop. Scale bar is 4 cm. (C) Incident solar flux, the outdoor ambient temperature the, purified water volume and temperature of the D-SH surface were recorded over time on a mostly sunny day from 8:00 to 20:00. (D) Evaluation of water purity using a multimeter with a constant distance between electrodes.

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A simple-to-install solar still water purification prototype using D-SH was deployed on

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the roof of the Engineering Teaching Center (ETC) in the University of Texas at Austin in

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August 2018, the representative mostly sunny days with sunlight flux from 0.5 to 0.8 kW m-2.

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As shown in Figure 5A, B, black and flexible large sheets of D-SH (Figure S2, S7) are floated

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in a container of brine (diameter 24 cm). The evaporated water condenses on the transparent

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condenser and the water condensate, guided by the gravity of the weight, flows to the purified


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water container located in the middle of the brine container. The clear plastic wrap was used

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as a transparent condenser and a 400 mL of glass beaker was used as a purified water container.

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The demonstration was carried out from 8:00 to 20:00 under natural sunlight with an average

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incident solar flux of ~ 0.58 sun (~ 58 mW cm-2), and the outdoor ambient temperature and

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humidity (Figure S8), the volume of purified water, temperature of D-SH surface and

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condenser were recorded (Figure 5C). The water purification rate is ~15 L m-2 day-1 (Figure

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5C, blue shade), which is adequate for daily family drinking needs.41 Compared with an

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outdoor ambient temperature of air with a temperature of the condenser (Figure 5C, green line

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and pink line), D-SH presented a strong solar-to-thermal ability using activated carbon powder

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coated kitchen paper towel. The surface temperature of the D-SH increased ~74 °C (Figure 5C,

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wine line), indicating a slower evaporation process compared to the laboratory testing condition

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due to a weakened evaporation cooling effect. This is because the proposed demonstration is a

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closed system where the humidity can easily get saturated to restrain the evaporation of water.

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Since PVA hydrogel is chemically crosslinked, D-SH is able to keep its mechanical robustness

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and perform stable under presented temperature condition. The outdoor rooftop prototype

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reveals a significant potential for clean water collection technology for an urgent need. The

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water quality was directly presented by a resistance since the ion concentration defines the

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conductivity of their aqueous solution (Figure 5D). The seawater (~3% salinity) and purified

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water showed resistance values of 100.13 kΩ and 0.5019 MΩ, respectively, indicating the

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effective (see Supporting Information S2.9) seawater desalination.

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Conclusion

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Highly efficient solar vapor generation was achieved by tailoring the surface topography of

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water in the hydrogel which significantly increases the heat flux at evaporating front. In situ

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template-assisted gelation is used to fabricate PVA hydrogel with designed surface topography.

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Coupled with commercialized carbon paper, the resultant hydrogel-based solar evaporator

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enables a rate of ~2.6 kg m-2 h-1 with energy efficiency up to ~91% and favorable salt-fouling

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capability. An easy-to-deploy solar water purification prototype is also demonstrated to deliver

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fresh water with a daily yield of 14.5 L m-2. Our work not only proves the water evaporation

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rate can be increased using hydrogel with tailored surface topographies but also reveals a new


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possibility of engineering the water vaporization surface that may contribute to a wide range

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of water-energy applications such as atmospheric water harvesting, humidity regulation and

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environment cooling.

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Associated Content Supporting Information The details of chemicals and materials, the fabrication of SHs and controlled samples, and other characterizations are provided, including eight supporting figures, and one table. These materials are available free of charge via the Internet at http://pubs.acs.org. Author Information Corresponding Author *Email: [email protected] Author Contributions #These authors contributed equally to this work. Notes The authors declare no competing interests. Acknowledgments We thank Prof. Joan Brennecke for providing the equipment for contact angle measurements. G.Y. acknowledge the financial support from the Lockheed Martin Corp, Sloan Research Fellowship, and Camille-Dreyfus Teacher-Scholar Award.

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Nano Letters

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Tailoring Nanoscale Surface Topography of Hydrogel for Efficient

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