Fiber-Based, Double-Sided, Reduced Graphene Oxide Films for

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Fiber-Based, Double-Sided, Reduced Graphene Oxide Films for Efficient Solar Vapor Generation Ankang Guo, Xin Ming, Yang Fu, Gang Wang, and Xianbao Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07759 • Publication Date (Web): 17 Aug 2017 Downloaded from http://pubs.acs.org on August 19, 2017

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ACS Applied Materials & Interfaces

Fiber-Based, Double-Sided, Reduced Graphene Oxide Films for Efficient Solar Vapor Generation Ankang Guo, Xin Ming, Yang Fu, Gang Wang and Xianbao Wang* Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Key Laboratory for the Green Preparation and Application of Functional Materials, Ministry of Education, Hubei Key Laboratory of Polymer Materials, School of Materials Science and Engineering, Hubei University, Wuhan 430062, China

ABSTRACT Solar vapor generation is a promising and whole new branch of photothermal conversion for harvesting solar energy. Various materials and devices for solar thermal conversion were successively produced and reported for higher solar energy utilization in the past few years. Herein, a compact device of reduced graphene oxides (rGO) and paper fibers was designed and assembled for efficient solar steam generation under light illumination, and it consists of water supply pipelines (WSP), a thermal insulator (TI) and a double-sided absorbing film (DSF). Heat localization is enabled by the black DSF due to its broad absorption of sunlight. More importantly, the heat transfer, from the hot DSF to the cold base fluid (water), was suppressed by TI with a low thermal conductivity. Meanwhile, bulk water was continuously transported to the DSF by WSP through TI, which was driven by the surface energy and surface tension based on the capillary effect. The effects of reduction degrees of rGO on the photothermal conversion were explored, and the evaporation efficiency reached 89.2% under one sun with 60 mg rGO. This new micro-device provided a basic technical support for distillation, desalination, sewage treatment and related technologies.

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Keywords: Photothermal conversion; solar vapor generation; solar energy utilization; heat localization; reduced graphene oxide.

1. INTRODUCTION While the irreplaceable freshwater and the traditional energies like petroleum, mine and natural gas are almost exhausted, researches and applications about efficient solar desalination system are needed urgently1. Because it can directly produce desalinated water by using solar energy with very little carbon footprint, solar desalination is considered as one of the irreplaceable technologies to cover the pressing shortage of global water scarcity2-6. In the past few years, the principle and application of photothermal conversation for solar steam generation attracted extensive and in-depth researches, which promoted the rapid development of this field2-10. The research was firstly proposed and explained by Naomi. J. Halas and her research group2, 11. After that, a lot of photothermal conversion materials and devices were successively reported for a wide range of solar energy utilization12, especially in distillation2, 13, desalination5, 8-9, 14-16 and sewage treatment8, 17. The researches about photothermal conversation for solar vapor generation were divided into two main branches as follows: one is the nanofluids, in which the sunlightabsorbing metal nanoparticles2, 6, 11, 16-19, graphene10 or other nanomaterials9, 13, 20 were suspended or dispersed uniformly to be overall heated and produced solar steam; another one is the absorbing films3-5,

8-9, 14-15, 18, 21-24

or artificially-networked structures (such as aerogels,

carbonized wood and so on)7, 16, 25-26, with a strong absorptive capacity of solar spectra. These kinds of structures could float on the bulk water and confine the solar illumination in the surface of the films or networked structures, while the bulk water was maintained at room temperature.


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As the absorbed energy was confined in a thinner region, the absorbing films have higher efficiency for solar steam generation than nanofluids. And the dissipated heat, spreading to the environment and bulk water, was minimized3-4,

22, 25

. That is why the researches about the

absorbing films and heat localization occupied the main position, concluding porous carbon films3,

5, 8, 18, 22, 26

, artificially-networked structures3-4,

7, 15-16, 18, 22, 25, 27

, deposited metallic

absorbers9, 14, 21, 23-24 and compact solar desalination devices5, 8, 15-16. On the other hand, under very strong illumination (>50 suns), there is a larger difference of temperature between the absorbing film and fluid caused by heat localization. Hence the excess heat can not be used for vapor generation, which leads to high radiative losses. When the upper limit of concentration was exceeded, the efficiency of photothermal conversion of the absorbing film would not increase with the increasing of solar concentrations. At the same time, numerously basic researches about nanofluids for photothermal conversion were reported, including carbon materials10, metal nanoparticles2, 6, 11, 16-19, polymer nanoparticles20 and so on. However, in terms of practical applications, it is a trend to focus on the absorbing film for solar vapor generation, because the absorbing film has higher efficiency for solar vapor generation than nanofluids under natural sunlight (one-sun). In recent researches, most of the absorbing films directly floated on the surface of bulk water without any thermal insulator and water supply system. If so, two problems would be produced: one is that complex synthetic processes are needed to prevent materials from shedding out of the films and sinking into bulk water. Another one is that there is still a significant portion of the heat loss transferring to bulk water and dissipating through bulk water. The above problems promoted the involvement of insulating materials and water supply micro-device5, 25-26. Several kinds of thermal insular materials are currently reported, including carbon foam3, natural


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wood26, 28-29 and polymeric compound5, 30-34. The special porous structure, the natural wood26, 2829

and the carbon foam3 were also used to supply the water to the photothermal conversion

materials. Besides the wood and carbon foam, the cotton rod34, air-laid paper5 and mushroom30 were also reported to supply the water. Here we demonstrate that a thermal insulator (TI) is used to break off the direct contact between the absorbing film and bulk water, and the water path is confined to numerous water supply pipelines (WSP). Therefore, both of the suppressed heat transfer and continuous water supply can be simultaneously achieved. As shown in Figure 1, the fiber-based double-sided absorbing film (DSF) was made of reduced graphene oxide (rGO) and fiber, and the absorbing film was physically separated from bulk water by a TI (an expanded polyethylene foam, EPE) to cut off heat transfer. A continuous water supply is enabled by an inserted WSP, consisting of numerous capillaries. More specifically, the upper end of the pipelines is leveled with the top side of the TI neatly (see Figure S1). The entire device could float on the bulk water naturally due to its buoyancy, therefore rGO film could only contact with the top side of the TI. Pumped by a capillary force, a continuous and efficient water supply to rGO film was achieved by a confined water path within fifty capillaries. Compared with “directly floating on bulk water”, the heat dissipation from the absorber to bulk water could be minimized due to the separation between rGO film and bulk water. In addition, the heat absorber will be protected and not be affected by the cold bulk water. Therefore, with the specially designed water supply pipelines and thermal insulator, both suppressed heat transfer and continuous water supply can be achieved simultaneously. This new micro-device provides a basic technology support for distillation, desalination, sewage treatment and related technologies.


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Figure 1. The schematic diagram of the solar vapor generating device with three components: (i) a double-sided rGO film (absorbing film), including a light absorber (the obverse side, upper right) and a water absorber (the reverse side, upper left), (ii) a thermal insulator, for suppressing heat loss (lower left) and (iii) the water supply pipelines, inserted in thermal insular and pumped by capillary force (lower right).

2. EXPERIMENTAL SECTION 2.1. Materials. Graphene oxide (GO) was supplied by Suzhou HengQiu graphite Technology Co. Ltd,. Ascorbic acid (AA, AR, >99.0%), quantitative filter paper (Q5778) and spotting capillary tube (with the diameter of 0.5 mm) were supplied by Aladdin’s official site and used as received. The ultrapure deionized water was used for dissolving, dilution and synthesis.


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2.2. Preparation of fiber based rGO films. Firstly, in 200 mL deionized water, 200 mg GO was reduced by 50-400 mg (every 50 mg) ascorbic acid after being stirred and sonicated for 30 min using a microwave reactor (Sineo, MAS-IIPlus, China) with 2.45 GHz, 200 W, 95 oC and 10 min. And the products, collected by vacuum freeze drying, were added to deionized water (2 mol∙L-1) and sonicated for 30 min for preparation. Meanwhile, 100 mg filter paper was boiled into pulp in 100 mL deionized water for more than 30 min. And then, 30 mL (2 mol∙L-1) rGO dispersion was added to the boiling paper pulp dispersion with stirring for 30 min at 100 oC and cooled down naturally. Finally, the black pulp with rGO was filtered by a vacuum pump, and those remodeling rGO fibers film were dried in a vacuum oven at 35oC, as shown in Figure S2. 2.3. Characterization. Different degrees of reduction of rGO were defined and compared by the atomic ratio of carbon to oxygen measured by Electronic Differential System (EDS) based on Scanning Electron Microscopy (SEM) (JSM 7100F, Japan). Also, different rGO samples were compared and distinguished by Fourier Transform Infrared (FT-IR) spectra (NICOLET Is50, USA). The surface morphology and roughness of fiber based rGO films were observed by SEM and Atomic Force Microscope (AFM, NT-MDT, Solver Nano). The thermal images were captured by an IR camera (FLIR E4). The irradiation light was generated by a xenon lamp (CELHXF300, Education Au-light Co., Beijing, China).

3. RESULTS AND DISCUSSTION rGO samples with different degrees of reduction were synthesized by 200 mg GO and 50-600 mg (every 50 mg) ascorbic acid (AA). As shown in Figure 2a, the atomic ratio of carbon to oxygen increased with AA dosages in 50-400 mg, but reached a plateau (~ 10.15) after 400 mg. This phenomenon means that double quality of AA was enough to reduce the GO under a


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microwave reaction (more details in Figure S3). Based on the above results, three samples, shown as being circled out in the bottom of Figure 2a, were selected to further explore the relationship between the degree of reduction and the atomic ratio of C/O.

Figure 2. (a) The atomic ratio of C/O of rGO samples with different mass of reducers (ascorbic acid). (b) FT-IR spectra of four selected samples from (a) (GO, rGO-50, rGO-200 and rGO-400) and ball-and-stick model of the functional graphene oxide (insert). The ball-and-stick model of the functional graphene oxide was shown in the inset of Figure 2b, showing three kinds of oxygen-containing groups: (i) the carboxyl groups (blue model), of which the characteristic absorption band (stretching vibration of C=O bond) was distributed in 1726 cm-1, (ii) the hydroxyl groups (red model), of which the characteristic absorption band (stretching vibration of C-OH bond) was distributed in 3414 cm-1 and (iii) the cyclic ether, including cyclohexene oxide (green model) and pentamethylene oxide (yellow model), of which the characteristic absorption band (stretching vibration of O-H bond) was distributed in 11001030 cm-1 and 980-900 cm-1. In the presence of the ascorbic acid, the hydroxyl radicals (·OH), transformed from the carboxyl and hydroxyl groups, oxidized the dienol group of the ascorbic acid to diketone and fell from the graphene sheets35. The consumption of carboxyl and hydroxyl


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groups increased with ascorbic acid, and this consumption was reflected in the gradual contraction and smoothness of characteristic absorption band of the carboxyl and hydroxyl in Figure 2b, leading to the rise of the atomic ratio of carbon to oxygen.

Figure 3. rGO films. (i) Optical image (a), SEM image (b) and AFM image with root mean square roughness (c) of the observe side of rGO film. (ii) Optical image (d), SEM image (e) and AFM image (f) of the reverse side of rGO film. (iii) SEM image of the rGO-shell on the observe side (g) and a single fiber wrapped in rGO sheets (h). Three selected samples (rGO-50/200/400) and 100 mg fibers were mixed and filtered by a vacuum pump to make the absorbing film. Compared with pure fibers and rGO dispersions, the fibers with rGO are easier to sink in the water. And the upper liquid was more clear and transparent, which means the fibers and rGO were combined rather than piled up together without any connection (see Figure S4). As is shown in Figure 3, two sides of the film had


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obvious difference in roughness and color, which were compared at the macro and micron scale. The obverse side, closed to the filter in the process of filtered, was bright black, flat and rich in rGO (see Figure 3a and note S1). Under scanning electron microscopy, fibers were wrapped and covered tightly by rGO without any macro gap between fibers (see Figure 3b). The enriched rGO provided incalculable units, which absorbed the incident light energy and produced heat due to the electronic vibration and the transitions between energy levels for photothermal conversion. So heat localization and stream generation were achieved by the above functional structure. In contrast, the reverse side was off-white, porous and cotton-like (see Figure 3d). The fibers intertwined and formed the porous structure, providing the water passage from capillaries to the obverse of the film (rGO enriched) due to the capillary effect (see Figure 3e). Furthermore, the roughness of the two sides was further quantified and compared by analyzing the AFM images within 20×20 μm2. As shown in Figure 3c and 3f, the relative depth of the obverse was only ~ 3.5 μm (from -1.5 μm to 2 μm) while the depth of the reverse side reached ~ 16 μm (from -8 μm to 8 μm ). The root mean square roughness (RRMS) was calculated by the software named Nova_Px, and the RRMS of the obverse was 0.413 μm while the depth of the reverse side reached 2.248 μm (more detail see Fig S5 and table S1). More importantly, the outline of fibers and gaps were caught easily and clearly from Figure 3f, which were marked with white and black circle. The obvious difference of double sides in color, roughness and function results from three aspects: (i) the rGO resource, which means the weak connection within rGO sheets and between rGO and fibers provide the shedding rGO for rGO transfer, (ii) the impetus of the transfer, which means the vacuum filtration provides the power for rGO transfer, and (iii) the transfer channel, the large channels in the range of micrometer generated by paper fibers provide the transfer channel for rGO transfer. The above three factors are indispensable for the formation of the


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difference sides. Because of the presence of the thermal insulator (TI) and the water supply pipelines (WSP), the absorber could separate from bulk water with the connection by the WSP, which provided the development space to simplify the preparation process of the absorbing film. So the film was made from mixed dispersion including rGO and fibers without any crosslinking agents and adhesives, and that is why the connection of rGO and fibers was a fragile intermolecular force. After the removal process of oxygen-containing groups, most of interconnections among rGO sheets were achieved by the intermolecular force, due to its relatively pure GO structure. And the hydrophobic surface resulted in the agglomeration of rGO in water. Under the action of strong suction, most of the rGO sheets, which were attached to fibers, fell off from fibers and piled up to the obverse of the film. Furthermore, the collected rGO was squeezed and compacted by two opposing forces, which were suction force from vacuum pump and support force from the filter membrane, to form a “rGO shell” (see Figure 3g). As for the reverse side, however, fibers gradually settled down with the decrease of the solution during the filtration process, and formed a loose and porous surface structure naturally without the continuous squeeze. More importantly, it is noteworthy that the fibers were still wrapped tightly by the rGO sheets even under such strong suction, as shown in Figure 3g. And that means, the fibers and rGO sheets could still be tightly combined while facing the continuous suction (i.e., the separation and shedding occurred between the rGO sheets rather than fibers and the rGO sheets). When the absorbing film was placed on the thermal insulator embedded with capillaries, the porous reverse side of the dry absorbing film absorbed moisture quickly pumped by the capillary effect. Starting from the first contract (see Figure 4a ), the rGO film formed a moist area (a relatively bright black area, see Figure 4b) in a second, and spreading out quickly (see Figure


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4c). Five seconds later, as shown in Figure 4d, the whole film was wetted and inflated slightly, attached to the capillary tubes tightly under the action of gravity. It was evidenced that the film achieved the dynamic balance of water on the absorption film when the steam was produced under irradiation.

Figure 4. Water supply and temperature distribution of solar vapor generating devices. (a-d) starting from the first contract between rGO film and capillary tubes, wetted area of rGO film (circled out by white dashed lines) spread (the direction of water spread indicated by arrows) over time. (e-h) Temperatures of rGO film, thermal insulators and bulk water over irradiation times. Because the absorbing film and bulk water were separated by the thermal insulator, the heat loss from the hot film to cold water could be suppressed. Meanwhile, plenty of water was supplied continuously by capillaries to the absorbing film for efficient solar steam production. Thus, solar steam was generated continuously on the hot film while the temperature of bulk water was maintained at room temperature. Figure 4e-4h showed the temperature distribution of


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the film and the bulk water at different times. the maximum and minimum values of temperature were marked and measured within the box. As shown in Fig 4e–g, the surface temperature of absorbing film was soared to ~ 33.5 oC from ~ 18.1 oC under one sun irradiation, and stabilized at 25.2 oC after 40 min illumination.

Figure 5. Characterizations of light absorbing ability and thermogenic capacity. (a) the absorption spectra and absorption strength of rGO film in the wavelength range of 250 nm to 2500 nm. Mass changes (b) and evaporation efficiency (c) of the solar vapor generating device under 1 kW∙m-2. (d) The comparison of the evaporation efficiency between our work and the reported results. However, the thermal insulator was warming up slowly form ~ 18.1 oC to ~ 25.5 oC within 40 min. Furthermore, when the radiation time was prolonged to 90 min, Figure 4h shown that the temperatures of rGO film and thermal insulator were still stable at ~ 39.5 oC and 25.5 oC,


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respectively. In the course of 90-min irradiation, on the other hand, the temperature of bulk water was limited in 28 oC, and the surpass section of room temperature resulted from the warming of the glass beaker. This situation could be improved by blocking the excess irradiation, and if so, the bulk water could be kept at room temperature strictly. In spite of the slight warming of the bulk water, the temperature were still below 28 oC, which would be impossible if the thermal insulator and water supply pipers did not exist. The absorption spectra of rGO film with rGO-50/200/400 were measured by using the UVvisible spectroscopy from 200 to 2500 nm wavelength. As shown in Figure 5a, the efficient and broad range of absorption of rGO film was obvious, whether it is rGO-50, rGO-200, or rGO-400. And the absorption strength increased significantly along with the degree of reduction of rGO. In order to study the photothermal conversion of rGO film, the mass change was monitored and recorded by an electronic balance, and the readings were transmitted to experimental computer via data lines. Figure 5b shown the steam flow rate of water and films with rGO-50/200/400 under one-sun illumination (1 kW∙m-2). By comparing with each other, it could be found that the flow rate of the solar steam increased with the degree of reduction, from ~ 0.94 kg∙ (m2·h)-1 (rGO-50) to ~ 1.02 kg∙ (m2·h)-1 (rGO-200), and to ~ 1.14 kg/(m2·h)-1 (rGO-400). However, the mass change of water was only ~ 0.04 kg∙ (m2·h)-1. Furthermore, the evaporation efficiency (η) was calculated by the simple and universal formula: = ∆/∆ , in which is a constant for abiding test conditions, including specific heat capacity ( ), molar mass ( ), molar enthalpy of evaporation of water (∆ ), illumination area () and optical concentration ( ), and ∆/∆ is the mass change rate (more detail in note S2). As the trend of the steam flow rate, the efficiency of rGO films increased significantly with the degree of reduction of rGO, from ~ 74.1% (rGO-50) to ~ 81.1% (rGO-200) to ~ 90.2% (rGO-400), as shown in Figure 5c. And the


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above trend of evaporation efficiency of RGO samples matched the earlier report36. Compared with the currently reported results (Figure 5d), the evaporation efficiency of our new device was ranked highly in all the reported results7-9, 18, 37-39. More specifically, as the absorbing film was directly floated on the surface of bulk water without any thermal insulation and water supply system, the evaporation efficiency of double-layer system39 was only 60%. And it confirmed the necessity of the thermal insulator and the water supply system. Three non-negligible hypotheses were set up in process of calculation for evaporation efficiency: (i) assuming the evaporation of water occurs at 100 oC (~ 373 K), (ii) assuming the vaporized steam is not condensed back to the surface of rGO film and (iii) ignoring the temperature rise of bulk water during the irradiation process. However, actually, vaporization has occurred below 100 oC and the ∆ is a function of the temperature of water, so the temperature of steam need to be further explored for getting more accurate evaporation efficiency.

4. Conclusions In summary, with the aid of the thermal insulator and water supply pipes, an effective and efficient device for generating solar steam was designed, assembled and tested under one-sun irradiation. Compared to the absorbing film floating on the water directly, this entire device had higher evaporation efficiency due to the effective suppression of heat transfer from hot film to cold bulk water. More importantly, the reduction effect of rGO on photothermal conversion were further researched, and the results showed that the evaporation efficiency increased with the reduction degree of rGO (reached ~ 89.2% under one sun). This well-designed steam generating device provided a guidance and chance for many potential applications, such as distillation, desalination, sewage treatment and related technologies.


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ASSOCIATED CONTENT Supporting Information Supplemental information, including optical image of the device, schematic illustration for the synthesis procedure of the rGO film, the weight percent and atomic percent of each rGO sample (EDS), hydrophobicity characterization of the rGO film, height distribution statistics and height parameters of the rGO film, the calculation formula for evaporation efficiency (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Xianbao Wang) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is financially supported by National Key R&D Program of China (Grant: 2016YFA0200200) and National Natural Science Foundation of China (Grants 51272071, 51203045 and 21401049).

REFERENCES 1. Shannon, M. A.; Bohn, P. W.; Elimelech, M.; Georgiadis, J. G.; Mariñas, B. J.; Mayes, A. M. Science and Technology for Water Purification in the Coming Decades. Nature 2008, 452, 301-310.


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Table of Contents Graphic

Fiber-Based, Double-Sided, Reduced Graphene Oxide Films for Efficient Solar Vapor Generation Ankang Guo, Xin Ming, Yang Fu, Gang Wang and Xianbao Wang* Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Key Laboratory for the Green Preparation and Application of Functional Materials, Ministry of Education, Hubei Key Laboratory of Polymer Materials, School of Materials Science and Engineering, Hubei University, Wuhan 430062, China


Fiber-Based, Double-Sided, Reduced Graphene Oxide Films for

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