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The total amount of the globe water is about 1.4 billion km3, and all forms of .... seawater desalination were far below the ion concentrations limit ...
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Plasmon Ag Promoted Solar-Thermal Conversion on Floating Carbon Cloth for Seawater Desalination and Sewage Disposal Panzhe Qiao, Jiaxing Wu, Haoze Li, Yachao Xu, Liping Ren, Kuo Lin, and Wei Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20665 • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

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Plasmon Ag Promoted Solar-Thermal Conversion on Floating Carbon Cloth for Seawater Desalination and Sewage Disposal Panzhe Qiao, Jiaxing Wu, Haoze Li, Yachao Xu, Liping Ren, Kuo Lin, Wei Zhou* Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, Heilongjiang University, Harbin 150080, P. R. China KEYWORDS: Solar-thermal energy conversion; plasmon Ag; floating carbon cloth; surface plasmon resonance; seawater desalination

ABSTRACT: Using solar energy to achieve seawater desalination and sewage disposal has received tremendous attention for its potential possibility to produce clean freshwater. However, the low solar-thermal conversion efficiency for solar absorber materials obstacles their practical applications. Herein, Ag nanoparticles (NPs) modified floating carbon cloth (ANCC) are firstly synthesized via wet-impregnation, photoreduction and low temperature drying strategy, which could float on the water and absorb the solar energy efficiently. It is worth noting that vaporization rate of ANCC with a high wide-spectrum absorption (92.39%) for the entire range of optical spectrum (200 - 2500 nm) is up to 1.36 kg h-1 m-2 under AM 1.5, which corresponds to solar-thermal conversion efficiency of ~92.82% with superior seawater desalination and sewage disposal performance. Plasmon Ag promotes the conversion efficiency obviously compared to

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the pristine carbon cloth because the surface plasmon resonance (SPR) effect could increase the local temperature greatly. After the desalination, the ion concentrations (Mg2+, K+, Ca2+ and Na+ ions) in water are far below the limit of drinking water. Such high-performance floating ANCC material may offer a feasible and paradigm strategy to manage the global water contamination and freshwater shortage problem.

1. INTRODUCTION The total amount of the globe water is about 1.4 billion km3, and all forms of freshwater resources merely 2.53 % of the total global water. In addition, 68.7 % of the freshwater belongs to the solid glaciers, which are distributed in the hard to use mountain glacier, Antarctic and Arctic regions.1-2 Indeed, some freshwater resources are also stored deep underground, leading to difficult exploitation and huge mining costs. Besides, the problem of freshwater pollution has not been controlled and the possibility of pollution expanding is increasing. Protecting water resources and making rational use of water resources are the responsibility and obligation of all human beings, and the research topic of seawater desalination has been widely concerned in recent years. Solar energy is the most primitive and original energy on the earth. So the solar energy is applied in many aspects, such as solar-thermal conversion, photovoltaics and photocatalysis.3-6 Using solar energy as thermal has applied to many aspects of life, such as power station, household water-heaters, sewage disposal and desalination operation.7-10 Take advantage of solar energy seawater desalination and sewage disposal has received tremendous attention for its potential to produce clean freshwater.11-14 Recently, a wide variety of light absorbing materials such as TiO x nanoparticles (NPs), Au NPs, Ti 2 O 3 NPs, have served as solar absorber materials for high-performance solar-thermal energy conversion.15-17 In spite of

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the continuous improvement of solar absorber materials, some progress has been made in the absorption of solar energy, but many difficulties have not been solved, such as the high investment, unworkability, vulnerable, and low conversion efficiency.18-21 Fortunately, carbonbased materials have appealed to a great deal of attention over the last several years, probably owing to its inexpensive, excellent photothermal conduction, efficient wide-spectrum absorption, porous structure, and low thermal emission. Those are the essential conditions to guarantee the possibility of practical application of seawater desalination and sewage disposal.22-26 Carbon cloth (CC) has these intrinsic advantages of carbon materials, which can promise for solarthermal energy conversion. CC also have the exceptional condition that the already industrialization to produce carbon cloth of scalable size, which thus have enhanced their both scalability and feasibility. According to the existing literature, this work is the first time for seawater desalination using carbon cloth as substrate. Unlike other organic foams or powder materials, these materials require floating supports to achieve effective photothermal conversion. However, it can provide a new way for the self-supporting seawater desalination. Moreover, surface plasmon resonance (SPR) of Ag NPs provides a powerful way to confine and transform light energy into some thermal energy. In other words, energy is transferred from photons to surface plasmons, and most of the energy of incident light is absorbed by surface plasmon waves, resulting in a sharp reduction in the energy of reflected light.27-31 Therefore, the SPR effect of Ag NPs can produce hot electron, which can further improve the solar-thermal energy conversion efficiency. In addition, due to the hot electrons, the local temperature on the NPs surface is increased and is far higher than the temperature of the surrounding solution, which is of great significance for the solar-thermal conversion.

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In this study, we demonstrated that Ag NPs modified floating CC possessed excellent seawater desalination and sewage disposal performance, which greatly improved the solar-thermal conversion efficiency. The ANCC composite had a high wide-spectrum light absorption (92.39 %) for the entire range of optical spectrum (200 - 2500 nm). The vaporization rates of 1.36 kg h-1 m-2 g-1 was achieved under 1 sun intensity, and the corresponding conversion efficiency up to ∼92.82 %. And the conversion efficiency is more efficient than most published articles.17,36 The

ion concentrations (Mg2+, K+, Ca2+ and Na+ ions) in water (Simulated sea water and Songhua River) after seawater desalination were far below the ion concentrations limit set by the World Health Organization (WHO) and Environmental Protection Agency (EPA) standards for drinking water. CC had the advantages of wide spectrum absorption, excellent photothermal properties, long durability and recycling performance, which provided requirements for excellent seawater desalination and sewage disposal performance. This work provides a new opportunity for seawater desalination and sewage disposal, which will promote the solving dilemma of freshwater shortage in water shortage areas and polluted areas. 2. RESULTS AND DISCUSSION

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Figure 1. Typical XRD patterns (a), UV-vis reflectance spectra (b), SEM images (c), (d), and the corresponding elemental mapping of C (e) and Ag (f) of ANCC. The inset of (c) is the XPS spectra for Ag 3d. The crystalline composition and structure of the as-prepared samples are investigated by XRD measurements. The XRD patterns of CC and ANCC still show two obvious crystal peaks at 2θ = 24.6 and 44.7° (Figure 1a), which could be ascribed to the (002) and (100) planes for graphite.32 After being modified by Ag NPs, it can be easily observed as the Ag loading increases gradually. The peaks located at 38.0° which is indexed as the (111) inflections of Ag (PDF#04-0783), indicating that Ag NPs are anchored on CC.33 High wide-spectrum light absorption of optical absorber is essential to obtain superior seawater desalination performance. The optical absorption

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of the CC and ANCC has slight reduction along with the growth of long wavelength direction. However, the absorption is still more than 92.39 % for the entire range of optical spectrum (200 2500 nm). It obviously shows the SPR peaks after modified by Ag NPs (Figure S1, Supporting Information). The low-magnification SEM image as shown in Figure 1c reveals that the optimal ANCC is composed of a large quantity of nanowires liking a rope, and each nanowire is scattered with few NPs. And the SEM image of CC is shown in the Figure S5 (Supporting Information). From the high-magnification SEM image (Figure 1d), it can be clearly seen that the NPs are composed of a large number of irregular aggregated NPs. And the corresponding elemental distributions of C (Figure 1e) and Ag (Figure 1f) are obtained by the energy dispersive X-ray EDS elemental mapping. Obviously, Ag NPs are modified the nanowires uniformly, favoring the utilization of Ag NPs sufficiently. From the Ag 3d spectrum in inset of Figure 1c, it shows two obvious peaks of the Ag 3d 3/2 and Ag 3d 5/2 binding energy, locating at about 366.2 and 372.2 eV, respectively. And there is an obvious proof of the existence of metallic silver, and the spin energy separation is 6.0 eV, indicating that the elemental silver is efficiently modified to CC.34

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Figure 2. The evaporation rate of ANCC compared with different loading amounts of Ag under 1 sun intensity (a), Surface temperature changes (b), the evaporation rate changes (c) with the irradiation time of the water, CC, and ANCC under 1 sun intensity. The stable evaporation rates (left-hand side axis) and the corresponding evaporation efficiency (right hand side axis) of the water, CC, and ANCC under 1 sun illumination (d). Cycling performance of the ANCC under 1 sun irradiation (e), the evaporation rates (left-hand side axis) and the corresponding evaporation efficiency (right hand side axis) under various solar illumination intensities (f). Error bars represent average value for the same repeated measurements. Apparently, the seawater desalination performance is judged by actual apparent mass changes. It is estimated for the ANCC by simply placing sample on simulate seawater surface and the evaporation rates of the ANCC are recorded by gauging the mass loss of water as time goes on (Figure 2a). The rate of the evaporation for 0.42, 1.71, 2.77, and 3.43 wt% Ag are 0.78, 0.92, 1.11, and 1.36 kg m-2 h-1, which are measured after the rate is stable by 1 h desalination. It is

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apparent that 3.43 wt% Ag loading processes the best performance for the rate of the evaporation, and the ANCC described in this article refers to 3.43 wt% Ag loading. When the load increases to above 5 wt%, the CC can no longer float on the water surface, and thus can not carry out effective solar-thermal conversion. Moreover, we use an infrared thermometer to explore the surface temperatures of the water, CC and ANCC with illumination time under 1 sun intensity (Figure 2b).” The maximum temperatures among the water, CC and ANCC is growing fast at the beginning and become gradually smooth subsequently. The surface temperature of the water only rises to 4.5 °C, but the surface temperature of ANCC is about 19 °C which is 4 times higher for the water only and more than 2 times higher for CC (≈ 8.5 °C). The ANCC’s high temperature indicates the superior thermal aggregation and confinement characteristic. Figure 2c exhibited the evaporation rate along with irradiation time under 1-sun intensity. The evaporation rate reaches the maximum at about 15 minute and maintains the rate (1.36 kg m-2 h-1). It is 5 times higher than the pure water (0.25 kg m-2 h-1) and nearly 3 times higher than CC (0.51 kg m-2 h-1). Such excellent performance of the ANCC could attribute to the high wide-spectrum light absorption, low thermal emission, fine heat localization, SPR effect and excellent solar-thermal conduction. Exceptionally clear steam could be seen of ANCC, which is a clear naked-eye proof that ANCC has high solar-thermal conversion efficiency and the possibility of practical application of seawater desalination (Movie S1, Supporting Information). Accordingly, the steady evaporation rate and corresponding solar-thermal energy conversion efficiency of water, CC and ANCC under 1-sun intensity after 15 min is achieved (Figure 2d). The efficiency can be calculated to be 10.81, 29.74 and 92.82 % (equation S1 Supporting Information), respectively, and which is superior to other previous reports (Table S1, Supporting Information). Obviously, long-term and cyclic stability of ANCC is also essential for the practical application of seawater

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desalination. Therefore, cyclic stability test is carried out of the ANCC under 1-sun intensity. The rate maintains mechanically stable and almost no decline through 50 cycles (Figure 2e), indicating the superior recyclability and stability.35 In order to research the photothermal conversion performance of the ANCC under more circumstances, the ANCC is tested under different solar intensities (1, 2, 3, 5, 7, 10 sun). The evaporation rate gradually increases from 1.36 to 11.25 kg m-2 h-1, corresponding the solar intensity from 1 to 10. However solar-thermal conversion efficiency gradually decreases from 92.82 % in 1 kW m-2 to 79.45 % in 10 kW m-2, which may be attributable to energy not be fully utilized, and too much transmission into deep water and reflection into the air.

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Figure 3. Water purification performance by ANCC under solar illumination. (a) The contaminated water (MB, MO and RhB) is condensed by Solar-Thermal Energy Converter after solar irradiation; (b) the pollutant water (Atrazine and Metribuzin) is also condensed by SolarThermal Energy Converter after solar irradiation. After the solar treatment, the water contained no MB, MO, RhB, atrazine or metribuzin as evidenced by the near zero optical absorbance, showing an excellent sewage disposal performance. (c) The measured concentrations of four primary ions (Na+, K+, Mg2+, and Ca2+) in an actual saline water sample (from the simulated seawater and songhua river) before and after desalination. After the desalination, the concentration is far below the ion concentration limit set by the World Health Organization (WHO) and the US Environmental Protection Agency (EPA) standards for drinking water.

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The outdoor solar-thermal energy conversion for water purification and desalination is implemented (Figure S2, Supporting Information). It is carried out using an electronic balance with ANCC and natural sunlight outdoors (Harbin, May 19, 2018) to evaluate the open-air performance. At noon, the solar intensity reaches the maximum and the evaporation rate also achieves the maximum. From 7:00 in the morning to 18:00 in the afternoon, the total evaporate amount of water for one day is 4.49 Kg m-2, which fundamentally proves the practical applicability of ANCC’s seawater desalination and sewage disposal. The solar-thermal conversion performance of ANCC is tested at different wavelengths. We can see clearly the production of steam under different wavelengths lights from optical images (365, 420, 450, 520, 550, 590, 650, 700 and 950 nm) (Figure S3, Supporting Information), and the video is recorded (Movie S2, Supporting Information). The wavelength-dependent apparent quantum efficiency (AQE) for seawater desalination over ANCC is shown in the Figure S6 (Supporting Information), which could further confirm that the ANCC is an excellent photothermal material for full spectrum absorption. Weak steam could be seen under the different monochromatic light, further indicating the ANCC possess almost absorb whole spectrum, which could be corroborated by the UV-vis reflectance spectra. A high efficiency solar steam evaporation system is displayed for the seawater desalination and sewage disposal (Movie S3, Supporting Information). The steam is rapidly formed under irradiation and then condenses into a liquid upon reaching the top glass. The condensed water automatically flows into the bottom of the container along the surface of the container glass, and the clean water is collected into the beaker collector. The evaporation of sewage disposal is purified in the above method, and condensed water and original water samples are measured by optical absorption spectra (Figure 3a). The optical absorption of three kinds of wastewater (MB,

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MO, RhB solution) is tested after purification. And since the optical absorption is almost zero, the condensed water is substantially free of above dye. And the evaporation of sewage disposal (atrazine or metribuzin) is also purified in the same method (Figure 3b).36 After steam condensation process, the water contained almost no atrazine or metribuzin, showing excellent solar-thermal conversion performance for sewage disposal. The seawater desalination performance of the ANCC is systematically revealed in Figure 3c.37 The ion concentrations after desalination measured by the ICP-OES decreased significantly. The water from the simulated seawater and Songhua River are chosen as the water samples. The ion concentrations (Mg2+, K+, Ca2+ and Na+ ions) after desalination are measured by the ICP-OES. It is obvious that the concentrations of Na+, K+, Mg2+, and Ca2+ ions are significantly reduced. After the desalination, it is far below the ion concentration limit set by the WHO and the EPA standards for drinking water.38-40

Figure 4. IR images of the temperature of vertical view for ANCC under 1 sun illumination at various time points (a, b, c and d are 0, 5, 10 and 15 min, respectively), and IR images of the

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temperature of side view for the beaker under 1 sun illumination (e and f are 0 and 15 min, respectively). The ANCC placed in the beaker (100 mL) could float itself on the surface of the water without auxiliary support. In order to observe visually and clearly the change in surface temperature as the irradiation time goes, an infrared camera (IR camera) is applied to the experimental test. The surface temperature distribution of ANCC was tested before and during 1-sun irradiation (Figure 4a-d). The primary water temperature is approximately 17.0 °C, which is basically consistent with laboratory indoor temperature (Harbin, April 13, 2018). Under 1-sun illumination, the uniform and red temperature pattern is shown, and the temperature is rising rapidly and gradually to the maximum temperature of 36 °C from 0 to 15 min. Due to the SPR effect and the characteristic properties of CC, most of the heat is highly localized at ANCC-water interface area, it displays distinct temperature evolution from the top to bottom areas (Figure 4e-f). The result indicates heat localization or confinement at the top surface of the ANCC converter minimized heat loss, which ensures a high and stable evaporation rate and high solar-thermal conversion efficiency.

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Figure 5. IR images of the temperature of vertical view for ANCC after reaching temperature stable state under various solar illumination intensities (a, b, c, d, e and f are 1, 2, 3, 5, 7 and 10 sun, respectively). The temperature of the ANCC surface is recorded by an IR camera under different solar intensities (1, 2, 3, 5, 7, 10 sun). To further explore solar-thermal conversion performance of the ANCC, the surface temperature distribution of the ANCC is tested after stability in the Figure 5. The light intensity is 1, 2, 3, 5, 7 and 10 KW m-2, and in the 15 min, the corresponding temperature is 36.1, 40.2, 46.2, 50.7, 60.6 and 73.3 °C, respectively, which indicates that the surface temperature of the ANCC will also increase with the increase of light intensity. And this conclusion is consistent with the results obtained in Figure 2f. And the several major obstacles for practical application are the unworkability, high investment, vulnerable, low conversion efficiency of solar absorber materials. In this work, the CC-based material has succeeded in overcoming these problems. Photo image of commercial CC is cut manual different sizes, which overcomes several major obstacles toward practical application, such as the high cost,

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unworkability, vulnerable, low conversion efficiency of solar absorber materials (Figure S4, Supporting Information). In addition, the industrialized process of commercial CC has matured, which will provide great convenience and possibility for the practical application of solarthermal energy conversion for water purification and desalination.

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Figure 6. A schematic of solar-thermal conversion difference between CC and ANCC. (a) The energy transfer sketch map in the process of solar-thermal conversion; (b) a schematic of heat generation difference between CC and ANCC; (c) indoor laboratory testing diagram.

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Due to carbon materials’ low density and seawater’ high density, the ANCC can self-float on the seawater, and because of the porous structure, the ANCC could siphon from water independently. The heat transfer mechanism involved in the floating solar-thermal absorber material under 1 sun illumination is revealed in Figure 6. As shown in 6a, the entire process of solar-thermal conversion has the final evaporation (92.82%), radiative (~3.69%), convective (~2.12%) with the environment and conductive (~1.26%) of lower water. The energy loss of the whole process is the root cause of limiting the solar-thermal conversion efficiency. In this work, the Ag-modified CC is used as the research subject to overcome the shortcomings of other solar absorber materials, such as high reflectivity, high thermal conductivity, unworkability, high investment, vulnerable and low conversion efficiency. As shown in Figure 6b, the ANCC composite possesses secondary structures compared to CC. First, high wide-spectrum light absorption of the ANCC with the SPR effect guarantees efficient and wide optical absorption in the whole spectral range. Then, due to the SPR effect and the low thermal conductivity characteristic of carbon material and the exothermic heat dominated by nuclear motion of Ag, most of the heat is highly localized at ANCC-water interface area, which indicates heat localization or confinement at the top surface of the ANCC converter minimized heat loss. It ensures a high and stable evaporation rate and high solar-thermal conversion efficiency. And from the Figure 6c, it vividly shows laboratory and outdoor testing procedures for solar-thermal conversion experiments.

3. CONCLUSIONS In conclusion, we firstly synthesized plasmon Ag promoted CC-based solar-thermal energy conversion for seawater desalination and sewage disposal. The ANCC composite had a high

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wide-spectrum light absorption (92.39 %) for the entire range of optical spectrum (200 - 2500 nm). The vaporization rate was up to 1.36 kg h-1 m-2 under 1 sun intensity, which corresponded to conversion efficiencies of ~92.82% with superior water desalination and purification performance. After the desalination, the ion concentrations (Mg2+, K+, Ca2+ and Na+ ions) in water (Simulated sea water and Songhua River) were far below the ion concentration of lower limit of drinking water quality index formulated by the WHO and the EPA standard. For practical applications, low-cost metal such as Al may be better choice for high-performance solar-thermal materials than that of noble metals. Such carbon cloth based solar-thermal material can provide a paradigm solution to solve the water shortage in remote mountainous areas and water shortage areas.

4. EXPERIMENTAL METHODS 4.1 Materials Ethanol (EtOH, CAS 64-17-5, 99.7 %), AgNO 3 (CAS 16940-66-2, 99.8 %) were purchased from Tianjin Kermel limited company. Commercial hydrophobic carbon cloth was purchased from Shanghai Hesen Electric Co., Ltd. Deionized water was used for all experiments. 4.2 Synthesis of ANCC composite Carbon cloth were washed with acetone, 15% HCl, ethanol and deionized water with violent ultrasonication sequentially, and then dried at 60 °C in an oven overnight. A certain amount of AgNO 3 was dissolved in 100 mL of deionized water and 150 mL of ethanol, vigorously stirred for 10 min to form a homogeneous solution. Subsequently, 6 * 6 cm carbon cloth was added to the above solution, then fiercely ultrasoniced treatment for 1 h. After that, it was irradiated under

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a UV-lamp (300 W) for 1 h. Then, the suspension was washed by deionized water and ethanol three times to remove the remanent silver ions and unreacted silver nitrate and finally dried at 60 °C in air. We can cut the prepared carbon cloth into any shape by hand cutting. This work is cut into about 5.5 cm diameter circles. ASSOCIATED CONTENT Supporting Information Characterization, solar vapor generation experiment, calculation for conversion, the partially magnified (300-850 nm) UV-vis reflectance spectra of CC and ANCC, experiment of photothermal conversion in outdoor real solar light, optical images of vapor generation under different single-wavelengths of whole spectrum, photo images of commercial carbon cloth with different sizes, SEM image of CC, wavelength-dependent apparent quantum efficiency (AQE) for seawater desalination over ANCC, Movie S1, Movie S2 and Movie S3, the solar-thermal performance and structure properties of materials.

AUTHOR INFORMATION Corresponding Author Wei Zhou* E-mail: [email protected] Notes The authors declare no competing financial interests.

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ACKNOWLEDGEMENTS We gratefully acknowledge the support of the National Natural Science Foundation of China (21871078, 51672073). REFERENCES (1) Post, V.; Groen, J.; Kooi, H.; Person, M.; Ge, S.; Edmunds, W. Offshore Fresh Groundwater Reserves as a Global Phenomenon. Nature 2013, 504, 71-78. (2) Lewis, N. Research Opportunities to Advance Solar Energy Utilization. Science 2016, 351, aad1920. (3) Xu, N.; Hu, X.; Xu, W.; Li, X.; Zhou, L.; Zhu, S.; Zhu, J. Mushrooms as Efficient Solar Steam-Generation Devices. Adv. Mater. 2017, 29, 1606762. (4) Liu, H.; Chen, C.; Chen, G.; Kuang, Y.; Zhao, X.; Song, J.; Jia, C.; Xu, X.; Hitz, E.; Xie, H.; Wang, S.; Jiang, F.; Li, T.; Li, Y.; Gong, A.; Yang, R.; Das, S.; Hu, L. High-Performance Solar Steam Device with Layered Channels: Artificial Tree with a Reversed Design. Adv. Energy Mater. 2018, 8, 1701616. (5) Zhao, F.; Zhou, X.; Shi, Y.; Qian, X.; Alexander, M.; Zhao, X.; Mendez, S.; Yang, R.; Qu, L.; Yu, G. Highly Efficient Solar Vapour Generation via Hierarchically Nanostructured Gels. Nat. Nanotechnology 2018, 13, 489-495. (6) Zhu, L.; Gao, M.; Peh, C.; Wang, X.; Ho, G. Self-Contained Monolithic Carbon Sponges for Solar-Driven Interfacial Water Evaporation Distillation and Electricity Generation. Adv. Energy Mater. 2018, 8, 1702149.

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