A Bioinspired, Reusable, Paper‐Based System for High‐Performance Large‐Scale Evaporation

Abstract

A bioinspired, reusable, paper-based gold-nanoparticle film is fabricated by depositing an as-prepared gold-nanoparticle thin film on airlaid paper. This paper-based system with enhanced surface roughness and low thermal conductivity exhibits increased efficiency of evaporation, scale-up potential, and proven reusability. It is also demonstrated to be potentially useful in seawater desalination. Solar-enabled evaporation is a solar-energy-harvesting technology that can be used in modern power plants, chemical plants, and seawater desalination plants.1-6 Multiple technological advances such as optimizing the mechanical systems and improvement of facilities have been implemented to increase the evaporation efficiency (i.e., the ratio of the energy used for the vaporization of the liquid to the total energy input) of evaporators, together with a lower equipment cost for the evaporation process.7-9 The process, however, still costs the world thermal-power market billions of dollars due to energy loss and the maintenance of these facilities every year.7, 9 To improve the efficiency of the evaporation process, we recently developed a new evaporation system based on assembled plasmonic gold nanoparticle (AuNP) thin film, which was inspired by biological evaporation systems such as human skin and plant leaves.10, 11 Plasmonic NPs and their assemblies have attracted tremendous attention because of their unique optical, electrochemical, and photothermal properties.12-20 Herein, a plasmonic NP thin film is utilized as a light-to-heat converter during the evaporation.10 Halas and her co-workers previously reported that aqueous solution containing light-absorbing AuNPs exposed to sunlight can induce plasmonic heating, resulting in relatively efficient evaporation.21, 22 The AuNPs served as localized light-to-heat converters and were dispersed throughout the solution. Vapor bubbles were generated inside the solution and travelled to the water–air interface to release the steam trapped inside the bubbles. The heat transfer from the traveling vapor bubbles to the bulk water intrinsically limited the efficiency of evaporation applications where the heating of bulk water is not necessary. Our bioinspired floating thin-film based approach, however, focuses the intense plasmonic heating on the evaporative surface (water–air interface), resulting in the generation of vapor bubbles close to the evaporative surface. These bubbles do not need to travel within the bulk solution so the heat loss is reduced and the evaporation efficiency is improved.10 Though a high performance of evaporation can be achieved via illuminating the plasmonic thin-film system, in real applications, such as the desalination process, a retractable sheet/film that floats at the air–water interface would be preferred for the application and the reuse of the plasmonic system. At the present stage, it is not feasible to directly apply the assembled plasmonic film in such a form and reuse the system. Three main challenges impede the industrial application of this thin-film based evaporation approach: i) scale-up of AuNP films in a large evaporation system. The as-prepared AuNP film is fragile and hard to be transferred between various large evaporation systems; ii) reusability of the AuNP film. The fragility of the film also makes it impossible to recycle and reuse; iii) heat loss to the non-evaporative portion of the liquid due to thermal diffusion.23 As shown in our previous work, even though the free-floating AuNP film enables the reduction of heat loss occurs in the dispersed aqueous AuNP solution, thermal diffusion from the heated AuNP film to the non-evaporative portion of liquid beneath the thin film is inevitable.10 To address these challenges, we took the approach of examining biological evaporation systems for inspiration. In skins, the biological tissues support the evaporation system (sweat pores, sweat glands, and blood vessels) and provide the mechanical stability. The low thermal conductivity of these tissues (≈0.3 W m−1 K−1) also help reduce the heat transfer from the evaporative skin surface to the internal biological parts.24 Inspired by such biological design, in this work, we employed a paper substrate, which provides both mechanical stability and low thermal conductivity, as a large scale and transferable support for the plasmonic thin film (Figure 1a). Paper, high-abundance and low-cost, has drawn much interest in electronic devices, batteries, and analytical and clinical chemistry.25-32 In this study, airlaid paper, a textile-like porous hydrophilic material with low thermal conductivity (0.03–0.05 W m−1 K−1), is selected as the paper substrate for AuNP thin film.33 Airlaid paper not only offers a mechanically stable support but also enables the reusability and large-scale production of the plasmonic thin-film evaporation system. This work demonstrates that airlaid-paper-based AuNP film (PGF) is portable and can be recycled at least 30 times. The paper substrate, exhibiting increased surface roughness, can produce multiscattering of incident solar light, yielding high absorption of incident light. The innumerable microscale pores within the paper layer can potentially leverage capillary water flow to the hot zone, leading to rapid replenishment of surface water evaporated.34 The low thermal conductivity allows airlaid paper to behave as a good thermal insulator to prevent the heat transfer from the plasmonic film to bulk water. Such increased light absorption and reduced heat loss enable PGF to exhibit much higher evaporation efficiency (77.8%) than free-standing plasmonic film (47.8%). With the scale-up potential, demonstrated reusability and increased evaporation efficiency, the paper-based evaporation system shows a broad prospect in the commercial applications. The fabrication process of PGF is simple and straightforward (Figure 1b). AuNPs (diameter = 17.4 ± 0.3 nm) were synthesized via the citrate reduction method (see Figure S1, Supporting Information for synthesis details).35 The self-assembled AuNP films were formed by incubating aqueous AuNP solution overnight in the presence of formic acid. When the formic acid vapor diffused into the solution, the negative surface charge of the AuNPs would be balanced by the diffusing H+ dissociated from formic acid.35 Once the AuNPs lost surface charge, they would be destabilized and then trapped at the water/air interface, and they eventually formed a shiny metallic film floating on the solution surface (Figure 1c). After removing the excess solution, the preformed AuNP film was deposited on the airlaid paper and this as-prepared PGF would be stored up for further steam-generation experiments. We noticed that PGF exhibits both a dark purple color and some black dots after being dried completely (Figure 1d). Close examination of the scanning electron microscopy (SEM) image revealed a continuous self-assembled AuNPs film attached on individual paper fibers. AuNP films sitting on different fibers, however, are not connected. This microscale morphology leads to the increased absorption for PGF surface (Figure 1e). In our previous work, we demonstrated that a hot zone could be created by absorbing and confining solar energy within a AuNP thin film and a fairly high evaporation rate was achieved.10 Figure 2a and Figure S2, Supporting Information show that both airlaid paper and PGF exhibited a uniform surface temperature distribution around 27 °C before solar illumination. An IR camera was used to measure the surface temperature36, 37 and such measurement was calibrated using thermocouples (see Supporting Information). The measurement uncertainty of the calibrated IR camera is ≈1.0 °C. Water was filled close to the top of the container so all the IR images were taken as the films were floating near the top of the container. After sunlight illumination for 15 min at a power density of 4.5 kW m−2, hot zones were observed on both PGF and airlaid paper. IR images, however, show that the surface temperature of the PGF reached as high as 80 °C, while the surface temperature of plain airlaid paper remained at ≈40 °C (Figure 2b,c). We reason that localized plasmonic heating was induced by the AuNP thin film on PGF, which resulted in higher surface temperature for PGF than that for plain airlaid paper. The as-prepared PGF shows promising prospects in large-scale production and excellent recyclability, which will further enhance its applicability in industrial applications. Figure 2d demonstrates that we have the capability of producing PGF at relatively large-scale. To prove the reusability, the evaporation rate of PGF is measured by recording the weight change as a function of time, and the evaporation experiment was cycled for 30 times under the same conditions. For each cycle, PGF floating on the surface of water was illuminated by focused light with an optical density of 4.5 kW m−2 for 15 min. After 15 min, the wetted PGF was completely dried in a drying oven and prepared for the next cycle. The maximum, minimum, and average weight changes are plotted in Figure 2e as a function of time. Figure 2f shows the results as a function of weight change versus cycle number. The weight change has an average of 1.25 g and does not deviate far away from the first run even after 30 times cycled. We ascribe the high evaporation rate and efficiency of PGF under solar illumination to three merits of using airlaid paper: i) enhanced absorption due to the structure-induced multiscattering on the surface of the airlaid paper; ii) increased evaporative surface area due to the large surface roughness of the airlaid paper; and iii) restricted thermal diffusion due to low thermal conductivity of the paper substrate. First, the microstructured fiber support enabled multiscattering and increased the light absorption events and thus the absorption of the AuNP film. Figure 3b and Figure S4, Supporting Information show that PGF has relatively higher absorption (≈87%) and lower transmittance (≈6%) compared with the absorption (≈64%) and transmittance (≈25%) of AuNP film, implying that most of solar light has been absorbed by PGF. The major inefficiency for the free-standing AuNP film is due to the transmitted and reflected solar light (≈36%), while thermal diffusion also plays a minor role for the inefficiency. Airlaid paper, composed of disordered polyester and cellulose fibrils, possesses large-scale surface roughness. As shown in Figure 1e, once a AuNP film is attached to the surface of airlaid paper, the 2D thin film changes to a 3D layout and exhibits enhanced absorption. The 3D structure enhances the multiscattering of the incident light at the surface of PGF and allows PGF to absorb efficiently broadband solar light and achieve higher evaporation rate.39-42 The enhancement of absorption due to multiscattering also offsets the potential decrease of photothermal conversion efficiency induced by the aggregation of AuNPs in the PGF. Second, the porous microstructure of the airlaid paper also increases the evaporative surface of PGF and further enhances its evaporation rate. Noting that both the free-standing AuNP film and the AuNP film on the airlaid paper (PGF) have similar assembled structures, it is plausible that the intrinsic surface roughness of the NP films, whether they are free-standing or on the airlaid paper, should be similar. The NP films on the PGF, however, have extra microscale roughness due to the microstructures at the surface of airlaid paper. We employed 3D optical microscopy to examine such extra microscale roughness of PGF and compared it with that of the AuNP film (Figure 3c,d). The microscale surface roughness ratios (the ratio of the total surface area to the projected surface area) of PGF and free-standing AuNP film are ≈5.8 and ≈1.0, respectively. The increased microscale surface roughness of PGF adds extra evaporative surfaces at the water–vapor interface and helps improve the evaporation rate. Additionally, the porous structure of the airlaid paper at the air/water interface also generates enough capillary forces that enable the replenishment of water as PGF's surface water evaporates (Figure 1e). Last but not least, airlaid paper impedes thermal diffusion to the non-evaporative portion of the liquid. Light-absorbing AuNP film serves as localized light-to-heat converter but it still suffers from heat diffusion to bulk water beneath NP thin film.10 With the airlaid paper inserted between AuNP film and bulk water, heat diffusion, both through conduction and local convection, can be minimized and heat loss can be reduced. Dry airlaid paper has quite low thermal conductivity.33 In this work, airlaid paper was filled with water during the evaporation. We measured the thermal conductivity of wet airlaid paper and PGF using an IR camera (Figure 3e and Figure S5, Supporting Information for details). Both airlaid paper (k = 0.48 W m−1 K−1) and PGF (k = 0.49 W m−1 K−1) show lower thermal conductivity than pure water (k = 0.556 W m−1 K−1).43 The microstructures of airlaid paper can also help suppress local convection. Both the reduced thermal conductivity and the suppressed convection contribute to the reduced heat loss from the evaporative surface to the non-evaporative portion of the water. Evaporation driven by solar energy has been widely utilized in commercial applications, one of which is solar still desalination.1, 2 As a sustainable and pollution-free source, solar energy is used to desalinate seawater to produce high-quality water in many regions all over the world.44 An efficient evaporation process not only can enhance the production and reduce operational cost but can also provide clean drinking water for the world suffering from diminishing fresh water supplies.2 The direct solar desalination process has low productivity and such a major limitation has attracted intensive research efforts.9, 44 To improve the productivity and performance of solar desalination system, a number of designs for the evaporation system have been proposed in the industry.9, 44 The paper-based system developed in this work can enhance the efficiency of light-to-heat conversion and thus potentially increase the productivity. Inspired by biological evaporation systems, we have used a common and low-cost material as the support of a self-assembled AuNP film to fabricate a reusable and highly efficient evaporation system. Such a system can enable the application of plasmonic heating to desalination, fractionation, and sterilization at large scale. Compared to the conventional evaporation process, PGF generates localized plasmonic heating only at the surface of solution while the bulk liquid still remains at low temperature. Also, it maintains high evaporation efficiency even when it is recycled multiple times. Another advantage of PGF is that its fabrication process is simple, scalable, and does not require complex equipment. Furthermore, PGF can be integrated with many existing commercial evaporation systems, resulting in high productivity with minimum extra installation costs. Y.L. and S.Y. contributed equally to this work. This work was supported by National Natural Science Foundation of China (Grant Nos. 51420105009, 91333115, 21401129, and 51403127), Natural Science Foundation of Shanghai (Grant No. 14ZR1423300), the Zhi-Yuan Endowed fund from Shanghai Jiao Tong University, China Postdoctoral Science Foundation Funded Project (Grant Nos. 2014M560327 and 2014T70414), and the postdoctoral international exchange program. The authors thank the Instrumental Analysis Center of Shanghai Jiao Tong University for access to SEM. The authors also thank the State Key Laboratory of Metal Matrix Composites for access to the solar simulator, 3D microscopy, and the UV/vis/NIR spectrometer. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. 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