A broadband aggregation-independent plasmonic absorber for highly efficient solar steam generation

Zhongming Huang , Shengliang Li *, Xiao Cui , Yingpeng Wan , Yafang Xiao , Shuang Tian , Hui Wang , Xiaozhen Li , Qi Zhao and Chun-Sing Lee *
Center of Super-Diamond and Advanced Films (COSDAF), Department of Chemistry, City University of Hong Kong, Hong Kong SAR, 999077 P. R. China. E-mail: lishengliang@iccas.ac.cn; apcslee@cityu.edu.hk

Received 19th February 2020 , Accepted 23rd March 2020

First published on 24th March 2020


Achieving efficient solar steam generation under natural sunlight has huge potential for sewage purification and seawater desalination. Plasmonic resonance has been extensively exploited for enhancing and extending the range of optical absorption. Until now, most reported broadband plasmonic solar absorbers have been designed by compact aggregation or engineering plasmonic architectures. In this work, we develop a new plasmonic absorber using gold nanostructures with the shape of a trepang (nano-trepang). By rationally regulating anisotropy at the single nanoparticle level, the nano-trepang shows good optical absorption over the entire solar spectrum (92.9%) with no requirement of engineering nanoparticle aggregation or constructing plasmonic architectures. The nano-trepang was then loaded into a polymeric aerogel and the network showed an excellent solar-to-vapor energy conversion efficiency of 79.3%. Under 1 sun AM1.5 G irradiation, a stable solar evaporation rate of 2.7 kg m−2 h−1 can be achieved, with high performance anti-salt precipitation in practical seawater steam generation. This work shows a broadband plasmonic absorber with aggregation-independent performance for highly efficient solar stream generation and provides a new strategy for practical solar desalination.


Solar steam generation is considered a promising platform for seawater desalination,1 water purification,2 distillation and sterilization.3,4 As the core component in all solar steam generation systems, an ideal solar light absorber should satisfy several criteria: broadband absorption over the solar spectrum, higher photothermal conversion efficiency (PCE),5–7 good thermal insulation and porous networks for vapor escape.8 Over the past few years, a variety of photothermal absorbers have been designed and fabricated for solar steam generation applications, such as carbon-based materials,9 metallic nanoparticles,10 semiconducting materials,11 organic materials and plasmonic absorbers.12,13 Despite the rapid progress, some absorbers suffer from drawbacks such as a narrow-band spectral response, insufficient utilization of solar energy, a complicated preparation process and instability over time. Therefore, it remains urgent to develop photothermal convertors for overcoming these challenges.

Metallic plasmonic nanomaterials, such as Au, Ag, Cu, and Al, have attracted tremendous attention in the fields of photothermal conversion, photovoltaic cells and solar distillation because of their unique optical and electrical performances.14–17 As excellent photothermal converters, plasmonic nanomaterials can effectively harvest and trap solar photons for photothermal conversion through multiple surface plasmon resonances (SPR).18–21 However, most of these absorbers have a relatively narrow absorption range which hinders their wider applications in solar energy conversion.22–24 Recently, much effort has been put on developing novel plasmonic architectures for absorption spanning the solar spectrum.25–29 Compact aggregation or close-packed distributions of plasmonic nanoparticles have been demonstrated to be effective approaches for widening their optical absorption ranges.30–32 Another effective approach is to employ plasmonic architectures prepared by top-down methods. For instance, construction of arrays of subwavelength metallic gratings,33 metallic gap resonators,34 microcavities,35 and nanowire bundle arrays has been explored.36 The performance of solar absorbers prepared by both the above approaches depends critically on maintaining a precise spacing between the neighboring nanostructures. This not only puts stringent requirements on the fabrication processes, possible changes in the nanometer-scale gaps between the nanostructures during operation are also a potential threat to the system stability.35

Herein, we reported, for the first time, a broad-band aggregation-independent solar absorber integrating multi-resonance at the single-particle level instead of relying on resonance effects involving aggregation between multiple nanostructures. By manipulating the structural anisotropy and morphology of nanoparticles, we obtained gold nanostructures with the shape of a trepang that shows good optical absorption over the entire solar spectrum. Unlike previously reported plasmonic nanostructures with a wide frequency absorption range, the present gold nanostructures do not require aggregation to achieve wide-range absorption. Based on this full spectrum solar absorber, a plasmonic aerogel network with strong and broadband absorption is constructed for solar steam generation applications (Scheme 1).


image file: d0ta01980a-s1.tif
Scheme 1 Schematic process of solar steam generation by the plasmonic aerogel network.

Construction of the full-spectrum solar plasmonic absorber is schematically shown in Fig. 1a. The prepared gold nanorods (hereafter referred to as AuNR0) show uniform rod morphology without any surface extrusion (Fig. 1b, top-left). AuNR0 was then used as a seed and allowed to further grow after adding the chiral template (L-cysteine) into the growth solution. It is interesting that after addition of L-cysteine, further growth in the size of nanorods is accompanied by the formation of extrusions in the shape of “horns” near the tips of the nanorods (inset of Fig. 1b AuNR1, see more TEM images in Fig. S1). Further growth in size leads to the formation of more horns at the tips of the nanorods (Fig. 1b, AuNR2). At the end of the growth process, the horn structures covered the entire length of the nanorod (Fig. 1b, nano-trepang) leading to a structure with a similar shape to a trepang. Changes in particle size and aspect ratio are summarized in Fig. 1c. Effects of the morphology changes on the optical absorption of the particles are shown in Fig. 1d. It is evident that the optical absorbance of the particles increases with the growth duration. In particular, the absorbance of the nano-trepang is not only much better than the other samples, it also shows wide and featureless absorption from 300 to 1100 nm. Fig. S2 shows that the full-range absorption can be maintained even after 20× dilution of the original aqueous dispersion, confirming that broad-band absorption is achieved with the individual nano-trepang instead of solid aggregation.


image file: d0ta01980a-f1.tif
Fig. 1 (a) Schematic of the epitaxial growth process by the seed-induced method. (b) The SEM image of AuNR0; AuNR1; AuNR2; the nano-trepang (inset images are corresponding TEM images and photographs of the NP aqueous dispersion, scale bar, 500 nm). (c) Dimension and aspect ratio summary of NPs. (d) Absorption spectra from AuNR0 to the nano-trepang. (e) Photothermal performance of the nano-trepang aqueous dispersion (200 μg mL−1) under Xe lamp irradiation (5 kW m−2).

Encouraged by their desirable solar-absorption properties, photothermal heating abilities of the NP samples are explored under irradiation with a xenon lamp source (PLS-SXE300D/300DUV, Beijing Perfectlight, China) at an intensity of 5 kW m−2. As shown in Fig. 1e, all the NP samples (200 μg mL−1) exhibit rapid temperature rises. The nano-trepang shows the fastest temperature increase with a maximum temperature of 76.8 °C after 5 min irradiation. The good photothermal response motivates us to explore possible solar steam generation applications of the nano-trepang. We prepare a hierarchical plasmonic aerogel network (PAN) by in situ gelation utilizing polyvinyl alcohol (PVA) which is a popular network scaffold with high hydrophilicity and low thermal conductivity.8,37

The gelation process is activated by adding hydrochloric acid after the PVA solution is loaded with the nano-trepang. The final PAN is subsequently formed after lyophilization (Fig. 2a). Hybrid hydrogels with different concentrations of nano-trepang loading are prepared and listed in Fig. S3. For comparison, AuNR0-doped hybrid hydrogels are also prepared (also see in Fig. S3). The hybrid hydrogels can be molded into different shapes (Fig. S4) and the preparation process can be easily scaled up. The polymeric framework with porous structures endows the network with low mass density (Fig. 2b) for floating on water and provides capillary channels for water supply. The PANs also have good elasticity (Fig. 2c). The SEM image (Fig. 2d) reveals that the blank hydrogel network has a 3-dimensional (3D) interconnected architecture and disordered capillary channels with diameters around one hundred microns. In addition, pores with much smaller sizes of about several microns are distributed on the wall structure of the capillary channels (Fig. 2e). Fig. 2f and g show PAN loaded with the nano-trepang. Elemental mapping is further studied using an X-ray energy dispersive spectrometer attached to a SEM. Fig. S5 shows that the AuNRs have fairly uniform distribution in the PAN. For practical applications, it is important to ensure that the nano-trepang is firmly adhered to the PAN. To confirm this, we tore a piece of 0.6 wt% nano-trepang loaded PAN into several fragments and ultrasonicated them in a water bath overnight (Fig. S6a and b). After ultrasonication, the water showed no observable change in color or absorption which confirms no loss of nano-trepang from the PAN. Considering that the market price of PVA and gold salt is $1.8/25 kg and $52.5/1 g, respectively. The total cost of this PAN prototype with a thickness of 0.5 cm is estimated to be around $7.7 per m2. Moreover, the nano-trepang loaded PAN exhibits excellent light absorption capacity in the visible and near infrared spectra region owing to its low light reflectance (Fig. 2h), while the AuNR0-doped PAN network shows much higher light reflectance (Fig. S7). To investigate the solar-thermal behavior, the prepared PAN is exposed to simulated solar irradiation (3 kW m−2, i.e. 3 sun AM1.5 G). The temperature of the PAN rapidly rises up to 45.3 °C after 25 s irradiation (Fig. 2i) while there is a negligible temperature change in a blank PVA network under the same irradiation conditions. More details on the solar thermal conversion experiments are shown in Fig. S8.


image file: d0ta01980a-f2.tif
Fig. 2 (a) Schematic illustration of the PAN formation process. (b) Photograph of a piece of PAN loaded on a leaf. (c) Photographs of the prepared PAN compressed and released. SEM images of (d) a blank PVA network and (e) its wall structure. SEM images of (f) a piece of PAN doped with 0.6 wt% nano-trepang and (g) its wall structure. (h) Reflectance spectra of a blank PVA network (grey line) and a piece of PAN doped with 0.6 wt% nano-trepang (black line) (insets are their photographs). (i) Temperature profiles of the blank PVA network and the PAN after Xe lamp irradiation for 25 s at 3 kW m−2.

Fig. 3a shows a schematic diagram of the subsequent photothermal steam generation experiments. A piece of PAN (28 mm diameter, 9 mm thick) loaded with 0.6 wt% nano-trepang was floated on water in a 25 mL beaker with an internal diameter of 32 mm. A considerable amount of steam can be generated upon irradiation with a solar simulator at 5 kW m−2 (AM1.5 G) for 180 s (Fig. 3b and Video S1). To estimate the photothermal performance under natural sun irradiation, we reduce the irradiation intensity to 1 kW m−2. The surface of the PAN shows a uniform temperature of about 55 °C (Fig. 3c, the inset is the IR thermal image of PAN). Temperature distribution measured from the side along the thickness direction is shown in Fig. 3d (see IR thermal images in the inset). The results indicate that the heating effect is highly localized within the thickness of the PAN sample and has little spread into the water. The highly localized heating is beneficial for efficient steam generation. For comparison, we repeated the experiment with a blank PVA network and observed a negligible temperature rise (see IR thermal images in the inset of Fig. 3c and d). Corresponding photographs of the floating PAN and the 3D temperature profiles are shown in Fig. S9.


image file: d0ta01980a-f3.tif
Fig. 3 (a) Schematic illustration of the photothermal steam generation experiment. (b) Photograph of the water steam generation upon irradiation for 180 s with a solar simulator (AM1.5 G) at 5 kW m−2. (c) Surface temperature distribution of the PAN water–air interface and; (d) temperature profile on the side along the thickness direction upon 30 minutes solar simulator irradiation (AM1.5 G) at 1 kW m−2.

We next explore the influence of water submersion depth on the vaporization rate, the PAN was fixed at four submersion depths: fully submersed in water (H1 = 0 mm), freely floating (H2 = 2 mm), and suspended at certain heights (H3 = 5 mm and H4 = 9 mm) (Fig. S10a). The corresponding IR images were recorded (Fig. S10b) under one sun irradiation. The temperature of the H1 case is evidently lower (32.1 °C) compared with the other cases. This phenomenon can be ascribed to bulk water that hinders the amount of light arriving at PAN, which influences the light to heat conversion. The temperature profiles of the PAN water–air surface and mass change under solar simulator are summarized in Fig. S10c and d. Higher surface temperature can be achieved for the H4 case owing to the heat localization, however, such a height increases the supply path that reduces the water supply rate for surface vaporization, which leads to a decrease in vaporization efficiency. The results indicate that the highest vaporization efficiency can be achieved with the free-floating mode.

In a typical steam generation experiment, the prepared PAN floats on water under solar simulator irradiation, and the photographs and IR images of PAN with different mass loadings of nano-trepang are shown in Fig. 4a, namely PAN-1 (0.05 wt% nano-trepang), PAN-1 (0.2 wt% nano-trepang), and PAN-1 (0.6 wt% nano-trepang). The surface temperatures of porous networks sharply rise and reach a plateau at ∼33.3 °C (PAN-1), 44.7 °C (PAN-2), and 55.7 °C (PAN-3) (Fig. 4b). Water evaporation rates of PAN-1, PAN-2, and PAN-3 are measured to be 1.0, 1.9 and 2.7 kg m−2 h−1 after evaporation stabilization for 2 h (1 sun AM1.5 G), which are correspondingly 5.6, 11.0 and 16.1 times higher than that of the blank PVA network (Fig. 4c). Benefiting from the excellent absorption properties throughout the whole solar spectrum, the temperature of PAN-3 rapidly increases to 96 °C after 500 s irradiation (5 sun) (Fig. 4d). Furthermore, as shown in Fig. 4e, we also measure the evaporation rates of PAN-3 under 1 sun (2.7 kg m−2 h−1), 3 sun (7.2 kg m−2 h−1) and 5 sun (14.7 kg m−2 h−1), which are competitive with those of other state-of-the-art solar–vapor conversion systems41 (Table S2 in the ESI). Remarkably, the external solar-heat conversion efficiency reaches up to 79.3% (ESI). Comparison of the solar–vapor conversion efficiency of recently reported work is also listed in Table S2. The surface temperature and mass change of PAN-1 and PAN-2 under various optical densities are also shown in Fig. S11–S14.Fig. 4f summarizes the evaporation rates and efficiencies of the PAN with different weight loadings of nano-trepang under 1 sun AM1.5 G. The reusability test was carried out by recording the mass change rate of PAN-3 during six light on and off cycles (Fig. 4g), showing that PAN-3 is a stable device for continuous steam generation at different light densities.


image file: d0ta01980a-f4.tif
Fig. 4 (a) Photographs and IR images of the blank PVA network, PAN-1, PAN-2, and PAN-3 under irradiation with a solar simulator (1 sun AM1.5 G). (b) The surface temperature changes and corresponding mass changes (c) of different PAN over time under 1 sun AM1.5 G. (d) Surface temperature difference and mass change (e) of PAN-3 under different solar irradiation densities. (f) Summary of evaporation rate and energy efficiency under 1 sun AM 1.5 G. (g) Stability and reusability of PAN-3 under different solar simulation light intensities. Each cycle lasted for 0.5 h. All experiments were carried out at the ambient temperature of 20 °C and humidity of 55%.

Salt accumulation on the surface of the solar evaporator during the seawater desalination process will hamper solar absorption and clog the water replenishment, leading to compromised steam generation efficiency.38 In order to address this issue, artificial ordered drilling channel-arrays are made within PAN-3 to lower salt precipitation according to reported work.39 The photographs and IR images of bare PAN-3 and PAN-3 with channel arrays under solar irradiation are recorded in Fig. 5a, PAN-3 with channel arrays shows less salt precipitation (Fig. 5a), and much more stable steam generation (2.4 kg m−2 h−1) than that of PAN without channel arrays that decreases to 1.4 kg m−2 h−1 after irradiation for 180 min (Fig. 5b). These results indicate that the employment of channels arrays is helpful for anti-salt precipitation in seawater desalination. Furthermore, we also evaluated the water purification performance of PAN-3, and the concentration of primary ions (Na+, K+, Mg2+, and Ca2+) in seawater decreases sharply after evaporation showing that the evaporated water quality meets the WHO standards of drinking water (Fig. 5c).40 Moreover, we also studied the purification of industrial waste water containing mixed heavy metal ions, like Hg2+, Cd2+, and Ag+, and the concentration of heavy metal ions dropped to 0.01 mg L−1 after purification (Fig. S15), which demonstrates that this technique is promising for industrial sewage purification. With broadband solar absorption and anti-salt performance, our PAN also has a high evaporation rate of 2.7 kg m−2 h−1 in water evaporation and is successful in achieving seawater desalination and wastewater purification. Compared with other reported systems, preparation of the present system is relatively simple and shows a high evaporation rate. On the other hand, although the efficiency of the present system is not among the highest, we consider that the high evaporation rate is more important from the application point of view. Nevertheless, it has to be mentioned that as the present system uses gold as the main absorber, it might be less “green” and cost effective, compared to those using biomasses.


image file: d0ta01980a-f5.tif
Fig. 5 (a) The photographs and IR images of the hybrid hydrogel without and with channel arrays and the photographs of salt precipitated on PAN-3, scale bar: 10 mm. (b) The comparison of evaporation rate with and without a channel-array design. (c) Concentration change of major ions in true seawater samples without and with solar desalination.

Conclusions

In summary, we have demonstrated an aggregation-independent solar absorber by regulating the surface anisotropies of plasmonic nanoparticles with highly efficient absorption over the visible-infrared region for solar steam generation. We fabricated a free-floating, PVA polymer-based structural hierarchy network by incorporating the plasmonic absorber. The solar steam generation rate can reach 2.7 kg m−2 h−1 under one sun irradiation. Anti-salt precipitation in true solar seawater desalination was achieved by channel array administration. Furthermore, the solar water purification performance indicates remarkable potential for pollution abatement in industry. Considering the increasing demand for solar energy driven water purification, our plasmonic aerogel network holds great promise not only in solar steam generation but also for a wide range of applications such as solar distillation, photothermal therapy, sterilization and solar-thermal catalysis.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Hong Kong Innovation and Technology Commission (Project No. ITS/219/19) and City University of Hong Kong (ARG-CityU Applied Research Grant: Project No. 9667160 and 9667179).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ta01980a

This journal is © The Royal Society of Chemistry 2020