AU2018341405A1

AU2018341405A1 - Plasmonic absorber and solar-driven steam generation apparatus using same

Info

Publication number: AU2018341405A1
Application number: AU2018341405A
Authority: AU
Inventors: Ho-Suk CHOI; Quoc Chinh TRAN; Ilsun Yoon
Original assignee: Industry Academic Cooperation Foundation of Chungnam National University
Current assignee: Industry Academic Cooperation Foundation of Chungnam National University
Priority date: 2017-09-29
Filing date: 2018-09-17
Publication date: 2020-05-14
Also published as: KR20190038331A; KR102091594B1; AU2021103927A4

Abstract

The present invention relates to: a plasmonic absorber for a plasmonic solar-driven steam generation apparatus, the plasmonic absorber having high photothermal conversion efficiency and capable of being manufactured with a large area by a simple method; and a solar-driven steam generation apparatus using the same and, more specifically, to: a plasmonic absorber for a plasmonic solar-driven steam generation apparatus, in which plasmonic gold nanoparticles are uniformly dispersed and fixed to a porous hydrophilic polymer substrate; and a solar-driven steam generation apparatus using the same.

Description

PLASMONIC ABSORBER AND SOLAR-DRIVEN STEAM GENERATION

APPARATUS USING SAME [Technical Field]

The present invention relates to a plasmonic absorber for a solar-driven steam generation device, in which the plasmonic absorber has high photothermal conversion efficiency and can be fabricated over a large area by a simple method, and a plasmonic solar-derived steam generation device including the same.

[Background Art]

As interest in environmental problems has increased, development of pollution-free renewable energy, which is obtained by acquiring energy that is transformed or dissipated in nature and converting it into usable energy, has been actively made. Examples of renewable energy include wind energy, solar energy, tidal energy, wave energy, and geothermal energy, and it is expected that renewable energy can replace fossil energy such as coal and oil. Among them, solar energy has received the most attention due to its universal, clean, environmentally friendly and sustainable properties, and has been widely used for hydrogen production, power generation, photocatalysts, water purification, desalination, and the like. The annual solar energy reaching the Earth's surface is about 2.85 million EJ/year, and is a vast energy source whose quantity is equivalent to about 10,000 times the world's annual energy consumption. Nevertheless, the use of solar energy is still very limited.

Steam generation by water evaporation, which is a phase transition process, is industrially achieved by two methods. The first method includes heating large amounts of water in a steam engine using fossil fuels to generate steam, but causes environmental pollution. The second method is known as solardriven vapor generation. Conventional solar-driven steam generation systems used for power generation heat water to its boiling point by reflecting or condensing sunlight by a collector using various optical devices. Thermal energy generates electricity by a steam turbine. Many studies on the solar-driven steam generation systems have been conducted for decades, but the solar-driven steam generation systems still have problems in that they are cost-ineffective and have low efficiency.

In recent years, new solar-driven steam generation methods have been developed, called 'plasmonic solar-driven steam generation (PSSG) systems' using plasmonic photothermal materials with high efficiency of converting sunlight into heat. Plasmonic photothermal materials act as a converter that converts light into heat by rapid energy release through electron relaxation, and evaporate water quickly even only by a few seconds of illumination in an environment different from that for conventional solar-driven steam generation systems.

The PSSG systems can be classified into two types according to the position of photothermal conversion material in a liquid state. The first type is a suspension system in which small particles are dispersed in a working fluid, and the second type is a floating system in which aggregates of particles float on the surface of a working fluid. In the suspension system, a large volume of a working fluid has to be heated to its boiling point, and hence heat loss occurs due to heat radiation from a large volume of water, resulting in reduction in steam generation efficiency. In the floating system, heat is concentrated on the surface of water by floating a light absorbing material on the surface of the water, and thus heat loss by the bottom water can be avoided.

A typical one among the floating PSSG systems uses metallic plasmonic properties. Bae et al., Nat. Commun., 2015, 6, 10103, proposed an absorber composed of a black gold thin film, fabricated by a combination of anodization, wet etching and sputtering methods, and the photothermal conversion efficiency of the absorber was 26 to 45% at 1 kW/m² (1 sun) illumination. Deng et al., Small, 2014, 10, 3234-3239, fabricated a gold nanoparticle thin film by self-assembly of gold nanoparticles, and reported that the photothermal conversion efficiency of the gold nanoparticle thin film at 10.18 W/m² illumination was up to 44%, which is more than twice that of a gold nanoparticle suspension system. Furthermore, the same group developed a double-layer absorber composed of paper as a support and a heatinsulating layer, and a gold thin film as a photothermal converter, in order to compensate not only for energy loss caused by heat conduction to a portion of a working fluid, in which no evaporation occurs, but also for the instability of the selfassembled gold nanoparticle thin film (Adv. Mater., 2015, 27, 2768-2774). Specifically, the absorber was fabricated by fixing paper as a support in the middle of a beaker, placing a solution of gold nanoparticles, allowing a thin film to be formed by selfassembly of the gold-nanoparticles, and then removing the solution by a syringe, thereby forming a gold nanoparticle thin layer on the paper. The paper increases the absorption of incident light through diffuse reflection, provides water to be evaporated by capillary force, and also serves to increase the evaporation area of water. With this configuration, the photothermal conversion efficiency increased to 77.8% at 4.5 kW/m²illumination. Zhu et al., Science Advances, 2016, 2, el501227) fabricated an absorber by cathodic oxidation of an alumina nanopore template, followed by physical vapor deposition (PVD) of gold ions, and achieved photothermal conversion efficiencies of about 65% at 1 kW/m² (1 sun) illumination and about 90% at 4 kW/m² (4-sun) illumination.

Despite the active studies as described above, the photothermal conversion efficiency of plasmonic absorbers for solar-driven steam generation still has much room for further improvement. In addition, in order to use steam as a renewable energy source, a technology capable of fabricating a large-area absorber by a simple process at lower costs is required.

[Disclosure] [Technical Problem]

An object of the present invention is to provide a plasmonic absorber for a solar-driven steam generation device having excellent photothermal conversion efficiency.

Another object of the present invention is to provide a plasmonic absorber for a solar-driven steam generation device, which is easily fabricated using inexpensive materials over a large-area by a simple method.

Still another object of the present invention is to provide a solar-driven steam generation device including the absorber.

[Technical Solution]

The present invention for achieving the above objects is directed to a plasmonic absorber for a solar-driven steam generation device in which plasmonic gold nanoparticles are evenly dispersed and immobilized on a porous hydrophilic polymer substrate.

In the present specification, the solar-driven steam generation device is a device that generates steam by converting solar energy into thermal energy, and may naturally use artificial light instead of solar light.

Since the plasmonic absorber should evaporate water by quickly receiving the heat generated by the plasmonic material, the absorber itself needs to be made of a hydrophilic polymer, and also needs to have a porous structure so that the effective area for adsorption of the plasmonic material and evaporation of water by the plasmonic material may be maximized. The hydrophilic polymer may be a natural polymer or a synthetic polymer, and may include one or more of paper, cotton, cellulose resin, polyacrylonitrile, polyvinyl alcohol, polyamide, polyethersulfone, polyethylene glycol, and hydrophilic polyurethane, but is not limited thereto.

The first requirement for the plasmonic absorber of the present invention to effectively utilize sunlight is that the plasmonic absorber should have a high light absorbance. To this end, the light absorbance of the plasmonic absorber of the present invention in the ultraviolet to far infrared wavelength region (250 to 2,500 nm) is preferably 80% or higher. The higher the light absorbance is, the more sunlight may be used, and thus it is natural that the upper limit of the light absorbance is meaningless .

The second requirement is that the absorbed sunlight should be able to be effectively converted into heat which should in turn be used for steam generation. As can be seen in the Examples below, as the average particle diameter of gold nanoparticles became smaller, the generation of steam was more efficient, and the average particle diameter was preferably 1 to 25 nm. In addition, the coverage of gold nanoparticles on the surface of the plasmonic absorber also affects the efficiency of the steam generation by the plasmonic absorber. If the proportion of gold nanoparticles is excessively low, the efficiency of photothermal conversion decreases because the proportion of plasmons that cause photothermal conversion decreases, and if the proportion of gold nanoparticles is excessively high, the efficiency of steam generation decreases because of a decrease in the surface area of the hydrophilic polymer to which water is supplied and in which the water is evaporated into steam. According to the experiment, the coverage of the gold nanoparticles was preferably 30 to 70%. Where the size and coverage of the gold nanoparticles are evenly distributed within the above range, the gold nanoparticles absorb light of all wavelengths, so that the plasmonic absorber becomes a black gold absorber. As the size of the gold nanoparticles increases, the reflection of incident light increases, so that the gold nanoparticles have yellow color which is the typical gold color.

For example, the plasmonic absorber for a solar-driven steam generation device according to the present invention may be fabricated by a method including steps of: (A) preparing a porous substrate composed of a hydrophilic polymer; (B) absorbing a gold nanoparticle precursor solution into the substrate, and drying the substrate; and (C) treating the dried substrate with plasma at atmospheric pressure.

Hereinafter, each step will be described in detail.

Step (A) is a step of preparing a substrate for the plasmonic absorber, and preparation of the substrate includes pre-treatment procedures such as washing.

The thickness of the substrate is preferably 1 pm to 5 mm. If the thickness of the substrate is excessively small, the amount of plasmonic material adsorbed may be excessively small, and if the thickness of the substrate is excessively large, efficiency may be lowered due to heat loss. The optimal thickness of the substrate may be determined in consideration of the porosity, the size of the pores, and the spacing between the pores, depending on the material of the substrate, and will be easily determined by a person skilled in the art. The size of the pores, the spacing between the pores, and the porosity may also be easily set to optimal values depending on the material and detailed structure of the substrate.

Although paper was used as the substrate in the Examples below, a natural or synthetic hydrophilic polymer having a porous structure as described above may also be used as the substrate. In particular, the above-described method is suitable for immobilizing gold nanoparticles on the surface of the substrate without damaging the substrate when heat-labile materials such as paper or cotton are used as the substrate.

Step (B) is a step of loading a gold nanoparticle precursor into a substrate by absorbing a gold nanoparticle precursor solution into the substrate and drying the substrate. As the gold nanoparticle precursor, any one may be used as long as it may be reduced into gold nanoparticles by plasma treatment, and a conventional precursor may be used, which is used in the preparation of gold nanoparticles in a conventional art. More specifically, examples of the gold nanoparticle precursor include hydrogen tetrachloroaurate, gold trichloride, potassium tetrachloroaurate, gold hydroxide, gold oxide, or gold sulfide. The concentration of the gold nanoparticle precursor is preferably 10 to 1000 mM, more preferably 10 to 200 mM. The concentration of the gold nanoparticle precursor solution significantly influences the amount of plasmonic material to be contained in the absorber later, that is, the surface fraction of the gold nanoparticles in the absorber. As the concentration of the gold nanoparticle precursor increases, the surface fraction of gold nanoparticles increases.

In order to absorb the solution of the gold nanoparticle precursor into the substrate, various methods may be used, such as immersing the substrate in the gold nanoparticle precursor solution and then taking it out, or drop-coating the substrate with the gold nanoparticle precursor solution, or spraying the gold nanoparticle precursor solution onto the substrate, or applying the gold nanoparticle precursor solution by a brush or the like when the area of the substrate is large. Any method may be used as long as it can evenly absorb the solution into the substrate.

If the concentration of the gold nanoparticle precursor is excessively high, aggregation of gold nanoparticles may occur. It is more effective to repeat this step using a low concentration of the precursor solution than to treat the substrate with a high concentration of the precursor solution at one time. That is, it is more effective to repeat twice the process of treating the substrate with the gold nanoparticle precursor solution at a concentration of 100 mM and drying the substrate, compared to treating the substrate once with the gold nanoparticle precursor at a concentration of 200 mM and drying. However, as the concentration of the precursor solution decreases, the number of repetitive treatments for increasing the light absorbance of the plasmonic absorber increases. For this reason, in view of process economics, suitable conditions for about 2 to 5 repetitive treatments may be selected and used.

As a result of the treatment in this step, the substrate exhibits a light yellow color as the gold nanoparticle precursor solution is absorbed into the substrate.

Step (C) is a step of plasma-treating the gold nanoparticle precursor, loaded in the substrate in the step (B), thereby reducing the precursor into gold nanoparticles which are plasmonic material. As a result of the treatment in this step, the gold nanoparticle precursor loaded in the porous substrate is reduced into black gold nanoparticles which are, in turn, evenly dispersed. In a conventional art, although an absorber having a gold thin film formed by PVD is also coated inside a porous substrate, it exists as a thin layer, and hence evaporation of water absorbed in the substrate is not effective and only water present in the pores may be evaporated. However, the plasmonic absorbent of the present invention is in a form in which nanoparticles are dispersed and adsorbed on the substrate, and hence not only water present in the pores formed in the substrate, but also water absorbed in the substrate, may be effectively evaporated to generate steam. For this reason, the photothermal conversion efficiency of the plasmonic absorber, fabricated by Zhu et al. by forming a gold thin layer on an aluminum substrate by PVD, reached about 90% at 4-sun light intensity, but was only about 65% at 1-sun light intensity , whereas the plasmonic absorber of the present invention, which has dispersing black gold nanoparticles dispersed on the porous substrate, exhibited not only a photothermal conversion efficiency of 94% at 3-sun light intensity, but also a photothermal conversion efficiency of 90% at 1-sun light intensity, indicating that the effect thereof was very remarkable.

In the present invention, plasma treatment may be performed at ambient temperature and atmospheric pressure. However, it is not excluded to apply plasma in a vacuum or a pressurized state. Thus, if it is necessary to treat the substrate with plasma in a vacuum or a pressurized state, the substrate may be treated with plasma under predetermined conditions. The time of treatment with the plasma may be 1 minute to 100 minutes, and the power of the plasma may be 10 to 500 W. The intensity of the plasma affects the size of the produced gold nanoparticles, and the size of the gold nanoparticles decreases as the intensity of the plasma increases. Therefore, the size of the gold nanoparticles may be effectively controlled by adjusting the plasma treatment time or power.

Where the process of treatment with the gold nanoparticle precursor solution in step (B) is repeated 2 to 5 times, the plasma treatment process in this step may also be repeated with step (B) . That is, although plasma treatment may also be performed after repetitive treatments with the gold nanoparticle precursor solution in step (B) are finally completed, plasma treatment may also be repeated after each treatment with the gold nanoparticle precursor solution.

In addition, it is more preferable to treat each of the upper and lower surfaces of the substrate with plasma in this step. The light absorbance in the near infrared wavelength region (750 to 1,200 nm) greatly increased when the lower surface was additionally treated compared to when only the upper surface was treated.

The present invention is also directed to a solar-driven steam generation device including the above-described plasmonic absorber. More specifically, the solar-driven steam generation device of the present invention may include: a heat-insulating layer that floats on top of water; the above-described plasmonic absorber of the present invention, located on the heat-insulating layer; and a porous hydrophilic polymer layer, the upper surface of which is entirely in contact with the lower surface of the plasmonic absorber, and a portion of which is immersed in water under the heat-insulating layer, so that the porous hydrophilic polymer layer is capable of continuously supplying water to the plasmonic absorber.

The porous hydrophilic polymer layer absorbs water by capillary action and supplies water to the plasmonic absorber. At this time, the porous hydrophilic polymer layer may supply water to the plasmonic absorber without energy loss, only when the upper surface thereof is entirely in contact with the lower surface of the plasmonic absorber. The upper surface of the porous hydrophilic polymer layer means an upper surface parallel to the water surface. If a portion of the upper surface of the porous hydrophilic polymer layer is exposed to the outside without contacting the lower surface of the plasmonic absorber, heat loss through the exposed portion may occur, resulting in reduction in the efficiency of steam generation.

The solar-driven steam generation device may be installed on water existing in nature, such as the sea, rivers, or lakes, without a separate water container, and may also be installed in a container containing a limited volume of water. Specifically, the solar-driven steam generation device may be used as a component of a system that requires the generation of steam, such as a desalination system, a wastewater treatment system, or a distillation system.

In the solar-driven vapor generation device, steam may be generated with the highest efficiency when the amount of water supplied to the plasmonic absorber by the porous hydrophilic polymer layer and the amount of water evaporated by the plasmonic absorber are balanced. If the amount of water supplied by the porous hydrophilic polymer layer is smaller than the amount of water evaporated by the plasmonic absorber, the plasmonic absorber may be dried, so that heat may be radiated without being used for steam generation. Conversely, if the amount of water supplied is larger than the amount of water evaporated by the plasmonic absorber, excess water may be incorporated into the porous hydrophilic polymer layer or the hydrophilic polymer constituting the plasmonic absorber, so that heat loss may be caused by excess water that has not yet evaporated.

[Advantageous Effects]

As described above, the plasmonic absorber of the present invention has significantly better photothermal conversion efficiency than the previously reported plasmonic absorbers, and thus exhibit a photothermal conversion efficiency of 90% not only at high light intensity but also at 1-sun light intensity. Accordingly, the plasmonic absorber may be useful as an absorber in a solar-driven steam generation device or the like.

In addition, the plasmonic absorber of the present invention may be fabricated by performing plasma treatment at ambient temperature and atmospheric pressure by a simple process, and thus may be fabricated over a large area using an inexpensive substrate made of, but not limited to, paper, cotton or synthetic resin. Accordingly, it may be fabricated economically, and thus is suitable for industrial use.

In addition, the size or surface fraction of the gold nanoparticles may be easily adjusted depending on the porosity or thickness of the substrate, the concentration of the gold nanoparticle precursor solution used, and the plasma treatment conditions, and thus the solar absorptivity and heat emissivity of the plasmonic absorber may be easily controlled.

[Description of Drawings]

FIG. 1 shows a process for fabricating a plasmonic absorber and photographs of absorbers fabricated thereby.

FIG. 2 shows SEM and TEM images of a plasmonic absorber fabricated in one Example of the present invention.

FIG. 3 shows the XRD diffraction spectrum of a plasmonic absorber fabricated in one Example of the present invention.

FIG. 4 shows the XPS spectrum of a plasmonic absorber fabricated in one Example of the present invention.

FIG. 5 shows the FT-IR spectrum of a plasmonic absorber fabricated in one Example of the present invention.

FIG. 6 depicts graphs and tables showing the average size and coverage of gold nanoparticles depending on conditions used in absorber fabrication.

FIG. 7 shows the absorption spectra of plasmonic absorbers fabricated using various concentrations of a gold nano-ion precursor.

FIG. 8 is a graph showing the light absorbance of an absorber depending on conditions used in absorber fabrication.

FIG. 9 depicts photographs and graphs related to evaluating the efficiency of a solar-driven steam generation device including a plasmonic absorber fabricated in one Example of the present invention.

FIG. 10 depicts graphs showing the amount of solar-driven steam generation depending on the conditions and time used in absorber fabrication.

FIG. 11 depicts graph showing the effect of the supply rate of water on the amount of steam generation in a solar-driven seam generation device.

FIG. 12 is a graph showing the rate of steam generation depending on the operating time of a solar-driven steam generation device.

FIG. 13 depicts a graph showing the concentrations of metal ions in fresh water produced by a desalination system, and a photograph of an absorber after desalination.

[Best Mode]

Hereinafter, the present invention will be described in further detail with reference to the following examples. It is to be understood, however, that these examples are for illustrative purposes only and are not intended to limit or change the scope of the present invention. In addition, those skilled in the art will appreciate that various modifications and alterations are possible based on this illustration, without departing from the scope and spirit of the invention.

Examples

Example 1: Fabrication of Plasmonic Absorbers for SolarDriven Steam Generation

As a support (substrate) for absorber fabrication, Whatman #42 filter paper was used.

After the filter paper was cut to a size of 1.5 x 1.5 cm, it was immersed in 10% (v/v) HNOs solution for 12 hours, and cleaned repeatedly with DI water until the pH of the cleaning solution became equal to that of DI water. Subsequently, the resulting paper was immersed again in 10 g/L of NaHCOa aqueous solution for 1 hour, and then cleaned repeatedly with DI water until the pH of the cleaning solution became equal to that of DI water. Next, the paper was dried in an N₂ atmosphere at 50°C for 2 hours .

Hydrogen tetrachloroaurate (HAUCI4. xH2O) was dissolved in ethanol to a concentration of 10 to 100 mM, and 50 μΐ of the solution was dropped on the above-described cleaned and dried paper, followed by drying for 30 minutes. This dropping and drying process was repeated three times. Finally, the paper was dried at 50°C for 30 minutes.

The dried paper was placed in the sample holder of a plasma system, and adjusted so that the distance between the paper and the plasma electrode was 3 mm. The paper was treated with plasma at a plasma power of 150 W and an Ar gas flow rate of 5 L/min for 15 minutes. To remove the remaining gold ions, the paper was cleaned several times by immersion in ethanol.

After the paper was dried, it was turned over and placed again in the plasma holder again. Then, the paper was treated with plasma for 15 minutes to reduce the remaining gold ions into gold nanoparticles, so that both surfaces of the paper were treated with plasma. The prepared sample was dried in a vacuum at 50°C for 2 hours, and stored in a dark room before use.

FIG. la is a schematic view showing a process for fabricating the plasmonic absorber for solar-driven steam generation, and FIG. lb depicts photographs of the absorbers fabricated by the above-described method using various concentrations of hydrogen tetrachloroaurate.

Example 2: Analysis of Structural Characteristics of Plasmonic Absorber for Solar-Driven Steam Generation

To analyze the structural characteristics of the plasmonic absorber for solar-driven steam generation fabricated using 100 mM hydrogen tetrachloroaurate in Example 1 above, the surface of the plasmonic absorber for solar-driven steam generation was observed using a scanning electron microscope (SEM; JSM-7000F, JEOL, Japan) and a transmission electron microscope ((TEM; JEM2100F, JEOL, Japan) . FIGS. 2a to 2c are SEM images at various resolutions, FIGS. 2d to 2f are high-resolution TEM images at various resolutions, and FIG. 2g is a TEM-EDS mapping image.

Specifically, FIG. 2a shows the structure of paper composed of cellulose, and FIGS. 2b and 2c, which are enlarged views of FIG. 2a, show that the gold nanoparticles indicated by white dots are adsorbed on the surface of cellulose. The TEM images also show that the gold nanoparticles are adsorbed on the surface of cellulose. In addition, EDS mapping shows that the gold nanoparticles are evenly dispersed throughout the paper.

FIG. 3 shows the X-ray diffraction spectrum of the absorber, measured before and after cleaning of the filter paper used for the fabrication of the absorber. The peaks marked with the symbol in FIG. 3 correspond to the peaks of monolithic cellulose. In the absorber, peaks corresponding to the (111), (200), (220) and (311) planes of gold nanoparticles were additionally observed.

Whether the plasma treatment would affect not only the formation of the gold nanoparticles but also the composition of the cellulose filter paper was examined by XPS (X-ray photoelectron spectroscopy) using a Sigma Probe Thermo Fisher VG Scientific spectrometer (MULTILAB 2000, Thermo Scientific, USA), equipped with a monochromatic Al Ka X-ray source, and ATR-FTIR (Nicolet 6700 spectrometer). To prevent contamination by moisture, the sample was dried in a vacuum at 50°C for 2 hours before analysis, and the sample was handled under a dry N₂5 atmosphere. FIG. 4 shows the Cls-XPS spectrum of each of the filter paper (a) and the absorber (b) after cleaning, and FIG. 5 shows the FT-IR spectra of the filter paper and the absorber. Table 1 below shows the chemical compositions calculated from the XPS spectra. The cleaning process did not significantly affect 10 the chemical composition of the filter paper, but it can be seen in the XPS and FT-IR spectra that the C/O ratio significantly increased and C=O bonds were created after the plasma treatment. This is because the -OH group on the cellulose surface was oxidized.

Table 1

Samples	Chemical composition (atomic %)	C/O ratio
C	O	Au
Filter paper after cleaning	58.02	41.98	0	1.38
Filter paper before cleaning	57.85	42.15	0	1.37
Absorber	64.50	22.31	13.19	2.89

FIG. 6 depicts a graph and table showing the effects of the concentration of the gold nanoparticle precursor and the intensity of plasma on the production of gold nanoparticles. As can be seen in FIG. 6, an increase in the intensity of plasma led to a decrease in the size of the nanoparticles, but did not significantly affect the surface fraction of the nanoparticles, and an increase in the concentration of the nanoparticle precursor did not significantly affect the size of the nanoparticles, but led to an increase in the surface fraction of the nanoparticles is increased.

Example 3: Analysis of Optical Properties of Plasmonic Absorbers for Solar-Driven Steam Generation

In order to increase the efficiency of solar-driven steam generation, the light absorbance of the absorber should be high. Accordingly, the optical properties of the absorbers fabricated in Example 1 were analyzed. Since the size and density of gold nanoparticles in the absorbers will affect the light absorbance of the absorbers, the optical properties of the absorbers fabricated under various conditions were measured by transmittance and reflectance. Light absorbance (%A) was calculated from transmittance (%T) and reflectance (%R) using the following equation:

%A = 100 - %T - %R

Since the absorber for steam generation operates at the interface between water and air and acts in a substantially wet state, the light absorbance in the wet state is more meaningful than the light absorbance in a dry state. FIG. 7 shows the light absorbance in wet state of each of the absorbers fabricated using various concentrations of the gold nanoparticle precursor. Plasma treatment of each absorber was performed once only on the upper surface, and only in the case of 100 mM_ (2 + 1) indicated by the dotted line, plasma treatment was performed once on the upper surface and twice on the lower surface. As can be seen in FIG. 7, the light absorbance in the ultraviolet to far infrared wavelength region (250 to 2,500 nm) increased as the concentration of the gold nanoparticle precursor increased. In addition, the absorber fabricated using the gold nanoparticle precursor at a concentration of 10 mM showed a light absorbance of about 97% in the ultraviolet to visible wavelength region. Furthermore, the light absorbance in the near infrared wavelength region also significantly increased as the concentration of the gold nanoparticle precursor increased. In the case in which both the upper and lower surfaces were treated with plasma, the light absorbance of the absorber significantly increased in the near infrared wavelength region.

FIG. 8 shows the light absorbance in a wavelength range of 250 to 2,500 nm depending on the concentration of the gold nanoparticle precursor, the intensity of plasma and the size and coverage of the nanoparticles. As can be seen therein, the light absorbance increased as the size of the nanoparticles decreased and as the surface fraction of the nanoparticles increased. As the concentration of the nanoparticle precursor decreased, the intensity of plasma had a greater effect on the light absorbance.

To evaluate the durability of the absorber of the present invention, the light absorbance of the absorber fabricated using the 100 mM precursor at a plasma intensity of 200 W was measured. That is, the absorber was cleaned three times or more with distilled water, and then the light absorbance of the absorber in a wavelength region of 250 to 2, 500 nm was measured. As a result, it could be seen that the light absorbance of the absorber after one month was almost no different from that of the absorber immediately after fabrication (see FIG. 12). This shows that the gold nanoparticles in the absorber of the present invention are very strongly attached.

Example 4: Evaluation of Efficiency as Absorber for SolarDriven Steam Generation

Using the absorber (size 1.5 x 1.5 cm) fabricated using a hydrogen tetrachloroaurate solution at a concentration of 100 mM by the method of Example 1, efficiency as an absorber for solardriven steam generation was evaluated. The size of the absorber substantially effective in the device was 1.45 x 1.45 cm. In the Examples below, #40, #41 and #42 represent the # of Whatman filter paper. The characteristics of the Whatman filter papers are shown in Table 2 below.

Table 2

	Pore size (pm)	Thickness (pm)	Average weight (g/m²)
#40	8	210	95
#41	20 to 25	220	85

#42

2.5

200

100

To measure the water evaporation rate of the absorber, the inner wall of a 50-mL cup was coated with polystyrene foam. The cup coated with polystyrene foam was placed on a scale connected to a computer and a solar simulator was placed thereon. An polystyrene foam (EPS) dish expanded to a size of 2 x 2 cm was placed in the cup coated with polystyrene foam on the inner wall, and then white cellulose paper having a size of 2 x 4 cm was placed in the center of the EPS dish, and both ends thereof were folded inward so that the paper could be submerged in water later. The absorber was placed on the paper, and water was added to the cup so that both ends of the paper could be submerged in the water while the water level was below the EPS dish. When the absorber was completely wet, the solar simulator was turned on and the weight was measured every 30 seconds. Based on the measurements, the amount of evaporation was calculated. FIG. 9a is a schematic view showing the experimental device, and FIG. 9b is an actual photograph of the experimental device. In FIG. 9a, ® denotes steam generated by evaporation, (2) denotes the absorber of the present invention, (3) denotes cellulose paper, (4) denotes the EPS dish, and (5) denotes water.

FIG. 9c shows thermal analysis performed after 30 seconds at 3-run light intensity, and it can be seen that the temperature of the periphery of the absorber was 21 °C, whereas the temperature of the absorber increased to 68°C. FIG. 9e shows the temperature of the absorber as a function of time. FIG. 9d is a photograph after 15 seconds at 3-sun light intensity, and shows that steam was generated by water evaporation from the surface of the absorber.

FIG. 9f shows the cumulative amount of water evaporation with time at 3-sun light intensity, and FIG. 9g shows the evaporation rates of water at 1-sun and 3-sun light intensities.

The photothermal conversion efficiency (η) of the absorber can be calculated using the following equation:

mh_LVη „ / τη ( is evaporation rate) wherein hLv/I is the sum (2,582 J/K) of the enthalpy required for water heating from 21°C to 100°C (4.2 J/g-K χ (100-21)°C = 332 J/K) and the enthalpy required for water evaporation (2,250 J/K), and I is the intensity of incident light.

The photothermal conversion efficiency calculated for 3-sun light intensity (3 kW/m²) was 94%, and the photothermal conversion intensity at 1-sun light intensity was 90%. Thus, it could be confirmed that the absorber of the present invention was superior in efficiency to any previously reported absorbers.

In the following experiment, the effect of steam generation under a light irradiation condition of 1-sun light intensity was examined.

FIG. 10 depicts graph showing the results of measuring the amount of steam generation using the above-described device for absorbers fabricated on #42 Whatman paper using various concentrations of a gold nanoparticle precursor at various plasma intensities. Where the plasma intensity was changed, the concentration of the precursor was fixed at 100 mM, and where the concentration of the precursor was changed, the intensity of the plasma was fixed at 200 W. Table 3 below shows photothermal conversion efficiencies depending on absorber fabrication conditions (gold nanoparticle precursor concentration and plasma intensity). As can also be predicted from the light absorbances shown in FIG. 8, it was confirmed in FIG. 10 and Table 3 below that, as the plasma intensity increased and the concentration of the gold nanoparticle precursor increased, the amount of steam generation for the same time and the photothermal conversion efficiency also increased.

Table 3

	100 W	150 W	200 W
20 mM	75.22	83.29	84.77
50 mM	78.48	87.19	88.78
100 mM	89.10	90.33	91.07

In order to confirm the effect of water supply on steam generation in the solar-driven steam generation device, the amount of steam generation was measured while changing the size and material of the cellulose paper (3) of the device. Absorbers were fabricated using #42 Whatman filter paper and a 100 mM gold nanoparticle precursor at a plasma intensity of 200 W. As cellulose paper, #42 Whatman filter paper was used, and a device was designed such that the width of the water-contacting side of the filter paper was 1.5, 4.5 or 13.5 mm. To evaluate the effect of the material of cellulose paper, the width of the cellulose paper was fixed at 4.5 mm, and the material to be used was changed to #40, #41 or #42 paper. FIG. 11 depicts graphs showing the characteristics of Whatman filter paper used as cellulose paper, and graphs showing evaporation rate depending on the size and material of cellulose paper. First, the flux of water through the cellulose paper was expected to be affected by the pore diameter and porosity of the paper, and #41 paper showed the highest flux.

In all the devices with different paper widths or materials, the evaporation rate of water increased rapidly up to 10 to 15 minutes, but gradually decreased again when the width was excessively small or the flux of water was low. This is considered to be because the evaporation of water was initially smooth because the measurement started in a state in which the absorber was sufficiently wet, but water was not sufficiently supplied to the absorber while the amount of water supplied through the cellulose paper became smaller than the amount of water evaporation in the later stage. When the amount of water supplied through the cellulose paper was not less than the amount of water evaporated by the absorber, the equilibrium was maintained without almost affecting the evaporation rate, even if the width was further increased or the flux further increased due to the properties of the material.

FIG. 12 shows the change in the rate of water evaporation with time, and the insert in FIG. 12 is a graph showing light absorbance. It can be seen in FIG. 12 that even when the device was continuously operated for 100 days, the rate of steam generation did not change significantly.

Example 5: Application to Desalination Process

To examine whether the plasmonic absorber for a solardriven steam generation device, fabricated in Example 1 above, can be applied to a desalination process, seawater and the device of Example 4 were used. Evaporated steam was collected by capturing it in a clean capture bag, followed by cooling. FIG. 13 depicts a graph showing the results of analyzing the concentrations of metal ions contained in seawater and in water collected by a desalination system, and a photograph of the absorber after 4.5 hours. As can be seen therein, salt precipitated on the absorber as water evaporated from seawater.

From the graph at the top, it can be seen that the concentrations of metal ions in the evaporated water were reduced to about 1/1000 to 1/10000 of those in seawater, and were also lower than the acceptable levels of metal ions in drinking water, suggesting that desalination was successfully achieved.

Claims

[claims] [Claim 1]

A plasmonic absorber for a solar-driven steam generation device in which plasmonic gold nanoparticles are evenly dispersed and immobilized on a porous hydrophilic polymer substrate.
[Claim 2]

The plasmonic absorber of claim 1, wherein the hydrophilic polymer comprises one or more of paper, cotton, cellulose-based resin, polyacrylonitrile, polyvinyl alcohol, polyamide, polyethersulfone, polyethylene glycol, and hydrophilic polyurethane.
[Claim 3]

The plasmonic absorber of claim 1, wherein a light absorbance of the plasmonic absorber in a wavelength region of 250 to 2,500 nm is 80% or higher.
[Claim 4]

The plasmonic absorber of claim 1, wherein the gold nanoparticles have an average particle diameter of 1 to 25 nm, and a coverage of the gold nanoparticles is 30 to 70%.
[Claim 5]

The plasmonic absorber of claim 1, which is fabricated by a method comprising steps of:

(A) preparing a porous substrate composed of a hydrophilic polymer;

(B) absorbing a gold nanoparticle precursor solution into the substrate, and drying the substrate; and (C) plasma-treating the dried substrate at atmospheric pressure.
[Claim 6]

The plasmonic absorber of claim 3, wherein step (B) or steps (B) and (C) are repeated 2 to 5 times.
[Claim 7]

The plasmonic absorber of claim 3, wherein the plasmatreating in step (C) comprises plasma-treating each of upper and lower surfaces of the substrate.
[Claim 8]

A solar-driven steam generation device comprising the plasmonic absorber of any one of claims 1 to 7.
[Claim 9]

The solar-driven steam generation device of claim 8, comprising:

a heat-insulating layer that floats on top of water;

the plasmonic absorber located on the heat-insulating layer; and a porous hydrophilic polymer layer, an upper surface of which is entirely in contact with a lower surface of the plasmonic absorber, and a portion of which is immersed in water under the heat-insulating layer, so that the porous hydrophilic polymer layer is capable of continuously supplying water to the plasmonic absorber.

[Claim 101

The solar-driven steam generation device of claim 9,

5 wherein the rate of water supply by the porous hydrophilic layer is not lower than the rate of water evaporation by the plasmonic absorber.