AU2021103927A4

AU2021103927A4 - Plasmonic absorber and solar-driven steam generation apparatus using same

Info

Publication number: AU2021103927A4
Application number: AU2021103927A
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: 2021-07-07
Publication date: 2021-09-09
Anticipated expiration: 2026-09-17
Also published as: AU2018341405A1; KR20190038331A; KR102091594B1

Abstract

] 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 5 fabricated over a large area by a simple method, and a plasmonic solar-derived steam generation device including the same, and more particularly 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 10 substrate, and a plasmonic solar-derived steam generation device including the same. Au u Au3. EtOH Dried filter paper e Ar* Cellulose filter paper.4 b

Description

Au u Au3.

EtOH Dried filter paper e Ar*

Cellulose filter paper.4

b

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 solar

driven 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/m 2 (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/m 2 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 heat

insulating 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 self assembled 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 self

assembly 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 2 photothermal conversion efficiency increased to 77.8% at 4.5 kW/m

illumination. Zhu et al., Science Advances, 2016, 2, e1501227)

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/m 2 (1 sun) illumination and about 90% at 4

kW/m 2 (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 Model

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 Solar

Driven 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) HNO3 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 NaHCO3 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 N 2 atmosphere at 500C for

2 hours.

Hydrogen tetrachloroaurate (HAuCl4.xH20) was dissolved in

ethanol to a concentration of 10 to 100 mM, and 50 pl 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 500C 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; JEM

2100F, 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 500C for 2 hours

before analysis, and the sample was handled under a dry N2

atmosphere. FIG. 4 shows the C1s-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

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=0 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 0 Au Filter paper 58.02 41.98 0 1.38 after cleaning Filter paper 57.85 42.15 0 1.37 before cleaning 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 Solar

Driven 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 solar

driven 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, ) denotes the absorber

of the present invention, @ denotes cellulose paper, A denotes

the EPS dish, and @ 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 210C, whereas the temperature of the absorber increased to 680C. 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 (r)) of the absorber

can be calculated using the following equation:

mhV ( is evaporation rate) wherein hv/I is the sum (2,582 J/K) of the enthalpy required for

water heating from 210C to 1000C (4.2 J/g -K x (100-21) OC = 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/m 2 ) 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 ) 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 solar

driven 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]

A method for fabricating a plasmonic absorber for a solar

driven steam generation device 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 method of claim 5, wherein step (B) or steps (B) and

(C) are repeated 2 to 5 times.
[Claim 7]

The method of claim 5, wherein the plasma-treating 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 4.
[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 10]

The solar-driven steam generation device of claim 9,

wherein the rate of water supply by the porous hydrophilic layer

is not lower than the rate of water evaporation by the plasmonic

absorber.

Fig 1 1/13

Fig 2

Fig 3

Fig 4

Fig 5

Fig 6

Fig 7

Fig 8

Fig 9

Fig 10

Fig 11

Fig 12

Fig 13