VIETNAM NATIONAL UNIVERSITY, HANOI

VIETNAM JAPAN UNIVERSITY

VU TIEN DUNG

BIOMETAMATERIALS APPICATION
FOR SOLAR STEAM GENERATION
DEVICES

MASTER'S THESIS


VIETNAM NATIONAL UNIVERSITY, HANOI

VIETNAM JAPAN UNIVERSITY

VU TIEN DUNG

BIOMETAMATERIALS APPICATION
FOR SOLAR STEAM GENERATION
DEVICES
MAJOR: NANOTECHNOLOGY
CODE: 8440140.11QTD

RESEARCH SUPERVISOR:
Dr. PHAM TIEN THANH
Dr. BUI NGUYEN QUOC TRINH

Hanoi, 2020

ACKNOWLEDGMENTS
Firstly, I would like to extend my sincere thanks to Dr. Pham Tien Thanh, and Dr.
Bui Nguyen Quoc Trinh, my supervisors, working for Vietnam Japan University,
for their enthusiasm, encouragement, and patient guidance during the preparation of
my master thesis.
Moreover, I would also like to express my great appreciation to Prof. Dr. Kotaro
Kajikawa, working for the Tokyo Institute of Technology, who gives me a lot of
valuable suggestions and teaches me with the necessary knowledge about the
science.
I take this chance to acknowledge the support provided by Assoc. Prof. Dr. Do
Danh Bich, Dr. Nguyen Duc Cuong, Dr Nguyen Viet Hoai, and Mr Nguyen Minh
Tuan. The advice given by them has been a great help in my research.
Finally, I especially wish to thank my mom, my dad, my brother, and friends, who
are always by my side, have supported and encouraged me throughout my life. My
life will be incomplete without them. Thanks.

Vu Tien Dung
Hanoi, 2020

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TABLE OF CONTENTS

Page
ACKNOWLEDGMENTS........................................................................................... i
TABLE OF CONTENTS ........................................................................................... ii
LIST OF FIGURES................................................................................................... iii
LIST OF TABLES ......................................................................................................v

LIST OF ABBREVIATIONS ................................................................................... vi
CHAPTER 1: INTRODUCTION ...............................................................................1
1.1. Clean water and Salination Issue + Desalination method ....................................1
1.2. Solar Steam Generation........................................................................................2
1.2.1. Mechanism of SSG ...........................................................................................2
1.2.2. Absorber Material .............................................................................................3
1.2.3. Purpose of this master thesis .............................................................................5
CHAPTER 2: EXPERIMENTAL METHOD.............................................................7
2.1. Carbonized pomelo peel synthesis and characteristics ........................................7
2.1.1. Fabrication procedure........................................................................................7
2.1.2 Carbonized pomelo peel characteristics .............................................................8
2.2. SSG System construction and evaluation ............................................................9
2.2.1. Construction of the SSG system .......................................................................9
2.2.2. SSG system evaluation ......................................................................................9
CHAPTER 3: RESULTS AND DISCUSSION ........................................................15
3.1. Carbonized pomelo peel .....................................................................................15
3.1.1. Physical characteristics of carbonized pomelo peel. .......................................15
3.1.2. Absorption properties ......................................................................................20
3.1.3. Solar heating behavior of materials under the sunlight ..................................24
3.2. Solar steam generation system ability ................................................................27
3.2.1. Vapor steam creation capacity ........................................................................27
3.2.2. Desalination and purification capacity of the SSG system .............................33
CHAPTER 4: CONCLUSION ..................................................................................35

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LIST OF FIGURES

Page

Figure 1.1: Drought in Vietnam (left), warning of saline intrusion in Ben Tre
province, and Vinh Long province on the television news .........................................2
Figure 2.1: Fresh pomelo (a) and (b), and Fresh pomelo peel (c) ..............................7
Figure 2.2: Pomelo peel and carbonized pomelo peel ................................................8
Figure 2.3: Light absorber and converter and water supply .......................................9
Figure 2.4: System to calculate the evaporation rate in the laboratory .....................10
Figure 2.5: Mechanism of the system in the real condition ......................................11
Figure 3.1: Porous Structure and Tube Structure of Fresh Pomelo Peel (left) and
Carbonized Pomelo Peel ...........................................................................................15
Figure 3.2: Changes in structural dimensions before and after carbonization process.
(a), (b), (c): Fresh Pomelo Peel; (d), (e), (f): Carbonized Pomelo Peel. ...................16
Figure 3.3: Water Capacity Ability of Carbonized Pomelo Peel ..............................17
Figure 3.4: XRD spectrum of Carbonized Pomelo Peel ...........................................18
Figure 3.5: Raman Spectrum of Carbonized Pomelo Peel........................................18
Figure 3.6: FTIR spectra of Fresh Pomelo Peel and Carbonized Pomelo Peel ........19
Figure 3.7: Absorption properties of carbonized pomelo peel with 1 hour of
annealing time ...........................................................................................................20
Figure 3.8: Absorption properties of carbonized pomelo peel with 2 and 3 hours of
annealing time ...........................................................................................................21
Figure 3.9: Absorption properties of carbonized pomelo peel and fresh pomelo peel
in the UV-Vis-IR region............................................................................................22
Figure 3.10: Absorption spectrum of carbonized pomelo peel samples before being
hydrated and after being hydrated .............................................................................23
Figure 3.11: Temperature of Fresh Pomelo Peel and Carbonized Pomelo Peel
Samples under an artificial sun .................................................................................24
Figure 3.12: Infrared image of sample carbonized pomelo peel placed under
artificial sunlight .......................................................................................................25
Figure 3.13: Temperature of Fresh Pomelo Peel and Carbonized Pomelo Peel
Samples under some real conditions (a): affected by wind; (b): affected by solar
intensity; and (c): affected by cloud ..........................................................................26

Figure 3.14: Vapor steam creation ability under an artificial sun .............................27
Figure 3.15: Vapor steam creation ability with different conditions of thickness
under an artificial sun. (a): Mass change within 1 hour; (b): Evaporation Rate within
1 hour; (c) Carbonized pomelo peel’s infrared photos when exposing to the sunlight
...................................................................................................................................28
Figure 3.16: Vapor steam creation capacity under the real condition ......................30
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Figure 3.17: Vapor steam creation capacity under different power intensity ...........31
Figure 3.18: Vapor steam creation capacity after 30 days ........................................32
Figure 3.19: Cation concentration (top), and Anion concentration (bottom) of Sea
water and Purified water compared to the Standard of drinking water ....................33
Figure 3.20: Wastewater Purification Application ...................................................34

iv


LIST OF TABLES

Page
Table 2.1: Some equipment used in this master thesis .............................................11
Table 4.1: The comparison of evaporation rate for each material and its
disadvantages ............................................................................................................35

v


LIST OF ABBREVIATIONS
CPP

NPs
PTC
SSG

Carbonized Pomelo Peel
Nanoparticles
Photo-thermal conversion
Solar Steam Generation

vi


CHAPTER 1: INTRODUCTION
1.1. Clean water and Salination Issue + Desalination method
In the recent time, one of global problem is crisis of the clean water. Water is
everywhere, but clean water is lacking. According to the World Economic Forum
2019, the clean water crisis is one of four global threats that have a great impact on
the lives of human beings [21]. Moreover, Water Center, University of Twente
(2016) showed us that over 65% of the world's population, have to face to the
shortage of clean water for at least one month a year as a result of climate change
and drought [14]. Vietnam is one of the country’s most vulnerable to climate
change and is currently facing a serious shortage of freshwater and irrigation due to
drought and especially surface intrusion over the years. According to Vietnam
Disaster Management Authority, in the Mekong Delta, nearly 160,000 households
are using polluted water, saline intrusion affects 40% of the fruit land area [19].
Figure 1.1 reveals drought and saline intrusion in several provinces in Vietnam. The
salinization (water with salinity of > 4‰) is alarming, and in some areas the saline
instruction occurs 80-100 km from the coast. Several predictions affirm that
Vietnam's GDP will be reduced by 10% by 2030 because of drought and saline
intrusion. Therefore, constructing a system to product the clean water from the sea

water is expected to be a valuable solution to face with the situation of clean water
scarcity.

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Figure 1.1: Drought in Vietnam (left), warning of saline intrusion in Ben Tre
province, and Vinh Long province on the television news
To find a solution with this situation, a lot of technologies for producing fresh water
from the saline water have been developed and applied, such as distillation, ion
exchange, membrane filtration, and so on [4], [7], [18]. However, these methods
have limitations, such as high cost, high consumption of materials and low
performance because of the sea water’s corrosion and salt precipitation. Nowadays,
technology for producing the clean water from the saline water using solar energy is
receiving much attention. The potential of this technology is to create an ecofriendly, cheap, high-performance system.
1.2. Solar Steam Generation
The solar energy is a kind of green energy which available in the nature. Moreover,
it is an endless source of energy for human life. However, we have not used the
solar energy optimally. According to California Institute of Technology, the amount
of sunlight energy that reaches the earth in 1 hour is equal to the total amount of
energy that humans use within 1 year [22]. In the world, Vietnam is one of the
countries with a lot of sunshine hours in a year (around 2000 to 2600 hours/year,
equivalent to 6-7 hours/day), which is a huge source of energy coming from the sun.
This is an extremely good condition, giving Vietnam many advantages when setting
up devices that use solar energy. Solar Steam Generation (SSG) is a system that
uses solar energy to turn water into steam. That steam is passed through a
condensation system to obtain the clean water. SSG system has many advantages,
such as no electricity in use, no CO2 emissions, simplicity, and competitive price.
With the sunshine hours of 6-7 h/d as in the South of Vietnam, a normal device can
produce 15-30 L/h, equivalent to the minimum water demand of a household per

day [25].
1.2.1. Mechanism of SSG
A complete SSG system is divided into 3 main components: the light absorber and
converter, (2) water supply system, and (3) the clean water collector [23]. The
principle of the system is the process of converting light energy into thermal energy.
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Absorber material receives sunlight and converts that energy into heat energy. The
surface temperature of the material goes up extremely high. The water supply
transports the water from the bottom to the surface of the absorber material through
a water transport system (usually a capillary system). A sufficient amount of water
when reaching the surface, will exchange heat with the absorber and converter
system, to receive energy to transform the state from liquid to vapor. Steam is
involved in the condensation process by a refrigeration device. Finally, clean water
is collected. The above process is operated continuously. Water is constantly being
transferred up to a specified amount so that the absorber layer’s surface temperature
is always at maximum. The performance of the system depends on the photothermal energy conversion efficiency of the absorber layer. In order to achieve the
highest photo-thermal conversion efficiency, light absorber materials must have
strong absorption in the sunlight range (from 300 nm to 3000 nm). Many categories
of materials have been used for developing light absorbers, such as metal
nanoparticles [2], [3], metal oxides [17], polymers [11], [20], semiconductors [6],
[26], bio-inspired materials [5], [10], [24], etc. The photo-thermal materials have
been designed with the porous and capillary structures or bio-inspired structures.
1.2.2. Absorber Material
Photo-thermal conversion (PTC) process has classified into 3 main types. The first
type is PTC based on the plasmonic localized heating of metals. There are two main
kind of materials that operate on this mechanism, metals nanoparticles and metal
oxides. Several metals with nanostructures such as gold, silver, and copper have
been utilized for absorber layer [1], [8]. They exhibit extremely strong absorption

due to the resonant effect of free electrons on the surface excited by the
electromagnetic of the incident light. Yang et al showed SSG system with copper
nanoparticles, which is a scalable and eco-friendly system. [13] The copper
nanoparticles exhibited strong absorption (around 97,7%) in the region of 200 nm to
1300 nm. The SSG based on Cu NPs had high efficiency up to 73% at 2 sun
(equivalent with P= 2 kW.m-2 ) illumination. Naomi et al developed the SSG using
SiO2/Au nano-shells particles [15]. This material strongly absorbs the light range

3


from 500 nm to 1200 nm, leading to the system’s conversion efficiency reached up
to 80% at 1 sun. However, the SSG system using these materials has limitations,
such as: complicated fabrication procedure, expensive, and so on. Constructing this
system on an industrial scale is an uneconomical option. For the metal oxides,
several groups published some initial results in the construction of the SSG system.
By using WO2.9 , SSG system manufactured by Wang et al can absorb more than
90% of sunlight. [16] PTC efficiency of the system reached over 85%, higher than
that of nanoparticles. Deng et al constructed the absorber layer by generation Fe3O4
nanoparticles on the surface of graphene [17]. The temperature of the water
contained this material rose to 10000C when exposing to sunlight. The photothermal materials based on metal oxides had higher light absorbability and greater
conversion efficiency than metal nanoparticles. Those materials get same
limitations of metal nanoparticles.
The second type of PTC is non-radiative relaxation of semiconductor.
Semiconductor materials absorb light to transfer electrons from the valence band to
the conduction band. At the conduction band, electrons perform non-radiative
relaxation before returning to the valence band. These relaxation causes the
temperature of the material to heat up, making the evaporation process faster. Hu et
al developed an SSG system using a membrane with CuFeSe2 nanoparticles
decorated wood. [12] CuFeSe2 had a narrow bandgap (0.45 eV) so it can absorb all

photons with energy greater than 0.45 eV (equivalent to wavelength shorter than
2755 nm). This SSG system achieved a solar thermal efficiency of 86.2% under 5
sun illumination. The evaporated water amount obtained with another
semiconductor materials was from 0.85 to 1.3 kg.m-2.h. SSG system using the
semiconductor as a photo-thermal material will be difficult to fabricate with high
price and be complicate to deal with large-scale system.
The third type of PTC is thermal vibration of molecules. Several materials have
extremely high absorption, and all absorbed photon energy will be converted to
thermal energy in the form of vibrations of the molecules of the material through
the photo-thermal conversion. Especially, carbon-based materials with zero band
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gap can absorb sunlight as a black body. They are cheap, stable and easy to
fabricate. Carbonization process, which can turn any material with a specific
structure into carbon-based materials with the same structure, plays an important
role in making carbon-based materials. Lin et al synthesized SSG system by
annealing to make the carbonized melamine foams, realizing highly efficient SSG
with a water evaporation rate of 1.270 kg.m-2.h-1 and an energy conversion
efficiency of 87.3% under 1 kW.m-2 solar illumination [9]. Zhu et al. developed the
SSG devices based on the natural materials, such as mushroom, wood, so on [24].
The mushroom was carbonized at high temperatures. As a result, the surface of the
mushroom turned into black and the absorbance of the carbonized mushroom
increased to above 95% in the wavelength range of 30-3000 nm. The SSG device
performed a high photo-thermal conversion efficiency as above 85%, and an
evaporation rate of 1.47 kg.m-2.h-1. The evaporated water quality met the standards
of WHO for the drinking (or clean) water. Carbonization material provides a good
performance for SSG system, simple fabrication process, environmentally friendly,
promising to be a key material for setting up SSG system. In particular,
carbonization materials of biological origin (naturally occurring) -named biometamaterials- are currently receiving much attention. Constructing the SSG system

using bio-metamaterial materials is a potential, and feasible direction.
1.2.3. Purpose of this master thesis
The pomelo is the most common fruit in Vietnam. The pomelo is not only a kind of
human food, other components of the pomelo also have many application in our life.
Pomelo's outer skin contains many essential oils, those essential oils are used as a
remedy for hair loss, sinusitis, etc. Pomelo peel is used as some traditional folk
dishes and also have applications in medicinal healing. In this master thesis, pomelo
peel have been considered because of their porous structure. With the porous
structure of the pomelo peel, the carbonized pomelo peel (CPP) promises not only
the good light-absorber material, but also the good ability to transport water onto
the surface itself. Carbonized pomelo peel promises to be a good bio-metamaterial
for constructing the SSG system. We hope that the SSG using the carbonized

5


pomelo peel as PTC material will have high absorbance, high efficiency of photothermal conversion, good ability in desalination and purification.
This master thesis aims at:
• Fabricating the photo-thermal conversion (PTC) material from natural sources by
the carbonization method.
• Studying on the PTC characteristics.
• Evaluating the absorption of bio-metamaterial under the sunlight.
• Developing a SSG system, using bio-metamaterial as a absorber layer (will
mention in the chapter 2)
• SSG system evaluation:
• Assessing the evaporation capacity of the system
• Demonstration in producing clean water from saline water using the Solar Steam
Generation system, which is based on solar energy, bio-inspired materials with light
absorbability and one-dimensional capillary sheets with high efficiency.


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CHAPTER 2: EXPERIMENTAL METHOD
2.1. Carbonized pomelo peel synthesis and characteristics
2.1.1. Fabrication procedure
• Fresh pomelo used in this master thesis was purchased from Vin-mart. Before
fabrication process, fresh pomelo is stored in the refrigerator at 500C.
• It, then, was cut into slices to take pomelo peel slices. (figure 2.1). Fresh Pomelo
slices used in SSG system have the dimensions from 2 cm to 3.5 cm, with the
thickness divided into 3 groups: thin groups with a thickness of 2 mm to 6 mm, the
average group with a thickness of 7 mm to 12 mm , and the thick group is 12 mm to
20 mm.

Figure 2.1: Fresh pomelo (a) and (b), and Fresh pomelo peel (c)
• Pomelo peel slices were washed by the solution of domestic water and absolute
ethanol 3 times with the stirrer at 200 rpm for 30 minutes at room temperature.
• After that, they were dried at 4000C for 12 hours. The moderate temperature helps
the water evaporate while keeping the sample from bending after drying.
• Dried-pomelo peel was taken part in a carbonization process at controllable
temperature and time in the nitrogen atmosphere. The carbonization process was
performed as follows:
1. Put the pomelo peel sample into the annealing tube, the lock the valve.

7


2. Vacuum for 5 minutes. Then blow nitrogen into the tube until the pressure is
equal to atmospheric pressure. Do this step at least 3 times to make the annealing
tube clean.

3. Blow nitrogen so that the pressure is maintained at 1.5 atmospheres.
4. Increase the temperature with the speed of 500 per minute.
5. When the system reaches the desired temperature, keep the system at that
temperature for a specified time.
6. Stop heating the system, wait until the system temperature returns to room
temperature, then take the sample. Figure 2.2 shows a sample of carbonized pomelo
peel.
7. Store the sample in a dry condition.

Figure 2.2: Pomelo peel and carbonized pomelo peel
2.1.2 Carbonized pomelo peel characteristics
• To study the structure and composition of materials, the following methods were
used:
1. SEM to observe the material morphology.
2. XRD, FTIR, Raman to study on the structure and composition of materials.
• To investigate other properties of the material:
1. Absorption properties of carbonized pomelo peel.
8


2. Solar heating behavior of carbonized pomelo peel.
3. The stability of materials
2.2. SSG System construction and evaluation
2.2.1. Construction of the SSG system
Figure 2.3 describes the structure of light absorber and converter part and water
supply part of the SSG system. Carbonized pomelo peel is placed at the top of the
beaker containing water. The area of the sample is designed to be equal to the
surface area of the beaker. Separating the water supply and carbonized pomelo peel
has a Polystyrene Foam sheet. This sheet helps prevent the heat exchange between
materials and the water supply. Surrounding the foam sheet is a gauze pad, the

gauze pad acts as a water transport channel, transport water from the water supply
to the carbonized pomelo peel. Water is transported to the surface through the
porous structure and tube structure of the material. Finally, the baker is wrapped
around with aluminum foil.

Figure 2.3: Light absorber and converter and water supply
2.2.2. SSG system evaluation
• To study the SSG system evaluation, the following methods were used:
1. Evaluate the evaporation index of the system.

9


Figure 2.4 shows the experimental diagram for determining the evaporation rate of
the SSG system. The beaker (constructed in section 2.2.1) is placed on the
electronic balance. The mass of the system will be collected at different times, to
determine the amount of evaporated water. After that, the evaporation rate of the
system is equal to the lost mass divided by the illuminated sample area.

Figure 2.4: System to calculate the evaporation rate in the laboratory
2. Evaluate the desalination and purification ability
For the desalination and purification processes, all experiments are done in the real
condition. The baker containing the carbonized pomelo peel is placed into a sealed
glass box designed as in Figure 2.5. The sun shines on the system, and makes the
water evaporate. That vapor will exist in a sealed glass box. A cooling system,
placed next to that glass case (dry ice), leading to the condensation of the steam
inside the box. Finally, the steam condenses into water and flows out through the
hole at the bottom of the box. For the desalination, sea water will be selected as a
source of input water. After taking enough amount of purified water, the ion
concentration of both solutions is analyzed via SW-846 Test Method 6010D by

using Skalar ++ CP-OES. For the purification process, crystal violet and methyl
orange solutions are pick as the wastewater. The color and transparency of the
purified solution will be assessed to evaluate the performance of the system.

10


Figure 2.5: Mechanism of the system in the real condition
2.3 Equipment
Table 2.1 shows some equipment used in measurement in the thesis.
Table 2.1: Some equipment used in this master thesis
Equipment

Note
UV-Vis Lambda 950 for observing
the absorption properties of materials
in the range of 200 nm to 2500 nm

UV/Vis Jasco V670 (Jasco, Japan)
for observing the absorption
properties of materials in the range of
200 nm to 900 nm

11


X-ray Diffraction (XRD Mini Flex
600, Rigaku, Japan) spectra of
sample were obtained using a Bruker
AXN model


Fourier- transform infrared (FTIR4600, Jasco, Japan) spectra of
samples were recorded on a Nicole
380 spectrometer.

Raman Labram evolution (Horiba)

12


FLIR infrared camera for calculating
the surface temperature of material

Ion concentration of solutions were
obtained via SW-846 Test Method
6010D by using Skalar ++ CP-OES

Annealing equipment for
carbonization process

13


SEM for observing the material
morphology

HOBO pyranometer for measuring
the sunlight intensity

14



CHAPTER 3: RESULTS AND DISCUSSION
3.1. Carbonized pomelo peel
3.1.1. Physical characteristics of carbonized pomelo peel.
3.1.1.1. SEM image

Figure 3.1: Porous Structure and Tube Structure of Fresh Pomelo Peel (left) and
Carbonized Pomelo Peel
Figure 3.1 describes the morphology of fresh pomelo peel and the CPP. The
annealing condition of this CPP was: 4000C for 2 hours. On a large scale, both
materials exhibit porous and tube structures. Tube structure is surrounded by porous
structures and is a minority. This proves that, after participating in the carbonization
process, the structure of the material remains unchanged. Moreover, the porous
structure, and especially the tube structure, can perform well as a water channel.
Water in the water supply will be brought to the surface of the absorption layer
through capillary force. The water can then absorb heat easily and evaporate.

15


Figure 3.2: Changes in structural dimensions before and after carbonization process.
(a), (b), (c): Fresh Pomelo Peel; (d), (e), (f): Carbonized Pomelo Peel.
Figure 3.2 shows the dimensions of the tube structure and the porous structure of
the material before and after the carbonization process. Fresh Pomelo Peel has a
porous structure with a hole diameter of about 300- 400 µm. After carbonization,
although the porous structure remains the same, the diameter of the holes is
significantly reduced. Most holes in the porous structure range in size from 75 to
150 µm. Similarly, the diameter of the tube mouth in the tube structure is also
reduced. This is explained by the evaporation of water in the material structure

during carbonization process. At high temperatures, the material loses water,
causing all structures to shrink from 2 to 4 times. As the size of the internal
structure of the material becomes smaller, it is beneficial to transport water. The
higher capillary force, the better water transport system, also the better water
retention ability.

16


Figure 3.3: Water Capacity Ability of Carbonized Pomelo Peel
The water capacity of carbonized pomelo peel is shown in Figure 3.3. By assessing
the mass ratio of the material before and after hydration, the water capacity ability
of the material is shown as the equation above. The variable y is mass of the
hydrated material and x is mass of the original material. It implies that the amount
of water that the material can hold can be more than 6 times its own weight. In
addition, water transport speed is also assessed through the change of average
weight over time of exposure to water of the material. The average water transport
speed reaches the speed of 0.1 kg.m-2.h-1. The above proves that the material is
carbonized pomelo peel capable of performing tasks as well as a water channel
system.

17