Fabrication of photothermal nature inspired materials application on highly solar steam generation

VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY NGUYEN MINH TUAN FABRICATION OF PHOTOTHERMAL NATURE-INSPIRED MATERIALS APPLICATION ON HIGHLY SOLAR STEAM GENERATION MASTER

VIETNAM NATIONAL UNIVERSITY, HANOI

VIETNAM JAPAN UNIVERSITY

NGUYEN MINH TUAN

FABRICATION OF PHOTOTHERMAL NATURE-INSPIRED MATERIALS APPLICATION ON HIGHLY SOLAR

STEAM GENERATION

MASTER’S THESIS

Hanoi, 2020

VIETNAM NATIONAL UNIVERSITY, HANOI

VIETNAM JAPAN UNIVERSITY

NGUYEN MINH TUAN

FABRICATION OF PHOTOTHERMAL NATURE-INSPIRED MATERIALS APPLICATION ON HIGHLY SOLAR

STEAM GENERATION

MAJOR: NANOTECHNOLOGY CODE: 8440140.11QTD

RESEARCH SUPERVISOR:

Dr PHAM TIEN THANH Associate Prof Dr MAI ANH TUAN

Hanoi, 2020

ACKNOWLEDGEMENTS

Firstly, I would like to extend my sincere thanks to Dr Pham Tien Thanh, working for Vietnam Japan University, for his enthusiasm, encouragement, and patient guidance during the preparation of my master thesis

Secondly, I would like to express my great appreciation to Assoc Prof Dr Mai Anh Tuan, working for National Center for Technological Progress (NACENTECH), Ministry of Science and Technology (MOST), for his enthusiasm guidance and inspiration throughout the implementation of the thesis

Moreover, I would like to express my great appreciation to Prof Dr Kazuo Umemura, working for Department of Physics, Faculty of Science Division II, Tokyo University of Science, who gives me a lot of valuable suggestions and teaches me with the necessary knowledge about the science

I would like to thank the respectful professors, lecturers, researchers, and staff in Master program in Nanotechnology, Vietnam Japan University, who help me accomplish this thesis

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 Vu Tien Dung The advice given by them has been a great help in my research

Finally, I especially wish to thank my mom, my dad, and friends, who are always by

my side, have supported and encouraged me throughout my life My life will be incomplete without them

Nguyen Minh Tuan

Hanoi, 2020

TABLE OF CONTENTS

ACKNOWLEDGEMENTS i

LIST OF TABLES iii

LIST OF FIGURES iv

LIST OF ABBREVIATIONS vi

ABSTRACT 1

CHAPTER 1: INTRODUCTION OF SOLAR STEAM GENERATION 1

1.1 The importance of converting seawater into freshwater 1

1.2 Desalinating seawater 2

1.3 Solar steam generation (SSG) 3

1.3.1 Types of photothermal materials 4

1.3.2 Solar steam generation devices design 7

1.4 Purpose of thesis 9

CHAPTER 2: EXPERIMENTS 11

2.1 Fabrication of photothermal materials 11

2.1.1 Chemicals 11

2.1.2 Preparation of natural porous materials 11

2.2 Characterization of photothermal materials 13

2.3 Solar steam generation systems 14

2.3.1 Construction of SSG systems 14

2.3.2 Evaluate the water evaporation ability of SSG system 15

2.3.3 Evaluate the desalination ability of the system 16

CHAPTER 3: RESULTS AND DISCUSSIONS 18

3.1 The surface morphologies of photothermal materials 18

3.2 Analyzing surface structure of photothermal materials 21

3.2.1 FT-IR spectra and EDS analysis 21

3.2.2 Iron-tannic acid complexes 23

3.3 The photothermal materials 25

3.3.1 Absorption properties 25

3.3.2 Evaluation of light to heat conversion 27

3.3.3 Evaluation of water transport 31

3.4 Performance of SSG devices 33

3.4.1 SSG devices under solar simulator 33

3.4.2 SSG devices under natural sun .35

3.4.3 The photothermal materials stability 36

3.4.4 Quality of freshwater collected from SSG systems 37

CONCLUSION 42

REFERENCES 43

APPENDIX 48

LIST OF TABLES

Pages

Table 3.1 Composition of the pristine pomelo, pomelo-TA, pomelo-TA-Fe3+ .23

Table 3.2 The comparison of evaporation rate for each material .41 Table S1 Composition of pristine crape myrtle wood, wood-TA, wood-TA-Fe3+ 49

Table S2 Composition of pristine corn stover, corn-TA, corn-TA-Fe3+ .49

Table S3 Composition of pristine fingered citron, fingered citron-TA, fingered

citron-TA-Fe3+ .50

LIST OF FIGURES

Pages

Figure 1.1 Distribution of Earth’s water 1

Figure 1.2 The photothermal material combined AuNPs and graphene oxide 5

Figure 1.3 Simulation of Tungsten oxides 5

Figure 1.4 Schematic of AuNps – Airlaid Paper and carbonized Airlaid Paper 6

Figure 1.5 Configuration of double-layer photothermal materials including polyacrylonitrile and carbon black NPs coating PMMA 8

Figure 2.1 Fabrication of (a) solar steam generation system and (b) photothermal materials 12

Figure 2.2 Some instruments used in this research (a) JSM-IT100 InTouchScopeTM Scanning Electron Microscope (b) Oriel® Sol1ATM Solar Simulators (c) FLIR C2 camera (d) Jasco V-730 UV-VIS Spectrophotometer (e) Drying oven Venticell LSIS-B2V/VC55, MMM Group .14

Figure 2.3 The structure of steam generation part of an SSG system .15

Figure 2.4 Evaluate the steam evaporation index of SSG systems in the laboratory .16

Figure 2.5 The experiment of collecting freshwater from seawater of SSG devices. .16

Figure 3.1 SEM images of natural porous materials (a-c) pristine pomelo peel, (d-f) pristine crape myrtle wood, (g-i) pristine corn stover, (i-l) pristine buddha’s hand 19

Figure 3.2 Images of natural porous materials at each step of functionalization: (a-d) natural samples, (e-h) pristine pieces, (i-l) materials modified by TA, (m-p) materials modified by TA and followed by Fe3+ From a to d, the materials are corn stover, crape myrtle wood, pomelo peel, and buddha’s hand, respectively 20

Figure 3.3 SEM images of functionalized materials (a, b) Pomelo-TA-Fe3+, (c, d) Wood-TA-Fe3+, (e, f) Corn-TA-Fe3+, (g, h) Fingered citron-TA-Fe3+ .21

Figure 3.4 FTIR spectra of the pristine pomelo, pomelo-TA, pomelo-TA-Fe3+ 23

Figure 3.5 Schematic representation of the background cellulose of natural porous materials with hydrogen bonded tannic acid .24

Figure 3.6 The possible complexation mechanism of TA with Fe3+ .25

Figure 3.7 Absorption of different chemically modified materials .26

Figure 3.8 Absorption of functionalized materials in the UV-Vis-IR region 27

Figure 3.9 The surface temperature rise of pristine materials and the resultant functionalized materials relative to heating time under 1 sun illumination .28

Figure 3.10 The surface temperature rise of functionalized porous materials relative to heating time under 1 sun illumination .29

Figure 3.11 IR images of chemically modified materials within 10 minutes under 1 sun illumination (a) Pomelo-TA-Fe3+, (b) Wood-TA-Fe3+, (c) Corn-TA-Fe3+ and (d) Fingered citron-TA-Fe3+ .30

Figure 3.12 Water capacity ability of chemically modified materials .32

Figure 3.13 The temperature of functionalized materials under 1 sun illumination relative to irradiation time .33

Figure 3.14 Vapor generation ability of various functionalized materials under 1 sun. .34

Figure 3.15 Vapor generation of functionalized material at difference moments .35

Figure 3.16 Images of materials before and after vibrating in ultra-sonic machine (a

– d) pomelo-TA-Fe3+, fingered citron-TA-Fe3+, corn-TA-Fe3+, wood-TA-Fe3+, respectively (e – h) pomelo-TA-Fe3+, fingered citron-TA-Fe3+, corn-TA-Fe3+,

wood-TA-Fe3+, respectively .36

Figure 3.17 Concentrations of primary ions in an actual seawater sample before and after desalination Anions (top) and Cations (bottom) .38

Figure 3.18 The freshwater from SSG system of each day (7 hours/day) 39

Figure S1 FTIR spectra of pristine and functionalized corn stover 48

Figure S2 FTIR spectra of pristine and functionalized wood .48

Figure S3 FTIR spectra of pristine and functionalized fingered citron .49

Figure S4 Vapor generation ability of pristine and functionalized materials under 1 sun illumination .50

LIST OF ABBREVIATIONS

AuNPs Gold nanoparticles

CDD Capillary driven desalination

CDWA Capillary driven water ascension

CS Carbon sponge

CuNPs Copper nanoparticles

DI Deionized

EDS Energy Disperse X-Ray Spectroscopy

FTIR Fourier-Transform Infrared Spectroscopy

GO Graphene oxides

LSPR Localized surface plasmon resonance

MTES Minimum thermodynamic energy of separation MWCNTs Multiwalled carbon nanotubes

NPs Nanoparticles

RO Reverse Osmosis

SEM Scanning Electron Microscope

SSG Solar steam generation

TiO2-NTs Titanium dioxide nanotubes

UV-Vis-NIR Ultraviolet-Visible-Near Infrarred

ZVI Zerovalent iron

ABSTRACT

Clean freshwater plays an essential role in human life and social development, which relates closely to many aspects such as potable water, food-producing, environment protection, ecological equilibrium, and so on As we known that 70% of Earth’s surface is covered by water, but only about 1.7% of that is freshwater being suitable for consumption Water scarcity has been considered as one of the most serious risks

in the world, which is originated from the unequal distribution of water over time and place on Earth Besides that waste, pollution, and unsustainable consumption are also known as causes from human activities leading to water scarcity Developing nanostructured materials-based methods for converting saltwater into freshwater has attracted the broad attention of the scientist, which is a potential approach to contribute to reducing consequences of water scarcity Thermal distillation method makes steam from salt water sources, and the condensation then generates the liquid phase of freshwater However, this method requires a large amount of energy Using renewable energy for thermal distillation is an effective solution instead of consuming traditional resources such as coal or fossil fuels Solar steam generation (SSG) system has been studied to exploit solar energy for producing freshwater In this system, photothermal materials are considered as a key component, which acts

to achieve a high yield for convert sunlight energy into thermal In addition, the photothermal material should possess the porous morphology that facilitates efficient water transport through capillarity, and enhances the speed of water evaporation Natural porous substances, such as pomelo, wood, corn stover, buddha’s hand fruit, reveal natural capillary infiltration due to the high density of porous media The application of natural substances for photothermal materials in SSG not only reduces the material preparation steps, but also offers an environmentally-friendly solution in material technology

This thesis with the title “Fabrication of photothermal nature-inspired materials application on highly solar steam generation” reports a relatively sufficient work for the development of the solar steam generation system based on natural porous materials For preparation of photothermal materials, natural porous sheets were

functionalized chemically using the iron-tannic complex This complex has an important role in enhancing solar absorptivity due to its strong absorption bands from the ultraviolet to near-infrared regions The effect of experimental conditions such as various natural porous substances, concentration of chemically precursors, and time for chemical functionalization, was investigated to prepare optimally photothermal materials The capability of the prepared materials to convert sunlight to heat was evaluated using photo-thermal imaging Setup of solar steam generation systems was conducted and applied with salt water to measure water evaporation rate under illumination of a solar simulator Subsequently, real tests were carried out by exposure to sunlight Furthermore, the quality of freshwater obtained from the SSG systems was determined that aims to evaluate the capability of potable water

CHAPTER 1: INTRODUCTION OF SOLAR STEAM GENERATION

1.1 The importance of converting seawater into freshwater

Water is a critical resource in all aspects of life on Earth Although about 71 percent

of the Earth’s surface is covered by water, most Earth’s water is salt water stored in the oceans The freshwater occupies 2.5 of the percent the total volume of water, and only 0.3 percent of that is in liquid form on the surface (Fig 1.1) Nowadays, the safety freshwater sources are being exhausted at an increasingly rapid rate because of pollution, climate change, industrial agricultural practices, unsustainable energy production, and population growth About half a billion people live with severe water scarcity every year [30], and many people have a serious difficulty in maintaining the standard water demand As for environmental, water scarcity on Earth is exhibited through negative environmental phenomenon including increasing salinity and decreasing area of the fresh water sources on the land surface, such as lakes, rivers and ponds [17] Furthermore, the wetlands and flood-plains, which play a role as a natural water filtration to support the growth of crops, are gradually narrowing area leading to danger for the habitats of many species [20] Water scarcity has been considered as a global risk, which can cause harmful impact over 10-year-round [45]

Figure 1.1 Distribution of Earth’s water [8]

Developing solutions to mitigate the rise of water scarcity is an essential problem to protect the environment, biodiversity, as well as residential communities on Earth Many strategies for reducing water scarcity have been proposed such as developing

water filtration systems, promoting water stewardship, protecting wetlands, improving irrigation efficiency, and increasing water storage in reservoirs Among these solutions, the application of technologies to produce freshwater from salt water has received wide attention during the past decades Enhancement of seawater-freshwater conversion yield accompanied reduction of consuming energy is the core value of an applicable technology

1.2 Desalinating seawater

The desalination process is known as one of the important approaches for turning seawater into freshwater There are many technologies that have been developed and applied, such as distillation, ion exchange, membrane filtration, etc [10, 16, 41] Generally, these technologies can be categorized into two groups based on their principle: physical processes and chemical processes As for chemical processes, the zerovalent iron (ZVI)-based distillation is a typical method, and it just start to be commercialized This material possesses high porosity and highly active oxidizing Iron (III), which allows storing and removing halite ions (for example, NaCl, MgCl2) from seawater [2, 3] Since chemical methods usually accompany the oxidation-reduction processes of the desalination materials, it is difficult to control product ions generated during converting seawater into freshwater Thus, chemical approaches do not seem approriate for drinking water

As typical physical processes, reverse osmosis is one of the desalination technique [42] However, RO-based technologies have a disadvantage that is difficult to reduce MTES Moreover, RO desalination requires consuming high-grade energy (electrical energy) and pre/post-treatment of salt water before using this method also consumes

a significant amount of energy [36, 37]

The heat-driven desalination technologies provides an applicable solution, because it can using the heat energy with medium temperature (400˚C) for evaporation [11, 32] Low-grade energy is thermal energy with low-to medium-temperature heat (up to 400˚C), which can be generated by the burning of fossil fuels like gas, coal, or oil The utilization of low-grade heat also encounters disadvantages such as difficult control, fast heat loss, and negative effects to the environment In thermal desalination, heat loss could be minimized by recovering the heat Then, the heat is used for the next desalination [19, 33]

The mentioned methods are considered as conventional approaches that are applied

in most countries for producing potable water However, they exhibit some limitations such as high cost, large consumption of energy, and problems in waste products This is a drive for efforts from science community to develop alternative methods Firstly, renewable energy, namely solar energy, offers a green solution to replace traditional resources of energy for thermal distillation Secondly, capillary-driven water ascension (CDWA) inspired by trees [52] has been considered as an environmentally-friendly technology in efficient energy harvesting, and capillary-driven desalination (CDD) [15, 28] Hence, exploitation of solar energy combined with CDWA has attracted broad attention of the scientist, which becomes a potential approach for making freshwater from salt water Nowadays, the solar steam generation system using solar energy is attracting much attention because of their advantages such as eco-friendly, cheap, and high-performance

1.3 Solar steam generation (SSG)

Solar energy is a renewable natural resource Annually, Earth receives a total of 173,000 TW from solar, equivalent to 10,000 times the amount of energy used by humankind [35] Vietnam is tropical country possessing an abundant source of solar irradiance (5 kW/h/m2) and high number of sunshine hours (2000-2600 h/y ~ 6-7 h/d) that facilitates all solar energy related technologies In recent years, solar steam generation (SSG) has obtained great attention because of significant advantages such

as no electricity use, zero CO2 emission, and simple fabrication It is expected to provide a useful solution for exploiting the natural energy sources With the sunshine hours of 6-7 h/d as in the South of Vietnam, the device can produce 15-30 L/h, equivalent to the minimum water demand of a household per day [49]

Solar steam generation (SSG) is a technology that provides the freshwater from salt water based on photothermal effect [43] The SSG is applied in many fields such as desalination seawater, purification waste water, thermoelectric system, and so on A solar steam generation system includes three essential components: (1) photothermal material, (2) water supply system, and (3) freshwater container [25] The principle of the SSG system is the solar-to-heat conversion process In this process, photothermal material absorbs the photons and converts light energy into thermal one With a large enough amount of heat the water (in liquid) is evaporated to vapor form with high

purity Later on, the vapor is condensed and collected into a tank In the SSG system, the water is continuously transported to the surface by several paths, one of which is through capillary forces Since the photothermal materials generate heat from solar energy without CO2 emission, the SSG system is expected to solve the clean water inadequacy with decreasing global warming In order to optimize the yield of freshwater generation, many research groups have been developed photothermal materials for SSG systems In the following section, various types of photothermal materials are categorized and their structure will be reported

1.3.1 Types of photothermal materials

There are various types of materials used for light-to-heat conversions such as: metallic nanoparticles [9, 27], metal oxides [39], polymers [44], semiconductors [46] and bio-inspired materials [12] (Fang et al., 2018)

a Metallic nanoparticles

At nano-scale, one of the interesting characteristics of metallic nanoparticles (NPs)

is the localized surface plasmon resonance (LSPR) This phenomenon is originated from the interaction between excited electrons on the surface of NPs and the incident light, and it helps NPs could display their high efficiency of the light absorption Many research groups have taken advantages of LSPR to develop materials that can

be used to absorb strongly solar light In 2017, copper nanoparticles (CuNPs) were fabricated by Yang and coworkers, which reveal the light absorption ability in the wavelength range from 200 to 1300 nm with a high level (~97.7%) [29] Using that materials, Yawen Lin developed a solar steam generation system that achieves the performance up to 73 % under 2 sun illumination [24] In another research, a combination of gold nanofluids and multiwalled carbon nanotubes (MWCNTs) for sunlight absorption have been investigated by Wen [5] Furthermore, Campo’s group combined metal NPs nanofluids and graphene oxides (GO), and pointed out that the materials bring the highly thermal conversion [6]

Figure 1.2 The photothermal material combined AuNPs and graphene oxide [4]

Plasmonic metal NPs modified fibers were used for steam generation, which revealed the average water evaporation rate of 1.4 kg m-2.h-1 under one sun illumination [18] Figure 1.2 shows a SSG system using the photothermal material that fabricated by a combination between AuNPs and graphene oxide [4] However, the integration of NPs to graphene is still a challenge the engineers in order to optimize the efficiency

of the SSG system

b Metal oxides

Metal oxides also exhibits strongly light absorption ability Tungsten oxide (WOx) has been widely used for SSG [31] WO2.9 has a great light absorption capacity with total absorption of 90.6% of the solar spectrum, and the light-to-heat conversion reaches 86.9% Using this material, as shown in Figure 1.3, in steam generation, water evaporation achieved 81% upon sunlight illumination [38]

Figure 1.3 Simulation of Tungsten oxides [38]

In other work, polydimethylsiloxane (PDMS)-modified Fe3O4 nanoparticles were dispersed on the surface of graphene sheets When exposing to sunlight, this material shows good light-to-heat conversion with the temperature up to 100˚C [39] Light trapping and LSPR properties were found at a hybrid material that combined Nickel nanoparticles (Ni-NPs) and titanium dioxide nanotubes (TiO2-NTs) This material possesses strong light absorption of 96.83% in the range of wavelength from 300 to

2500 nm [7] The strong absorption ability in the range of visible to NIR results in metal oxides can convert the absorbed light more efficient However, this material has some disadvantages such as complex and high-power density

c Carbon-based materials

In the past years, carbon nanostructures-based photothermal materials have been developed for the SSG system due to their effective characteristics with solar energy conversion and eco-friendly advantages The combination of carbon nanotubes and hydroxyapatite nanowires produced one-dimensional materials which has greatly flexible and thermally-resistant The 1D materials exhibited high water evaporation efficiency of 83.2% at 1 sun [46] In addition, carbon sponge (CS) at three-dimensional structures with highly porous media can be used for water evaporation that achieved the vaporization rate of 1.39 kg.m-2h-1 under solar illumination of 1 kW.m-2 [22]

Figure 1.4 Schematic of AuNps – Airlaid Paper and carbonized Airlaid Paper [23]

Carbon-based materials are originated from natural porous substances, providing a new route to fabricate solar-thermal driven conversion materials Many works focused on the carbonization of natural materials such as woods, mushrooms, lotus seedpods [47, 12, 52] For example, kelps possessed a hydrophilic surface due to their

porous structure at micro-scale After the carbonization, the materials reveal strong absorption with solar light Under one sun illumination, their water evaporation rate could achieve 1.351 kg.m-2h-1 with efficiency up to 84.8 percent However, the carbon-based materials are fabricated at high temperature with complicated instruments In addition, one of the disadvantages of this approach is the long time it takes to fabricate the materials

It is seen from the above mention that the fabrication of new photothermal materials with simple process and large quantities with suitable fabrication technique is essential In this work, both photothermal materials and fabrication method will be presented and discussed in the chapter number two

1.3.2 Solar steam generation devices design

In a high-performance solar steam generation system, two factors including (1) water transportation capacity and (2) the light-to-heat conversion ability of photothermal materials affect directly to the efficiency Each factor will be presented briefly in this section

a Rapid water supply and fluent steam channels

When an SSG system is performed, salt water is transported from the salt-water tank

to material’s surface and then turned into steam by heat of the photothermal material The capillary forces are primary factors which bring the water from the bottom to the surface of materials In order to ensure adequate water on the surface of materials for steam generation, the structure of photothermal materials should have porous media and hydrophilic surface The hydrophilic property of materials allows maintaining water at the interfacial surface For example, the SSG system in the report of Wang and coworkers was constructed by multilayer polypyrrole (PPy) nanosheets that exhibits highly hydrophilic surface The evaporation efficiency rose up to 92% under

1 sun illumination [13] The multilayer nanostructured materials with a strong hydrophilic property can absorb droplets of water dripped onto the surface in less than one second In other studies, various membranes combined with Cu NPs was used to optimize for SSG systems The result is that SSG system with cellulose membranes shows a great efficiency The performance of SSG could rise to 73% in

an irradiation power density of 2 sun [24]

In making freshwater from sea water, salt could cause a blockage of capillary channel that hinders water transportation within the photothermal material This problem could be significantly decreased when using hydrophobic materials structures Therefore, the combination of hydrophilic and hydrophobic structure or either of them depends on specific objectives in order to enhance the efficiency of the SSG system For example, in the SSG system of Zhu [48], photothermal materials were designed with double-layer that consists of polyacrylonitrile as hydrophilic layer and PMMA-coated carbon nanoparticles (C-NPs) as hydrophobic layer (Figure 1.5) With this structure, PMMA-CNPs prevent accumulation of salt at the surface of materials, while the polyacrylonitrile layer maintains adequate water for steam generation [48]

Figure 1.5 Configuration of double-layer photothermal materials including

polyacrylonitrile and carbon black NPs coating PMMA

In summary, the photothermal materials need to have a good water channel system

to ensure the rapid and efficient water supply In addition, using double-layer

consists of hydrophobic and hydrophilic layer is a method to prevent salt

accumulation from blockage the capillary pores Therefore, our research group used

a polystyrene foam covered by gauzed pads, as a double layer It could minimize the salt accumulation phenomenon while maintaining the water transport through the water channels

b Rational thermal management

As mentioned, photothermal materials play a role in light-to-heat conversion and the steam generation is carried out by heat The water cannot absorb entire heat from the photothermal material because the heat lost due to the thermal transfer to the ambient through main pathways including conduction, convection, and irradiation Thermal energy not only raises the surface temperature of materials, but also transfers heat to the bulk water below This means thermal energy is not fully transferred to the steam generation process, so the SSG system’s efficiency of water evaporation decreases Thus, reducing the heat loss means increasing water evaporation Ho and coworkers pointed out that the floating carbon sponges (CS) has the highest evaporation efficiency in three types of CS materials including floating, sinking, and suspending

CS [51] The authors explained that the floating CS processes the smallest contact area in three materials A small contact area leads to decreasing the heat transfers into

a bulk water, and reducing heat loss In another study, the heat loss of carbonized kelps is reduced by covering a thermal insulator [26] From that, our research groups used a polystyrene foam as an insulator for SSG system Because the chemically modified materials are separated from the water surface while the amount of water

on the surface is always maintained

Thus, it can be seen that water supply management and maximizing light-to-heat conversion are two essential aspects in fabrication of a SSG system In the forward aspect, enhancing transport of water supply can be solved by using porous materials and preventing salt accumulation in porous channels While the later aspect can be achieved by reducing the amount of heat transferred to the surroundings

In this section, we have summarized the potential of thermal distillation technologies for producing freshwater, essential components, and operation of solar steam generation (SSG) system, importance, and the main kinds of photothermal materials

in SSG, and the problems in fabrication of photothermal materials

1.4 Purpose of thesis

This thesis reports the fabrication of SSG structures using natural porous materials, such as pomelo, wood, buddha’s hand fruit, and corn stover, for preparation of photothermal materials Natural porous materials were chemically functionalized by iron-tannic complex that provides the strong absorption in the solar region The energy conversion from the absorbed solar radiation into the thermal energy on the

surface of materials was evaluated through measuring thermal images Performance

of fabricated SSG devices was investigated by calculating the rate of water evaporation in a solar simulator as well as in real conditions The purity of freshwater, which is obtained by SSG devices in real conditions, was determined to evaluate the capability of potable water

The master thesis aims at:

- Fabricating the photo-thermal materials from natural porous materials by the chemical method

- Studying the characteristics of the materials

- Evaluating the absorption of bio-metamaterial under the sunlight

- Developing a SSG system

- Evaluating the evaporation capacity of SSG system

- Demonstration in desalination seawater capacity of SSG system

CHAPTER 2: EXPERIMENTS

This chapter presents a fabrication process of photothermal materials using natural materials with porous media Material surface is functionalized by chemical methods using iron-tannic complexes Solar steam generation systems are built and performed for converting salt water into freshwater

2.1 Fabrication of photothermal materials

2.1.1 Chemicals

- Tannic acid (C76H52O46, powder), FeCl3 (powder) were purchased from Sigma Adrich (German)

- Ethanol, H2SO4, H2O2, and acetone were obtained from China

- Ultra-pure water, which is obtained from Water distill D4000 (VJU), was used

to prepare solutions

2.1.2 Preparation of natural porous materials

In this research, the natural materials including pomelo peel, corn stover, buddha’s

hand (scientific name: Citrus medica var sarcodactylis) and crape myrtle wood (scientific name: Lagerstroemia speciosa) was chosen to fabricate photothermal

materials The samples of those materials were cut into pieces with square-shape or circle They were immersed in absolute ethanol to remove unexpected substances such as contaminant, essential oils, and so on Then, the samples were rinsed by DI water to make a preparation for later fabrication (Fig 2.1)

Figure 2.1 Fabrication of (a) solar steam generation system and (b) photothermal

solution (solution B) at 0.012 M of concentration was obtained from dissolving 0.4 g

of FeCl3 in 200 ml of ultra-pure water The volume of solutions was calibrated in volumetric flask The pieces of natural materials, prepared as described in section 2.1.1, were chemically modified as following steps:

Step 1: Immerse samples in solution A and stirring in 4 hours

Step 2: After that, rinse samples with DI water

Step 3: Immerse sample-TA in solution B and stirring in 2 hours

Step 4: After that, rinse samples with DI water

Step 5: Dry the materials products at 75 – 80 ˚C in a furnace

The photothermal materials products was observed and analyzed surface structure by

these method such as SEM, EDS, FTIR, etc Finally, the SSG system was implemented in the different conditions to investigate the efficiency of the materials

2.2 Characterization of photothermal materials

The photothermal materials was observed and structure analyzed by some equipment

at Nanotechnology laboratory, Vietnam Japan University and Research Center for Environmental Monitoring and Modeling (CEMM), Hanoi University of Science (HUS), Vietnam National University (VNU) Figure 2.2 shows some instruments used in this research These instruments were used to structural analysis and morphological properties of the photo-thermal conversion materials

- Using the drying oven: to dry the material products

- Using SEM method: to observe the structure morphology of the materials

- Using camera FLIR C2: to investigate the change of temperature of materials under natural sun and artificial sun

- Using UV-VIS-NIR spectrophotometer: to measure the absorption of materials

- Using FTIR, EDS method: to determine the functional group and chemical compositions

- Using Oriel® Sol1ATM solar simulator: to evaluate the water evaporation ability of SSG system

Figure 2.2 Some instruments used in this research (a) JSM-IT100 InTouchScopeTM

Scanning Electron Microscope (b) Oriel® Sol1ATM Solar Simulators (c) FLIR C2 camera (d) Jasco V-730 UV-VIS Spectrophotometer (e) Drying oven Venticell

(VNU-UET)

Figure 2.3 The structure of steam generation part of an SSG system

Figure 2.3 illustrates the structural model and realistic photo of steam generator part

of SSG system The water supply part is composed of two parts: a beaker as a tank and a polystyrene foam wrapped by gauzed pads The polystyrene foam acts as an insulator to prevent heat loss from the photothermal materials while a gauzed pad ensures that the water is transferred to the surface of materials Finally, the functionalized materials are put on the insulator foam

2.3.2 Evaluate the water evaporation ability of SSG system

Figure 2.4 presents an experiment for determining the evaporation rate of SSG device The SSG device (described in section 2.2.1) was mounted on an electronic scale connected to a computer to record the mass change of water over time The mass change was recorded at equal intervals, then plotted on the computer and the water evaporation rate was calculated upon the decrease of water in SSG system This experiment was conducted under 1 sun illumination generated by a solar simulator at Faculty of Engineering Physics and Nanotechnology, VNU University of Engineering and Technology (VNU-UET)

Figure 2.4 Evaluate the steam evaporation index of SSG systems in the laboratory

2.3.3 Evaluate the desalination ability of the system

The desalination experiment was conducted under natural sun with the water input source being seawater as shown in figure 2.6

Figure 2.5 The experiment of collecting freshwater from seawater of SSG devices

A beaker (100 cm2 in area) was placed inside a compartment of a glass box and exposed to natural sunlight throughout the duration experiment During this time, seawater was brought up on the surface of the material via capillary channels and

turned into vapor because the materials has a good light-to-heat conversion The steam then condensed on the top glass case and flowed down to the water tank compartment due to the slope of the roof To enhance the performance condensation,

a refrigeration consist of dry ice was placed next to the glass box, which contributed

to speed up the vapor-liquid phase transition The freshwater would escape through the outlet hole at the bottom of the glass box The ion concentrations of freshwater were analyzed at Research Center for Environmental Monitoring and Modeling (CEMM), HUS

CHAPTER 3: RESULTS AND DISCUSSIONS

This chapter will discuss the characteristics of photothermal materials synthesized by the chemical method as described in chapter 2 Characterization is focused on important properties of a photothermal material when it is applied in the SSG system, including ability of water transport and degree of light to heat conversion Furthermore, the performance of the fabricated SSG system will be investigated under various conditions of experimental protocol Finally, freshwater obtained by the SSG systems will be evaluated, which aims to compare with drinking-water quality standard

3.1 The surface morphologies of photothermal materials

Figure 3.1 shows the surface morphologies of natural porous materials before functionalization Pomelo peel, crape myrtle wood, corn stover and buddha’s hand (another common name: the fingered citron) were known as natural porous materials

On a large scale, all pieces of materials exist the region of porous structures interwoven with tubular structures As can be seen, these tubular constructions occupy a small proportion to the porous structure of natural materials, except for wood In general, the structure of crape myrtle wood consists of the alternative arrangement of large and small canals with the diameter of each pore about 6 – 20

µm Each small pore has a diameter of about 7 – 10 µm while the size of large pore

is about 15 – 20 µm of diameter In the structure of other materials, the pore aligns closely and surround each other to form regions of canals In pomelo peel, the pore size is in the range of 8 – 20 µm and a large part of pores is approximately 15 µm of diameter For corn stover, every region of canal structure composes of small pores surrounding three super-large others Most pores range in size from 10 – 15 µm while the large other have a size from 30 µm to 50 µm For the fingered citron, each region structure composes of big pores surrounding smaller pores The biggest pores have a size about 35 – 45 µm, the medium pores range in size from 25 – 30 µm, and the smallest pores have a diameter of about 10 – 15 µm of each

Because the diameter of canals of objects is at micro-scale, the capillary force has a primarily role for transport water inside materials The combination between the uniform porous structure and a large of canal regions allows more effective water transport This is one of the important properties of photothermal materials in the steam generation

Figure 3.1 SEM images of natural porous materials (a-c) pristine pomelo peel, f) pristine crape myrtle wood, (g-i) pristine corn stover, (i-l) pristine buddha’s hand

(d-Figure 3.2 Images of natural porous materials at each step of functionalization: d) natural samples, (e-h) pristine pieces, (i-l) materials modified by TA, (m-p) materials modified by TA and followed by Fe3+ From a to d, the materials are corn stover, crape myrtle wood, pomelo peel, and buddha’s hand, respectively

(a-Figure 3.2 illustrates the images of natural porous materials at each step of functionalization The raw objects were cut into pieces of sample and then chemically modified It is clearly seen that the size of sample remained during functionalization process The color of materials change from bright to black after modified by TA and followed by Fe3+, indicating that Fe(III)-tannin complexes appeared on surface of materials As is well known, the photothermal materials with black body are necessary for thermal absorption ability After functionalization, surface