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 i 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 ii LIST OF TABLES Pages 3+ Table 3.1. Composition of the pristine pomelo, pomelo-TA, pomelo-TA-Fe . .....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 iii 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 iv 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 v LIST OF ABBREVIATIONS Abbreviation Description 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 vi 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 1 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]. 2 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 3 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]. 4 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 (WO x) 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]. 5 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 6 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]. 7 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 8 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 9 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 10