LITERATURE REVIEW
Nanostructures in nature
Life-forms own numerous magnificent structures in order to serve their life activities such as hooks in fore- and hindwings in sawfly or suctions cups on tentacles of Octopus However, not only did a selection of macrostructures evolve but also selected structures at even smaller scale too, producing a ton of nanostructure on living things’ organs with a size range about 1 nm to 100 nm in at least one dimension
The male blue morpho (Morpho rhetenor) boasts breath-taking blue wings
Figure 1.1 illustrated images for the blue morpho’s wing and nanostructure on the surface of the scale [11] Multilayer nanoscale patterns found on every scale which is the secret to creating iridescent blue wings It absorbs selective light in the visible region and reflects almost completely blue, so male blue morpho’s wings have a single blue colour This optical behaviour is caused by physical structures not for pigments so its product of this method is known as physical colours This phenomenon is formed by random scattering or interference and is quite popular in insect groups such as adult Lepidoptera, Odonata or Coleoptera
The structure on surface of dragonfly’s wings is another example in the nanoscale pattern in nature [18] Recent studies showed the bactericidal ability of nanostructures on dragonfly wings Instead of killing bacteria with antibacterial compounds or growth inhibitors such as antibiotics, they kill bacteria by physical mechanisms [1] Nanostructures have the ability to penetrate the cell wall when bacteria try to move on this surface Therefore, this structure limits the number of bacteria and prevents them from forming biofilm on the wing surface of dragonflies These structures inspired the antibacterial materials to be born with the surface covered with nanostructures similar to those on dragonfly wings With the exception of dragonflies, similar structures are also common in both animals and plants From the examples above we can imagine the diversity of nanostructures on the surface of animals Thus, we want to rely on these special surface structures to create new materials with features that can be applied to everyday life As characteristics of a tropical country, Vietnam with a high level of biodiversity is an advantage to discover new surface structures in nature In this study, our team focused on plant objects with leaf surfaces with the appearance of nanostructures
Figure 1.1: Morpho rhetenor butterfly a) A picture of half of butterfly b) The surface on the wing c) An individual scale d) The reflection spectrum of the butterfly’s wing e) & f) Scanning electron micrograph (SEM) of a scale Scale bars (b, c) 50μm, d) 5μm, e) 2μm
Figure 1.2 Some common nanostructure on the surface of animal and plant a) Nanostructures on fruit fly eyes (Drosophila melanogaster) b) mold fibers with micrometer size with nanostructures on the cell surface c) Hook hair on two- spotted spider mite (Tetranychus urticae) d) & c) Structure on water fern (Azolla filiculoides) f) The structure on rose leaves (Rosa Chinensis)
Plant leaves surface
The surfaces of leaves display a number of functional interfaces between the plants and their environment, both biotic and non-biotic things Plant leaf surfaces have evolved to adapt thousands of different living conditions leading to the diversity of their surfaces Many surfaces present a large variety of features such as super-hydrophobicity, self-cleaning, super-hydrophilicity and reduction of adhesion and light reflection, and absorption of harmful ultraviolet (UV) radiation, based on the existence of three-dimensional waxes For example, lotus (Nelumbo nucifera) leaves are known as an icon for a self-cleaning and superhydrophobic surfaces, and have resulted in the concept of the “Lotus effect” Scanning electron microscopy (SEM) images allowed to be seen microstructure and nanostructure in of lotus leaves’ epidermis layer which is an outermost complex tissue with protecting and gas exchanging function Covered on structures are cutin, a hydrophobic composite material consisting of nonacosane-10-ol and nonacosanediol in lotus, which are responsible for the superhydrophobic and self-cleaning ability of leaves Edelweiss or Leontopodium nivale inhabits at high altitudes of about 3000m and is the symbol of the Alps where UV radiation index reaches high-risk level The higher the UV radiation index, the greater the potential for harm to the cells and deoxyribonucleic acid (DNA) To survive in the alpine zone, this plant develops a thousand of white tiny hairs with nanoscale patterns around 100-200 nm in size to cover its flowers
These structures are capable of absorbing the UV light, protecting the flower from burning in the sun and also reflect all visible light Therefore, hierarchical structures play an important role in wetting behaviour, light reflection, and absorption of plant surfaces Three-dimensional wax crystals on the cuticle such as platelets, filaments, rods, crusts, and tubules often occur in the size range from 100 nm to 1000 nm so microscopic techniques are really useful to investigation of this epicuticular waxes
However, on a small group of the wax film has some special few molecular layers bringing about hardly visible in the SEM Thus, in some cases, atomic force microscopy (AFM) can be useful to investigate this wax film formation on a living plant surface Such results of studies displayed wax films consisted of various monomolecular layers with thicknesses up to a few hundred nanometers According to the thickness of wax films, it is classified into two groups: first is called three- dimensional (2-D) thin wax films with the size range up to 0.5μm and second is three-dimensional (3-D) waxes with size range about 0.5-1μm [2]
Inhabitation, plants are host to infection by a lot of pathogenic vectors that could do damage to organs or even kill the plant Therefore, all plant organs are covered with layers of hydrophobic compound that diminish dehydration and hinder the pathway of pathogenic microorganisms Several types of research have shown that the chemical compounds of plant waxes belonged to alkanes, alcohols, esters and aldehydes groups Other compounds are determined as ketones, β-Diketones, flavonoids, and triterpene The length of the hydrocarbons chain in all compounds is around 20 to 40 atoms, in some case with esters about 60 atoms These compounds are arranged in crystal form which plays a role as a water-repellent coat Previous studies on hydrophobic surfaces on leaves showed that water-resistant surfaces frequently appear with micrometer or nanometer-sized structures [8] Thus, we can choose samples through the water resistance of the surface
Table 1.1 The common chemical compounds in plant waxes [2]
Aliphatic compounds Frequently existing in waxes, but mostly as minor compounds Chain length
Alkanes CH3-(CH2)n-CH3 Odd C19 – C37
Primary alcohols CH3-(CH2)n-CH2-OH Even C12 – C36
Esters CH3-(CH2)n-CO-O-(CH2)m-CH3 Even C30 – C60
Fatty acids CH3-(CH2)n-COOH Even C12 – C36
Aldehydes CH3-(CH2)n-CHO Even C14 – C34
Rarely existing in waxes, but if present, than major wax compounds Ketones CH3-(CH2)n-CO-(CH2)m-CH3 Odd C25 – C33 β-Diketones CH3-(CH2)n-CO-CH2-CO-(CH2)m-CH3 Odd C27 – C35
Secondary alcohols CH3-(CH2)n-COH-(CH2)m-CH3 Odd C21 – C33
Cyclic compounds Flavonoids e.g Quercetin Triterpene e.g β-Amyrin
Metamaterials
Metamaterials are a group of artificial materials capable of interacting with electromagnetic waves in the desired way On the surface of metamaterial is often made a structure layer with a smaller size than the wavelength being considered
Therefore, with this group of materials, scientists pay more attention to their surface structure which is capable of interacting with electrical and magnetic components of light rather than their chemical composition [9] Basically, each artificial structure has the basic properties of natural atoms that act as in common materials
Nevertheless, when interacting with the components of electromagnetic waves, it generates a completely extraordinary property Because of this special property, metamaterial has the potential to be applied in many areas such as medical devices, remote aerospace applications, solar batteries, high-frequency information transmission, blackbody or absorber [4], [3], [15], [10] Recently, lenses allow both the light intensity and the direction of the incoming light to be developed by metamaterials [13] This allows the user to re-focus the image and then refactoring the depth of field information The surface of the metalens array is covered by nanotennas made of gallium nitride (GaN) which allow the tube to record bright field information
For electromagnetic waves to be able to interact or penetrate the homogeneous structure of metamaterials in an efficient and accurate way, the structures on the surface of the metamaterial must be much smaller than the size scale of the wavelength Electromagnetic absorbing materials can be divided into two main types: resonant absorbers and broadband absorbers [16] While the resonant absorber is based on an interaction with a specific appropriate frequency, the broadband absorber is usually a frequency-independence material and therefore absorbs radiation with a wide absorption spectrum than the other one
Metamaterials are designed based on the determination of magnetism permeability and electric permittivity so they can interact directly with the two components, electric and magnetic of light [19]
One of the most contributing applications of metamaterials is solar cells It is capable of helping the surface increase the ability to absorb light in solar panels by trapping light from all corners without a concentrator or monitoring system [17]
Besides the technology of using solar in the solar steam-generation device is also a promising direction Therefore, the team focused on making the surface of the material capable of absorbing the energy of light and then converting that energy into thermal energy [7] Some researchers chose simple natural or artificial structures used as light-absorbing materials are being developed as environmentally friendly solutions, [12] For example, the use of mushrooms after carbonization to increase the evaporation efficiency by 3 times in the artificial sunlight conditions of the authors from China [21] Recently, studies of sputtering of metals such as gold on natural surfaces such as taro leaves or lotus leaf have created super-materials with good absorption of visible light [6] Therefore, our team focused on finding and developing metamaterials that use nanostructures in nature to apply them to solar steam-generation technology.
METHOD AND MATERIAL
Fabrication of bio-metamaterial
Figure 2.1 Some equipment used in this research a) JSM-IT100 InTouchScope™
Scanning Electron Microscope b) JED-2300 Analysis Station Plus c) Syskey sputtering coater d) NanoMap-500LS Contact Surface Profilometer
Figure 2.2 Process of conducting experiments
2.1.1 Sputtering in the air at low pressure
The samples were prepared by the following procedure: (i) The young developed leave of both water cabbage and purple bauhinia were collected in wild at Hanoi (ii) After treating samples with deionized water (DIW), it was nipped off and fixed on a glass slide (iii) A thin copper film was deposited on the leaves The copper coating was carried out using sputtering in the air at low pressure For control, we prepared the samples of copper-sputtered leaves of rosy periwinkle (Catharanthus roseus) and (Rosa Chinensis) After preparation of the sample, the sample is put into the chamber of a sputtering coater in order to vacuum
The conditions during sputtering operation
Pressure before sputtering 10 -6 torr Pressure during sputtering 10 -3 torr
2.1.2 Analysing surface structures (EDS, SEM, FT-IT, Spectrometer)
The optical consideration was performed by a similar system which shows in Figure 2.3 The reflection spectra were recorded with MCPD – 3000 spectrometer (Otsuka Electronics) using a halogen lamp as a light source For the measurements of reflectance, the light was conveyed to the samples with Y-type optical fiber and the reflected light was collected by it The angle of incident light is 0 o The commercial aluminum sample film is used as a reflectivity reference For the measurements of scattering, the light from light source was conveyed by an optical fiber to the sample with angle equal 0 o The back-scattered light was collected by another optical fiber and transferred to the spectrometer An SRS-99 diffused reflectance standard (Labsphere) was used as reflectance reference The scattering angle was approximately 60 o with respect to the surface normal SEM pictures were performed and analysed with a JSM-IT100 InTouchScope™ combined with JED-
Figure 2.3 Experimental model for the record of reflection and scattering spectra with an MCPD-3000 spectrometer (Otsuka Electronics) using a halogen lamp
2.1.3 Checking of efficient solar absorption
All the object samples are placed on a thermal insulator that made of polystyrene foam The temperature is determined by FLIR C2 thermal camera The sun is used as the light source of the experiment Benetech GM1010 device is used to measure light intensity.
Prediction the low reflectivity by Finite-difference time-domain (FDTD)
The finite-difference time-domain (FDTD) method is arguably the simplest, both conceptually and in terms of implementation, of the full-wave techniques used to solve problems in electromagnetics The FDTD method can solve complicated problems, but it is generally computationally expensive Solutions may require a large amount of memory and computation time The FDTD method loosely fits into the category of “resonance region” techniques, i.e., ones in which the characteristic dimensions of the domain of interest are somewhere on the order of a wavelength in size If an object is very small compared to a wavelength, quasi-static approximations generally provide more efficient solutions Alternatively, if the wavelength is exceedingly small compared to the physical features of interest, ray- based methods or other techniques may provide a much more efficient way to solve
There are 3 simulation models to predict the absorption capacity of the bio- metamaterials and the 30 nm copper thin layer:
Nanostructure pattern with 30 nm copper thin layer with the height is
3 layers: the bottom is 30 nm copper thin layer, the center is nanostructure pattern with the height is 500 nm, and the top is 30 nm copper thin layer
Nanostructures pattern used in the model is constructed from the actual structure contained in SEM images SEM images are transferred to black and white images then used to create 3D nanostructure models in FDTD software with the same length and width compared to the original image
Figure 2.4 SEM images changes to black and white colours, then it uses to 3D structure in FDTD modelling.
Solar steam-generation system
50 ml beaker used in the experiment marked with lines to indicate 10, 20, 30,
40 and 50ml of volume Cotton pad-wrapped polystyrene foams had a thickness around 1.5 cm which covered fully the brim of the beaker Bio-metamaterials are fixed between two layers of cotton that play a role as a capillary path Beakers are placed on an insulating foam that made of polystyrene
Figure 2.5 Solar steam-generation device.
RESULTS AND DISCUSSION
Fabrication of absorbers based on some nanostructure in nature
Figure 3.1 (a) Bauhinia purpurea, (b) Pistia stratiotes
Based on the ability of water resistance, two kinds of leave, Bauhinia purpurea and Pistia stratiotes, were selected for the experiment Water cabbage
(Pistia stratiotes) belongs to a genus of aquatic plant in the arum family, Araceae, and Bauhinia purpurea is a member of the family Fabaceae with a common name, purple bauhinia Their photographic images are shown in Figure 3.1 (a, b) The surface of a water cabbage leaf and a purple bauhinia leaf is also highly water- repellent, although the surface nanostructure two both of leaves completely differs from that of lotus leaves
Figure 3.2 shows the photographic images of copper-coated leaves There are four samples, including of Bauhinia purpurea, Pistia stratiotes and two controls which have really low water resistance: Catharanthus roseus and Rosa Chinensis
Two controls leaves have the same colour with thermal tape after sputtering in argon at low pressure However, water cabbage and purple bauhinia, two object leaves turned into black colour after sputtering a thin layer of copper on leaves’ surface with a thickness of copper coating about 25 nm or more
Figure 3.3 displays the structures on surface of four samples The surface of rose periwinkle leaf has no micro- and nanoscale patterns and the surface of the rose leaf have microstructure patterns However, purple bauhinia shows plate-like nanostructures with the thickness about 100 nm while water cabbage shows root- hair-like nanostructures with the thickness about 80 nm The estimated height of each wax is about 1 μm This structure is quite similar to the nanostructure in taro leaves which was studied It suggests that the mechanism of colour of Bauhinia purpurea leaves can be similar to taro leaves In the case of Pistia stratiotes leaves, the leaf surface has many roots in micro scale which include hair-like nanostructure having predicted length is 80nm Both structures in Bauhinia purpurea leaves and Pistia stratiotes leaves have random orientation and dense distribution
The colour of the object leaves became different when changing sputtering time Figure 3.4 and Figure 3.5 reveal the colour of the copper-coating on the surface of purple bauhinia and water cabbage In case of purple bauhinia samples, there are five samples of 2, 3, 4, 5, and 6 with sputtering times of one minute, two minutes, three minutes, four minutes and five minutes respectively As we observe, the colour of the sample varies from back to bronze when viewed from sample 2 to sample 6 With water cabbage, the results also were the same with purple bauhinia
However, Nanostructures on the leaf surface change when the sputtering time changes especially on the thickness of the structure tends to increase (Figure 3.5)
Therefore, the colour of copper thin-film depends on the thickness of the coating A purple bauhinia leaf with in a range 20 nm to 40 nm copper layer has the best ability to absorb light while a water cabbage leaf needs in a range 30 to 50 nm of a copper- coated layer to achieve the best light absorption
Figure 3.2 Photo images of (1) a rose periwinkle leaf (2) copper-coated rose periwinkle leaf, (3) a rose leaf (4) copper-coated rose leaf, (5) a water cabbage leaf
(6) copper-coated water cabbage leaf (7) a bauhinia leaf (8) copper-coated bauhinia leaf
Figure 3.3 Scanning Electron Microscope (SEM) images show the surface of all the samples
Figure 3.4 40 nm and 100 nm copper-covered on water cabbage leaves
Figure 3.5 Copper-covered bauhinia leaves with different time sputtering
Figure 3.6 SEM images (a) 30 nm copper-covered, (b) 100 nm copper-covered water cabbage leave (c) 30 nm copper-covered, (d) 100 nm copper-covered purple bauhinia leaves
Figure 3.7 Fourier-transform infrared (FT-IR) spectra (Z;\classes\spectroscopy\all spectra tables for web DOC.)
The results of the analysis of the surface composition of natural leaves indicate the appearance of hydrophobic organic groups Figure 3.7 shows the peaks of Fourier-transform infrared spectra at 2920 and 2850 cm -1 , 1398 and 1318 cm -1 that display the carbon chain The peaks at 1735 cm -1 and 1075 cm -1 confirmed the existence of the ester group However, after covering copper thin-layer on the surface of leaves, the spectra did not show new peaks especially the signals related to copper while the EDS method determines the presence component of copper layer on the surface of the sample
Figure 3.9 and Figure 3.10 display reflection spectra of the copper-covered purple bauhinia leaf and natural purple bauhinia leaf Visible spectrum with the
Bauhinia purpurea Pistia stratiotes Copper-covered Bauhinia 30nm Copper-covered Pistia 30nm
(C-O)(C=O) wavelengths from 380 nm to 760 nm, the natural leaf has two wide peaks at 540 nm and longer than 680 nm The reason is that some pigments such as Chlorophyll a, b and β-Carotene in chloroplast can absorb most of the visible wavelengths in order to use for photosynthesis reaction but except for the wavelengths in the green and far- red Meanwhile, a copper-covered leaf absorbs the visible light completely and reflectivity below 2% Thus, all object samples have low reflectance over the range of 380 to 760 nm and are black It means that the optical properties of coated leaves are decided not by the material of leaves The coated Bauhinia purpurea leaves, which correspond to copper coating 30 nm thick, have the lowest reflectance and the lowest scattering efficiency compare with the other samples of this type of leaf
In the case of Pistia stratiotes leaves, the lowest reflectance and the lowest scattering efficiency are achieved with 40 nm thickness of the copper coating The reflectance spectrum of 30 nm of copper coating and the scattering efficiency spectrum of 30 nm and 40 nm of copper coating show some similar peaks as in uncoated leaves This is a storage problem with Pistia stratiotes samples Few days after fabrication, the black color of Pistia stratiotes gradually disappears However, the lower reflectance and scattering efficiency of Pistia stratiotes in comparing with
Bauhinia purpurea’s suggest that it still has high potential The transmittance of these coated leaves also was determined and it can be neglected So, the absorbance is more than 90% entire visible region It can compare with the other absorber So, we conclude that the copper coated Bauhinia purpurea and Pistia stratiotes leaves are good candidate applying to absorber for visible-near infrared region and promise able for the infrared region
Figure 3.8 Energy dispersion spectrometry (EDS) of the copper-covered leaf
Figure 3.9 Reflection spectra from the copper-covered bauhinia leaf and bare baubinia leaf without coating Scattering intensity spectra from the copper-covered purple bauhinia and purple baubinia leaf with no copper-coating
Figure 3.10 Reflection spectra from the copper-covered water cabbage leaf and bare water cabbage leaf without coating Scattering intensity spectra from the copper-covered water cabbage leaf with no copper-coating
3.3 Prediction the low reflectivity by finite-difference time-domain
Figure 3.11 shows the calculated reflectance (R), transmittance (T) and absorption (A) spectra of flat copper thin films The absorption A is calculated following the relation A = 1 – T – R, because there is no scattering in flat copper surface The calculation was done using the Finite-difference time domain with refractive index data of software The reflectance is more than 20% over the visible spectral range with cases of thickness which are calculated Although reflectance and transmittance vary with the thickness of copper, the absorption is almost independent with changing of thickness In the wavelength range from 380 nm to about 780 nm, the absorption is about 4% It explains the disappearance of the peak in range 500-600 nm in reflectivity and scattering spectrum of sputtered Bauhinia purpurea leaves The absorption is weak (< 4%) in range of wavelength over than
680 nm Meanwhile, the reflectivity and scattering intensity of sputtered leaves are much lower than both non-sputtering leaf and copper thin layer in this range of wavelength, indicating that the Pistia stratiotes leaf and Bauhinia purpurea leaf with copper coating has considerably low reflectivity because of not only the nature of copper layer but also the metallic surface structure With the incident light in the visible region, we predict that it has three phenomena in the surface of coated leaves One is the light absorption by copper The other is the interference due to the reflection inside the nanostructure of leaves The last is localized surface plasmon resonance which causes greater absorption at the plate edges Figure 3.12 show calculation of nanostructured simulation model with 500 nm height with 30nm thick copper coating capable of absorbing light in high visible light Through the model, we realized that the copper coating layer changed the reflection and transmittance that did not show in the model of a copper thin layer or natural leaves
Figure 3.11 Calculated reflectivity R, transmittance T and absorption efficiency A for (a) a flat 30-nm thick gold thin film using FDTD method, (b) The model of two- layer copper with nanostructure pattern
Figure 3.12 Calculated reflectivity R, transmittance T and absorption efficiency A for the model with three layers
After assessing the sample's ability to absorb light, we conducted an investigation of energy conversion in the form of heat Figure 3.13 and Figure 3.14 show the process of changing the temperature of the samples under sunlight condition During the first 100 seconds, the temperature changes rapidly in all samples, then the temperature is maintained at a stable level at a later time In the bio-metamaterial sample, the observed temperature range is the largest and when the maximum temperature is reached the object samples have a temperature higher than the control samples over 10 ºC With purple bauhinia, the highest temperature measured about 54 ºC under luminous intensity condition about 0.0124 W/cm 2 Meanwhile water cabbage, the highest temperature measures about 65 ºC under luminous intensity condition about 0.018 W/cm 2 According to the chart, the temperature of the thin copper layer does not change much and when it reaches the maximum temperature, it only stops at about 29 ºC Control sample with 30 nm coating, the maximum temperature reached about 33 ºC Natural purple bauhinia also had a larger range of temperature changes than the reference sample, the temperature recorded at the peak temperature of 43 ºC Meanwhile, purple bauhinia with 30 nm copper-covered thin layer had a maximum temperature near 55 ºC and the one with 100 nm copper-coated thin layer reached about 46 ºC This again demonstrates that the effect of the copper layer on the surface of the sample depends greatly on the thickness of the sputtering layer In compare with two research group [21] and [12], the maximum temperature of the biomaterial sample is about two times the maximum temperature of the two object samples in the previous study In addition, the metamaterial sample has a rapid heating capacity, which takes only 100 seconds to reach the maximum temperature while the two samples of the previous study object took about 1000s to reach the saturation threshold Thus, bio-metamaterials show the promising potential use in the solar steam-generation devices With good stability and heating ability, this material sample can be selected as a cheap and environmentally friendly material In addition, with a higher heating capacity than the control sample and a thin copper layer, the metamaterial sample exhibited the property of interacting with light waves at least in the visible region After that, they transformed that energy in the form of heat Therefore, we can use bio-metamaterial in many other applications
Prediction the low reflectivity by finite-difference time-domain
Figure 3.11 shows the calculated reflectance (R), transmittance (T) and absorption (A) spectra of flat copper thin films The absorption A is calculated following the relation A = 1 – T – R, because there is no scattering in flat copper surface The calculation was done using the Finite-difference time domain with refractive index data of software The reflectance is more than 20% over the visible spectral range with cases of thickness which are calculated Although reflectance and transmittance vary with the thickness of copper, the absorption is almost independent with changing of thickness In the wavelength range from 380 nm to about 780 nm, the absorption is about 4% It explains the disappearance of the peak in range 500-600 nm in reflectivity and scattering spectrum of sputtered Bauhinia purpurea leaves The absorption is weak (< 4%) in range of wavelength over than
680 nm Meanwhile, the reflectivity and scattering intensity of sputtered leaves are much lower than both non-sputtering leaf and copper thin layer in this range of wavelength, indicating that the Pistia stratiotes leaf and Bauhinia purpurea leaf with copper coating has considerably low reflectivity because of not only the nature of copper layer but also the metallic surface structure With the incident light in the visible region, we predict that it has three phenomena in the surface of coated leaves One is the light absorption by copper The other is the interference due to the reflection inside the nanostructure of leaves The last is localized surface plasmon resonance which causes greater absorption at the plate edges Figure 3.12 show calculation of nanostructured simulation model with 500 nm height with 30nm thick copper coating capable of absorbing light in high visible light Through the model, we realized that the copper coating layer changed the reflection and transmittance that did not show in the model of a copper thin layer or natural leaves
Figure 3.11 Calculated reflectivity R, transmittance T and absorption efficiency A for (a) a flat 30-nm thick gold thin film using FDTD method, (b) The model of two- layer copper with nanostructure pattern
Figure 3.12 Calculated reflectivity R, transmittance T and absorption efficiency A for the model with three layers.
Efficient solar absorbers
After assessing the sample's ability to absorb light, we conducted an investigation of energy conversion in the form of heat Figure 3.13 and Figure 3.14 show the process of changing the temperature of the samples under sunlight condition During the first 100 seconds, the temperature changes rapidly in all samples, then the temperature is maintained at a stable level at a later time In the bio-metamaterial sample, the observed temperature range is the largest and when the maximum temperature is reached the object samples have a temperature higher than the control samples over 10 ºC With purple bauhinia, the highest temperature measured about 54 ºC under luminous intensity condition about 0.0124 W/cm 2 Meanwhile water cabbage, the highest temperature measures about 65 ºC under luminous intensity condition about 0.018 W/cm 2 According to the chart, the temperature of the thin copper layer does not change much and when it reaches the maximum temperature, it only stops at about 29 ºC Control sample with 30 nm coating, the maximum temperature reached about 33 ºC Natural purple bauhinia also had a larger range of temperature changes than the reference sample, the temperature recorded at the peak temperature of 43 ºC Meanwhile, purple bauhinia with 30 nm copper-covered thin layer had a maximum temperature near 55 ºC and the one with 100 nm copper-coated thin layer reached about 46 ºC This again demonstrates that the effect of the copper layer on the surface of the sample depends greatly on the thickness of the sputtering layer In compare with two research group [21] and [12], the maximum temperature of the biomaterial sample is about two times the maximum temperature of the two object samples in the previous study In addition, the metamaterial sample has a rapid heating capacity, which takes only 100 seconds to reach the maximum temperature while the two samples of the previous study object took about 1000s to reach the saturation threshold Thus, bio-metamaterials show the promising potential use in the solar steam-generation devices With good stability and heating ability, this material sample can be selected as a cheap and environmentally friendly material In addition, with a higher heating capacity than the control sample and a thin copper layer, the metamaterial sample exhibited the property of interacting with light waves at least in the visible region After that, they transformed that energy in the form of heat Therefore, we can use bio-metamaterial in many other applications
Figure 3.13 Temperatures of the samples over irradiation time MB0: Bauhinia leaf, MB30: 30 nm copper-coated bauhinia leaf, MB100: 100 nm copper-coated bauhinia leaf, Ct30: 30 nm copper-coated rose periwinkle leaf, Cu100: 100 nm thin- film of copper
In the case with water cabbage, due to adapting to underwater life, their leaves contain air chambers This helps the natural leaves to be able to regulate its own temperature The temperature of the natural water cabbage sample recorded at a stable time is 32 ºC Meanwhile, the temperature of the object sample sputtered with a copper layer of 40 nm thickness was recorded as 65 ºC and higher than the temperature of natural samples by 30 ºC In addition, we also conducted experiments to compare the temperature of two object samples under the same conditions Figure 3.17 and Figure 3.18 shows the process of changing the temperature of two object samples under the condition of luminous intensity is 0.014 – 0.015 W/cm 2 The temperature of the water cabbage with 40 nm copper- covered thin layer sample is about 62 ºC higher than the purple bauhinia sample with the temperature about 52 ºC under the same conditions Maybe due to the density of the wax on the water cabbage leaves’ surface combined with microstructures that increase the surface area, the copper-coated water cabbage is possible to absorb higher than copper-coated purple bauhinia on the same area
Thus, the temperature on 40 nm copper-coated water cabbage is higher than the temperature on 30 nm copper-coated purple bauhinia
MB0 MB30 MB100 Control Cu100
Figure 3.14 Maximum temperatures of the samples MB0: natural Bauhinia purpurea leaf MB30: 30 nm copper-coated Bauhinia purpurea leaf MB100: 100 nm copper-coated Bauhinia purpurea leaf Control: 30 nm copper-coated
Catharanthus roseus leaf Cu100: 100 nm Copper thin-layer on glass’s surface
Through three experiments investigating the temperature change of samples under sunlight conditions, we can evaluate the ability to convert light energy into thermal form of bio-metamaterials The temperature of bio-metamaterial always is higher than the temperature of natural leaves at least 10 ºC Therefore, they can be used as a sunlight absorbing material In the next experiment, I will use this bio- metamaterials in a system that evaporates water based on sunlight energy
Figure 3.15 Temperatures of the samples over irradiation time BC0: water cabbage leaf, MB30: 30 nm copper-coated water cabbage leaf, MB100: 100 nm copper-coated water cabbage leaf, Ct30: 30 nm copper-coated rose periwinkle leaf, Cu100: 100 nm thin-film of copper
Cu100 BC40 BC100 BC0 Control
Figure 3.16 Maximum temperatures of the samples BC0: natural Pistia stratiotes leaf BC30: 30 nm copper-coated Pistia stratiotes leaf BC110: 100 nm copper- coated Pistia stratiotes leaf Control: 40 nm copper-coated Catharanthus roseus leaf Cu100: 100 nm Copper thin-layer on glass’s surface
Figure 3.17 Temperatures of the samples over irradiation time MB30: 30 nm
Figure 3.18 Maximum temperatures of the samples MB30: 30 nm copper-coated
Bauhinia purpurea leaf BC40: 40 nm copper-coated Pistia stratiotes leaf
Figure 3.19 shows the process of dehydration of the evaporating system during the investigation in one hour under the luminous intensity is 0.1 kW/m 2 After a one-hour investigation, the amount of water lost in the sample using the metamaterial-based evaporation system is more than the control sample
Specifically, the control sample lost 0.8 kg m -2 h -1 while the object sample lost 1.5 kg m -2 h -1 The efficiency of solar steam-generation system compared to the free evaporation rate of water is more than 75% in the same condition The results between use of purple bauhinia with 30 nm copper-coated and water cabbage with
40 nm copper-coated are near the same In compare with solar steam-generation devices based on the carbonized mushroom, our object sample and carbonized mushroom in one hour lose the same amount of water, about 1.5 kg m -2 h -1 [21]
However, in this experiment, the authors used normal solar illumination with luminous intensity 1kW m -2 , more than 10 times in our conditions
Figure 3.19 Mass change of water under the sun
Our research team found two suitable objects: Bauhinia purpurea and Pistia stratiotes for the aim of metamaterial fabrication The nanostructures of these two leaf sheets are periodic The thickness of the nanostructure layer on these two types of leaves is predicted from 80-100 nm We also showed the ability to absorb light well when leaf samples are coated with copper metal with a thickness of 20-80 nm
Absorption capacity predicts about 90% of light entering the structure A purple bauhinia leaf with about 30 nm copper layer has the best ability to absorb light while a water cabbage leaf needs about 40 nm of a copper-coated layer to achieve the best light absorption After sputtering a copper layer, two object leaves are capable of converting light energy into heat and are potential in the solar steam- generation system
1 Our research team will optimize the solar steam-generation device based on bio-metamaterials to improve system performance
2 In addition, our research team would like to fabricate the imitation of the models based on the nanostructure in Bauhinia purpurea and Pistia stratiotes leaves’ surface in order to create a material with the same properties but more stable than natural object leaf samples
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