3.1.1.1. SEM image
Figure 3.1: Porous Structure and Tube Structure of Fresh Pomelo Peel (left) and Carbonized Pomelo Peel
Figure 3.1 describes the morphology of fresh pomelo peel and the CPP. The annealing condition of this CPP was: 4000C for 2 hours. On a large scale, both materials exhibit porous and tube structures. Tube structure is surrounded by porous structures and is a minority. This proves that, after participating in the carbonization process, the structure of the material remains unchanged. Moreover, the porous structure, and especially the tube structure, can perform well as a water channel.
Water in the water supply will be brought to the surface of the absorption layer through capillary force. The water can then absorb heat easily and evaporate.
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Figure 3.2: Changes in structural dimensions before and after carbonization process.
(a), (b), (c): Fresh Pomelo Peel; (d), (e), (f): Carbonized Pomelo Peel.
Figure 3.2 shows the dimensions of the tube structure and the porous structure of the material before and after the carbonization process. Fresh Pomelo Peel has a porous structure with a hole diameter of about 300- 400 àm. After carbonization, although the porous structure remains the same, the diameter of the holes is significantly reduced. Most holes in the porous structure range in size from 75 to 150 àm. Similarly, the diameter of the tube mouth in the tube structure is also reduced. This is explained by the evaporation of water in the material structure during carbonization process. At high temperatures, the material loses water, causing all structures to shrink from 2 to 4 times. As the size of the internal structure of the material becomes smaller, it is beneficial to transport water. The higher capillary force, the better water transport system, also the better water retention ability.
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Figure 3.3: Water Capacity Ability of Carbonized Pomelo Peel
The water capacity of carbonized pomelo peel is shown in Figure 3.3. By assessing the mass ratio of the material before and after hydration, the water capacity ability of the material is shown as the equation above. The variable y is mass of the hydrated material and x is mass of the original material. It implies that the amount of water that the material can hold can be more than 6 times its own weight. In addition, water transport speed is also assessed through the change of average weight over time of exposure to water of the material. The average water transport speed reaches the speed of 0.1 kg.m-2.h-1. The above proves that the material is carbonized pomelo peel capable of performing tasks as well as a water channel system.
18 3.1.1.2. XRD, FTIR, and Raman spectra
Figure 3.4: XRD spectrum of Carbonized Pomelo Peel
Figure 3.4 shows X-ray diffraction (XRD) of the carbonized Pomelo Peel. It exhibits a weak and broad signal at the 2-theta degrees of 230, that describes the typical reflection of graphite-like structure [7]. This is understandable, because after carbonization process, most of the material has been carbonized, and the main constituent material is carbon. Moreover, that are also confirmed by the Raman spectrum of Carbonized Pomelo Peel (figure 3.5).
Figure 3.5: Raman Spectrum of Carbonized Pomelo Peel
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The Raman spectrum exhibits two prominent peaks at 1350 cm-1 (D) and 1590 cm-1 (G), which represented the graphitic lattice vibration mode and disorder in the graphitic structure of the biochar [24]. Specifically, peak D refers to disordered sp2- hybridized carbon atoms with vacancies and impurities, while peak G causes from the stretching of sp2 atomic pairs in the carbon atom ring or carbon chain. It can be assumed that the carbon atom is bonded to a sp2 hybridized covalent bond, while electrons which are not involved in hybridization form a π bond. However, the intensity of G-band is much higher than that of D-band. It implies that the amorphous carbon plays an important role in the formation of carbonized pomelo peel.
Figure 3.6: FTIR spectra of Fresh Pomelo Peel and Carbonized Pomelo Peel Another method to examine the physical characteristic of the material is FTIR.
Figure 3.6 show FTIR spectra of Fresh Pomelo Peel and Carbonized Pomelo Peel in the region of wavenumber from 400 cm-1 to 4000 cm-1. Fresh Pomelo Peel FTIR spectrum reveals vibration peaks of some functional groups of the carbonized pomelo peel. Distinguishing vibration peaks of C-O bond, C=C bond, C=O bond, C-H bond and (N-H)/(O-H) bond are located at around 1000 cm-1, 1500 cm-1, 1800
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cm-1, 2900 cm-1, and 3300 cm-1, respectively. On the other hand, the FTIR of Carbonized Pomelo Peel saw the significantly reduce of the intensity of (N-H)/(O- H), (C-H), (C=O), (C-O) due to carbonization process. From the results of XRD, FTIR, Raman, the elemental composition and structure of the carbonized pomelo peel are clearly shown.
In summary, after heating the sample at high temperature for a defined time, the CPP sample shrinks but still retains its structure. After carbonization process, the main component of the CPP is carbon, the carbon-carbon bonding is predominant.
With carbon as the main ingredient, the CPP material promises to exhibit a strong sunlight absorption. Section 3.1.2 will study the light absorption properties of CPP material in the ultra violet, visible and infrared regions.