002
Int ens it y ( a.u )
ZnO
101 102
30
010 110 103
80 ZnO/C-dots 4 Layer ZnO/C-dots 6 Layer
ZnO/C-dots 2 Layer Co-Doped ZnO/C-dot Mn-Doped ZnO/C-dot
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Table 1 Texture Coefficient (TC) Value of ZnO nanorods composite ZnO/C-dots with various C-dots layer
Amount of C-dots on ZnO nanorods surface (layers)
Texture Coefficient (TC)
(100) (002) (101) (102) (103) (112)
0 0.492 1.566 0.301 0.497 0.298 0.525
2 0.687 1.491 0.674 1.113 0.667 1.577
4 0.269 2.801 0.385 1.234 0.677 0.477
6 0.303 1.573 0.477 1.638 0.932 1.072
Co doped ZnO 0.048 2.648 0.928 0.056 0.815 0.166
Mn doped ZnO 0.349 2.765 0.456 0.234 0.765 0.198
According to Table 1, it is clearly indicated that the (002) crystalline plane for pure ZnO has TC value greater than 1. This result proves that ZnO nanorods without addition of C-dots particles growing more uniform on (002) crystal plane. Meanwhile TC value of all the peaks from ZnO/C-dots with addition of 4 and 6 layer of C-dots particle are varied; it reveals the preferred growth orientation of the sample is not particular towards c-axis direction since the other crystal plane having TC value greater than. Furthermore, ZnO/C-dots heterostructure with 2 layer of C-dots dispersed solution exhibits the lowest TC value of the (002) crystalline plane.
The TC value for preferred orientation of (002) crystal plane was then used to further calculate the lattice parameters such as d spacing, crystallite size and FWHM. The parameters are presented in Table 2.
Table 2. XRD data of ZnO/C-dots preferred (0 0 2) crystal plane
From Table 2, it can be observed that the measured lattice parameters for all ZnO samples are similar, namely, a = 3.25 Å and c = 5.20 Å. These results are in good agreement
Amount of C-dots on ZnO nanorods surface (layers)
(2 theta) FWHM d-spacing ( (Å)
Crystallite
size (Å) a=b (Å) c (Å) Volume (Å^3) 0 34.029 0.0836 2.6345 64.869 3.252 5.204 47.67 2 33.990 0.1547 2.6362 43.803 3.300 5.181 48.86 4 34.011 0.2344 2.6339 23.049 3.300 5.181 48.86 6 34.057 0.0086 2.6324 67.024 3.256 5.256 48.26 Co 33.990 0.1338 2.6375 37.101 0.300 5.181 48.86 Mn 33.994 0.1020 2.6351 48.971 3.265 5.219 48.18
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with the standard data for wurtzite ZnO (a = 3.249 Å, c = 5.205 Å). The heterostructure with 6 layers of C-dots possesses the highest average crystallite size.
The FESEM micrographs showing the morphology of ZnO and ZnO/C-dots heterostructure with various C-dots layer are shown in Figure 8. The average grain size of all ZnO nanorods sample was in the range of 109-184 nm, while the density of the sample is 137 rods/mm2. The morphology of all samples was found to be uniform throughout the substrate. As can be observed from Figure 8, there are considerable differences between surface morphology and density of ZnO nanorods with and without addition of C-dots. ZnO without addition of C-dot dispersed solution exhibits high density of hexagonal nanorods and grown almost perpendicular to the substrate. Similar kind of morphology was also clearly observed on the ZnO/C-dots composite that was prepared with addition of C-dots dispersed solution. However, the growth orientation of sample is random or not exactly perpendicular to the substrate. The Brownian motion effect from C-dots dispersed solution as result of slow drying process when deposited C- dots into ZnO nanorods surface. Moreover, thin layer of C-dots dispersed solution was also found to cover the entire ZnO nanorods. This result is in accordance with the XRD pattern in Figure 4 which exhibits that the TC value of (002) plane decreases with the increase of C-dots dispersed solution loaded onto ZnO nanorods surface which means the growth orientation of the sample is random.
In the other hand, as the amount of C-dots dispersed solution increases, the shape of particle was found significantly changed due to many horizontal and vertical nanorods were accumulated, and then aligned themselves into aggregates forming nanoflowers like morphology, especially for the sample loaded with 4 and 6 layer of C-dots dispersed solution. However, it is obvious to notice that those non-uniform nanorods are coupled with two or more particles to form interlinked clusters and then nanoflowers like become bigger once the sample loaded with 6 layers of C-dots dispersed solution. Furthermore, the grain size of ZnO sample becomes inhomogeneous and particles distribution is not packed since there are remain spaces due to the formation of central-radial pore structure between other nanoflowers. The ZnO nanorods are also found to be almost indiscernible due to the C-dots particles distribution and covers ZnO nanorods surface.
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Figure 8. FESEM micrograph of ZnO nanorods and heterostructure ZnO/C-dots with various C- dots layer; (a) Pure ZnO (b) 2 layers (c) 4 layers and (d) 6 layers
The energy dispersive spectroscopy (EDS) spectra of ZnO nanorods and heterostructure ZnO/C-dots sample with various C-dots layers are shown in Figure 6. The EDX pattern of ZnO that was prepared without addition of C-dots dispersed solution shows the existence of the element of Zn and O. Meanwhile, the EDX pattern of ZnO/C-dots nanocomposite detects the existence of Zn, O and C element on the whole surface, confirming the presence of these elements in the sample. No other impurities are detected in ZnO/C-dots sample. It also shows that sample contains mixed carbon materials and ZnO nanorods.
1 μm
a
c
b
d
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Figure 9. Energy Dispersive Spectroscopy spectrum of ZnO nanorods and heterostructure ZnO/C-dots sample with various C-dots layer
The FESEM images and EDS spectra in Figure 10 shows the morphology and elemental composition of heterostructure Co-doped ZnO/C-dots and Mn-doped ZnO/C-dots. The samples have morphology of hexagonal nanorods. The average grain size of Co-doped ZnO nanorods sample is in the range of 47.7-235 nm and the density of the particle is 187 rods/mm2. Meanwhile, the particle distribution of Mn-doped ZnO/C-dots is found to be more uniform compared to Co-doped ZnO nanorods with the particle average size and distribution in the range of 87 nm-145 nm and density the sample is 167 rods/mm2. The EDX pattern of heterostructure Co-doped ZnO/C-dots nanocomposite detects the existence of Zn, O, C and Co element on the whole surface, confirming the presence of these elements in the sample. Meanwhile, the heterostructure Co-doped ZnO/C-dots nanocomposite detects the presence of Zn, O, C and Mn element. No other impurities are detected from ZnO/C-dots sample. It also shows that sample contains mixed carbon materials and ZnO nanorods. (BERULANG)
Element Weight % Atomic %
Zn L 67.58 66.22
O K 32.42 33.78
Element Weight
% Atomic % Zn L 96.8 85.30 O K 0.48 1.71 C K 2.71 12.99
Element Weight % Atomic % Zn L 98.14 91.09
O K 0.4 1.53
C K 1.46 7.38
Element Weight
% Atomic % Zn L 95.48 80.03
O K 0.57 1.96
C K 3.95 18.01
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Figure 10. (a) FESEM micrograph and (b) Energy Dispersive Spectroscopy spectra of heterostructure Mn doped ZnO/C-dots and Co doped ZnO/C-dots with 4 layers of C-dots
The photocatalytic performance of all samples is shown in Figure 11. The methyl blue solution in the absence and presence of ZnO sample was kept in dark condition for 30 minute to allow ZnO sample absorb the methyl blue molecule. The result clearly shows that the photocatalytic degradation of all the ZnO/C-dots heterostructures is considerably higher compared to pristine ZnO nanorods. Particularly, the ZnO/C-dots heterostructures sample with 4 layers of C-dots exhibits the highest activity due to 74.98% of methyl blue solution was found to be degraded after illumination with UV light for 70 minutes.
co
1 μm
Co Mn
Element Weight % Atomic % Zn L 69.49 33.95
O K 22.13 44.17
C K 8.19 21.77
Co K 0.19 0.10
Element Weight % Atomic % Zn L 67.20 31.49 O K 23.19 44.40
C K 9.40 23.99
Mn K 0.21 0.12
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Figure11. Photodegradation rate of methyl blue under UV light in the presence of heterostructure ZnO/C-dots with various layers of C-dots
The photocatalytic activities of ZnO nanorods and heterostructure ZnO/C-dots samples with various number C-dots layers provides the information of the best content of C-dots to prepare the ZnO/C-dots heterostructure sample with good photocatalytic property to degrade organic pollutant. The degradation rates of the ZnO/C-dots, Mn-doped ZnO/C-dots and Co- doped ZnO/C-dots with 4 layers of C-dots are 75.02, 80.84 and 81.13%, respectively. The enhancement of photocatalytic activity of Co-doped ZnO/C-dots is attributed to the incorporation of 4 layers C-dots created favorable bonding with Co-doped ZnO nanorods, which subsequently encourages the efficient transfer of photogenerated charges at the surface of ZnO sample. As reported in the previous study, the conjugated p structure of C-dots was reported to adsorb the organic pollutant since this functional group in carbon atom would facilitate easy way to bind with other chemically reactive groups, inducing the surface passivation and functionalization in catalyst to reduce various organic, inorganic, polymeric or biological materials in pollutant solution [27]. In addition, C-dots act as reservoir of electron to give fast response of the sample
Degradati on ( % )
0