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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
20
to receive and transfer electrons, then stimulates the surface redox reactions at their surface [13][28].
Figure12. Photodegradation rate of Methyl blue under UV light in the presence of Co-doped ZnO/C-dots and Mn-doped ZnO/C-dots heterostructure with 4 layers of C-dots
The electronic interaction by chelation between ZnO and C-dots in ZnO/C-dots heterostructure was found to be essential to enhance the photodegradation rate of methyl blue solution. It is believed that the good photocatalytic activity of metal doped ZnO/C-dots especially Co-doped ZnO/C-dots sample under UV light irradiation originates from their outstanding unique property which stimulates the separation process of photogenerated electron- hole pairs based upon C-dots. This result is good agreement with the data analysis from optical properties (UV-Vis absorption and PL spectra) of the sample. The absorption spectrum from UV-Vis analysis reveals that attachment of the C-dots on the surface of ZnO/C-dots heterostructure slightly decreases the energy bandgap of ZnO. Meanwhile, PL spectra indicates that doping with metal element and the incorporation of C-dots particles increase the crystal defects in ZnO nanorods structure, especially the oxygen vacancies in ZnO surface. This crystal
Time
(Minute)5 10 20 25 30 35 40 45 50
4 L Mn Co
MB
55 60 65 70
15 90
Degradati on ( % )
0 0 10 30 40 50 60 70 80
20
21
defect will act as electron reservoir, which could prevent the recombination process of photogenerated charges and extend the electron lifetime which will play as an important role in the degradation process of methyl blue. In other words, increasing the charge separation efficiency of its electron-hole pairs significantly enhances the visible light photocatalytic activity of the sample. C-dots are believed as a light source for the excitation of electrons in ZnO, which then act as reservoir of electrons after excitation.
CONCLUSIONS
We have successfully synthesized carbon nanodots (C-dots) via a facile and green chemistry technique through simple carbonization of cassava peels at low temperature. The C- dots dispersed solution were coated onto ZnO nanorods surface to form ZnO/C-dots heterostructure and then used as good photocatalyts to degrade methyl blue under UV light illumination. The experimental results reveals that the incorporation of C-dots particles and doping with element Co or Mn into ZnO lattice modifies their morphology, surface area and crystallinity. The PL analysis reveals that doping with Co or Mn element into ZnO nanorods lattice and then coated it with 4 layers of C-dots increases the crystal defects in ZnO nanorods structure, especially the oxygen vacancies. The photodegradation rate of sample was found to depend on the layers of C-dots. The photocatalytic activity of Co-doped ZnO/C-dots heterostructure coated with 4 layers of C-dots particles has the highest efficiency in degradation of methyl blue solution
ACKNOWLEDGEMENTS
This research was financially supported by Hibah Penelitian Unggulan Perguruan Tinggi 2019 (No. 21/AKM/PNT/ 2019) from the Ministry of Research, Technology and Higher Education Indonesia.
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