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Suyitno, et al. /International Energy Journal 18 (2018) 53 – 60 3.2 Cyclic Voltammetry Test of Curcumin-Dyes Figure 6 shows the cyclic voltammetry (CV) curve for curcumin-dyes. The shape of CV curve for curcumin- dyes when subjected to lights and heats for 0, 100, and 200 hours are similar. CV test measures the oxidation and reduction energy level of curcumin-dyes corresponding to HOMO and LUMO. HOMO is the highest occupied molecular orbital, while LUMO is the lowest unoccupied molecular orbital. Electrons from the dyes can be efficiently injected to the TiO2 semiconductors when the LUMO energy of dyes is more negative than the conduction band (CB) of TiO2 [26]; [27]. The electron regeneration is effective when the HOMO energy of dyes is more positive than the potential energy of the redox electrolyte as shown schematically in Figure 7. The HOMO and LUMO 57 energies of dyes were determined from the respective oxidation and reduction potential value compared to ferrocene using a reference electrode of Ag/AgCl. The LUMO, HOMO, and band-gap energy of curcumin-dyes are -3.62 eV, -5.55 eV, and 1.93 eV, respectively. When subjected to the treatment of light and heat at 50°C for 100 and 200 h exposures, the LUMO decreases and the HOMO of the dyes increases. It causes the ability of the dyes to inject and regenerate electrons decreases. The auto-oxidative degradation process of curcumin dyes introduces the degradation of the C=C and C=O functional groups and the rise of the new functional groups such as C-O or C-O-C. Because the ionic charge of O is higher than that of C, then the LUMO levels of curcumin were relegated to decline. Fig. 6. Cyclic voltammetry curve for curcumin-dyes subjected to lights and heats for 0, 100, and 200 hours. -3 -3.62 -3.5 -4 -4.5 -5 -5.5 -6 -6.5 -7 -4.14 -4.22 Injection -3.76 -3.90 -4.12 Excitation -5.01 -4.50 Photo Regeneration -4.80 -5.12 -5.01 -5.28 -5.22 -5.55 0 h 100 h 200 h LUMO (eV) HOMO (eV) Fig. 7. HOMO-LUMO energy of curcumin-dyes. www.rericjournal.ait.ac.th TiO2 Curcumin Energy Level (eV) Curcumin Curcumin ElectrolitePDF Image | Curcumin-Dye-Sensitized Solar Cells
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