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Besides, the NH3-tratment remarkably modifies the surface chemical states and, consequently, the optical properties of the TiO2. For example, the color of the rutile’s deposit, which is completely white in the case of the initial TiO2 sample (Figure 6.28a) and it turns bluish on the deposit after NH3-treatment at 500oC (Figure 6.28b), due to defect formation on the rutile215,216 caused by the reducing effects produced by the presence of ammonia. The annealed sample at 700oC (Figure 6.28c) is changed to black due to a deeper defect formation. Finally, the case of 900oC (Figure 6.28d) turns to a brown color as a result of the titanium nitride (TiN) formation. The following reactions explain the annealing processes that the electrodes are subjected to in presence of ammonia when reaching certain temperature217: 2NH3 TiO2 + H2 TiO + NH3 TiO2 + NH3 2TiO + N2 N2 + 3H2 TiO + H2O TiN + H3O+ TiON + H3O+ 2TiON (6.3) (6.4) (6.5) (6.6) (6.7) Initially, when ammonia gas is heated it is decomposed to hydrogen and nitrogen gas (6.3). Hydrogen partially reduces the titanium dioxide phase to titanium monoxide (6.4) which in the presence of the acidic electrolyte will form the oxyhydroxide. However, in order to produce titanium nitride, the heating temperature should reach at least 800oC. Below that temperature reaction (6.5) does not take place, therefore TiN is not formed. Intermediate reactions (6.6) and (6.7) takes place from 700oC to form the oxynitride phase218. In order to confirm these findings the UV-vis spectra, shown in Figure 6.28e, is done for the as-prepared electrodes. Considering the CF@TiO2 electrode as the initial spectra, there is broad absorbance decay, from 400 to 850 nm wavelength, corresponding with the visible and Near-IR region. It increases to maximum absorbance when the sample is annealed in ammonia at 500oC. When annealed at 700oC, the absorbance is lowered around 5% relative to the CF@TiO2 N500 sample, from 400 to around 700 nm. Finally, the absorption after nitride formation appears to be stronger in the visible range (400-550 nm) for the sample annealed at 900oC in presence of ammonia (CF@TiO2 N900). Afterwards has a linear decreasing absorbance, from 550 to 850 nm, as the color of our electrode’s deposit turns from black to dark brown219,220. 115PDF Image | Redox Flow Batteries Vanadium to Earth Quinones
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