In this study, the doped V ions migrated from the bulk lattice to the TiO2 surface above 300 °C, thus increasing their surface concentration by 7.6 times as the temperature increased to 600 °C. The concentrated V ions at the surface induce V2O5 crystals formed in the TiO2
surface region. The incorporation of V ions into the TiO2 lattice accelerated the phase transformation and resulted in a lower phase transit temperature of 500 °C. The bandgap energy of the TiO2 was greatly reduced from 3.1-3.3 eV to 1.6 eV when 1.00 at.% V ions are doped. The photocatalytic activity of the TiO2 samples for RhB degradation increased with calcination temperatures because of improved crystallinity. Doping trace amounts of V ions enhanced the oxidative activity. However, high concentrations of V ions in the bulk lattice induced severe charge recombination and reduce numbers of effective charge carriers. The pure TiO2 calcined at 600 °C, which comprised of anatase and rutile phase, showed the highest oxidation efficiency for RhB. However, the TiO2 sample calined at 500 °C exhibited the highest activity for CO2 reduction. The high concentrations of V-doped TiO2
increased the yield of CH4 with increased calcination temperature and reached the highest yield of CH4 at 500 °C. Methane yield by high concentrations of V-doped TiO2 was 1.68 times higher than the pure TiO2. The QE of CH4 was reached to 0.66%. However, the initial yield of CH4 was 0.89 times lower than the pure TiO2. This study showed that the V3+ and V4+ ions in the bulk of TiO2 suppress the ability of the electron transfer efficiency because V3+ and V4+ ions acted as recombination centers. However, V2O5, formed by increasing calcination temperature, on TiO2 was helpful to preserve methane from reoxidation because it decreased the ability of oxidation of TiO2. The formation of reduced intermediates was observed to occupy the surface to prevent the following interactions of CO2 and water with the surface charge carries.
68
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Appendix A. XPS Analysis
(a)
1000 800 600 400 200 0
-500 0 500 1000 1500 2000 2500 3000 3500 4000 4500
Intensity
eV 1.00 at.% V-TiO2-200 oC
(b)
470 465 460 455
0 2000 4000 6000
Intensity
eV 1.00 at.% V-TiO2-200 oC
Appendix A-1 The XPS spectra of the 1.00 at.% V-doped TiO2 at 200 °C. (a) survey and (b) Ti (2p).
77
(a)
1000 800 600 400 200 0
-500 0 500 1000 1500 2000 2500 3000 3500 4000 4500
Intensity
eV 1.00 at.% V-TiO2-700 oC
(b)
470 465 460 455
0 2000 4000 6000
Intensity
eV 1.00 at.% V-TiO2-700 oC
Appendix A-2 The XPS spectra of the 1.00 at.% V-doped TiO2 at 700 °C. (a) survey and (b) Ti (2p).
78
(a)
1000 800 600 400 200 0
-500 0 500 1000 1500 2000 2500 3000 3500 4000 4500
4.00 at.% V-TiO2-200 oC
intensity
eV
(b)
470 465 460 455
0 2000 4000
Intensity
eV 4.00 at.% V-TiO
2-200 oC
Appendix A-3 The XPS spectra of the 4.00 at.% V-doped TiO2 at 200 °C. (a) survey and (b) Ti (2p).
79
Appendix B. EPR Analysis
2.1 2.0 1.9
600 oC 500 oC
300 oC 400 oC
200 oC
Intensity
g-factor
Appendix B-1 EPR spectra of pure TiO2 at different calcination temperature at 77K in the dark.
80
Appendix C. Degradation of RhB
(a)
Appendix C-1 The degradation of 0.01 mM RhB by (a)the pure TiO2 and (b)the 0.01 at.%
TiO2.
81
0 5 10 15 20 25 30
0.5 0.6 0.7 0.8 0.9 1.0
C/C 0
time(min) 900 oC
700 oC 200 oC 300 oC 600 oC 400 oC 500 oC
Appendix C-2 The degradation of 0.01 mM RhB by 1.00 at.% TiO2.
82
Appendix D. Calibration Curve
Appendix D-1 The calibration curve of CH4.