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4.3 Transition metal doped titania
4.3.1 Effect of transition metal doping
0 20 40 60 80 100 120 140 160 0.000
0.005 0.010 0.015 0.020 0.025
Pore volume (cc/gm)
Pore diameter (nm)
70
Fig.4.19 shows the XRD patterns of M-TiO2 catalysts. Anatase phase was observed to be dominant in all samples with peaks at 2θ = 25.3°, 36.9°, 48.0°, 53.9°, 55.1° and 62.1° (JCPDS No: 21-1272-TiO2-Anatase). No characteristic peaks corresponding to transition metal oxides were detected in any of the samples suggesting that the metal oxides were in a highly dispersed state. The lattice parameters and lattice distortion values of the samples are given in Table 4.7.
Table 4.7. Surface area, pore volume and lattice parameters of M-TiO2, M-1N- TiO2, Cu-xN-TiO2 (M = Cu, Fe, Ni; N = nitrogen; x = 0.5, 1, 1.5 at.%)
The lattice distortion values correspond to the maximum strain observed in the crystal lattice due to incorporation of dopant metals. The origin of lattice distortion for doped TiO2 can be attributed to the difference between the ionic radii of dopant ions and Ti4+
ion. The effective ionic radii of used transition metals ions are; Cu2+: 0.073 nm, Fe3+: 0.064 nm, Ni2+: 0.072 nm and Ti4+: 0.068 nm (Hsiang and Liu, 2009; Ganesh et al., 2012). The higher lattice distortion as observed for Cu-TiO2 and Ni-TiO2 may be attributed to the higher ionic radius of copper and nickel ions as compared to that of
Catalyst
BET surface area (m2/g)
Pore volume
(cc/g)
Lattice parameters
Anatase Lattice
distortion a (Å) (101) c (Å) (004)
Cu-TiO2 41 0.17 3.8390 9.5440 0.317
Fe-TiO2 20 0.18 3.8027 9.5685 0.248
Ni-TiO2 25 0.15 3.7885 9.5197 0.309
Cu-1N-TiO2 49 0.23 3.8747 9.5416 0.486
Fe-1N-TiO2 30 0.60 3.8036 9.5807 0.415
Ni-1N-TiO2 46 0.53 3.8375 9.5685 0.497
Cu-0.5N-TiO2 54 0.70 3.8580 9.5220 0.432
Cu-1.5N-TiO2 26 0.14 3.8876 9.6178 0.743
TiO2-Anatase 84 0.97 3.7852 9.5139 -
20 30 40 50 60 70 80 0
200 4000 200 4000 200 400
Intensity (a.u)
2 (Degree)
Cu-TiO2
Fe-TiO2 Rutile
Ni-TiO2
Anatase
titanium ions. The Cu2+ and Ni2+ ions are more likely to reside in the interstitial sites of the TiO2 lattice due their larger ionic radii compared to that of Ti4+. The value of lattice parameters of the M-TiO2 catalysts were higher compared to that of TiO2 due to the distortion of the lattice.
Fig.4.19. XRD patterns of M-TiO2 (M = Cu, Fe and Ni)
The FESEM images of M-TiO2 samples are shown in Fig.4.20 (a, c and e). All the catalysts were in agglomerated state. For Cu doped titania, the particles were less agglomerated and had spherical shape. For the Fe doped TiO2, the agglomeration seemed to be highest.
72
Fig.4.20. FESEM images of (a) Cu-TiO2 (b) Cu-1N-TiO2 (c) Fe-TiO2 (d) Fe-1N- TiO2 (e) Ni-TiO2 and (f) Ni-1N-TiO2
The optical absorption properties of M-TiO2 and TiO2 samples are shown in Fig. 4.21 (A). The M-TiO2 samples exhibited significant absorption in visible light region compared to TiO2 sample. The band gap energy of the oxides was estimated by the Kubelka-Munk method by extrapolating the straight line portion of (αhν)1/2 vs hν plot to the energy axis. The Kubelka-Munk plots of M-TiO2 samples are shown in Fig.4.21
250 300 350 400 450 500 550 600 650 0.0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Absorbance (a.u)
Wavelength (nm)
(a) Cu-TiO2 (b) Fe-TiO2 (c) Ni-TiO2 (d) TiO2
(b) (a) (c) (d) (A)
2.0 2.5 3.0 3.5 4.0 4.5
0 1 2 3 4 5 6 7
(d) (c) (b)
(h)1/2 (eV1/2 . cm-1/2 )
heV) (a) Cu-TiO2
(b) Fe-TiO2 (c) Ni-TiO2 (d) TiO2
(a)
(B)
(B). The band gap values obtained for the oxides were 3.0 eV (TiO2), 2.3 eV (Cu- TiO2), 2.5 eV (Fe-TiO2) and 2.55 eV (Ni-TiO2). The shift toward visible light absorption for doped titania has been attributed to the formation of intra band states by dopant transition metals within the titania bandgap.
Fig.4.21. (A) UV-Visible diffuse reflectance spectra and (B) Kubelka-Munk plot of undoped TiO2 and M-TiO2 (M = Cu, Fe and Ni)
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Fig.4.22 shows the PL spectra of M-TiO2 and TiO2 samples. For all samples signals were obtained in the range of 350 to 600 nm. Six distinct peaks at about 360, 395, 450, 470, 481 and 492 nm wavelengths were observed for TiO2. The corresponding transition energy values are 3.44, 3.13, 2.76, 2.65, 2.8 and 2.52 eV, respectively. As discussed in section 4.2.1, these peaks corresponded to band gap transition of anatase (395 nm) and emissions of free and bound excitons.
Fig.4.22. Photoluminescence spectra of undoped TiO2 and M-TiO2 (M = Cu, Fe and Ni)
Addition of transition metals to titania drastically reduced the intensity of the peaks suggesting reduction in the rate of radiative recombination of photogenerated electrons with valence band holes. This can be explained as follows. The transition metals can exist in multiple valance states such as Cu+2/Cu+1/Cu0, Fe+3/Fe+2/Fe0 and Ni+2/Ni0. These transition metals when incorporated into the TiO2 crystal lattice can act as trap for photogenerated electrons resulting in non-radiative combinations as shown below.
( 1)
n n
M
e
M
350 400 450 500 550 600
0.0 0.5 1.0 1.5 2.0
(d)
(c) (b)
Intensity (a.u)
Wavelength (nm)
(a) Cu-TiO2 (b) Fe-TiO2 (c) Ni-TiO2 (d) TiO2
(a)
Where ‘n’ represent the valence state of metal ‘M’ and ‘e’ is the photogenerated electron. This trapping mechanism reduces the rate of radiative recombination of photogenerated electrons with holes in valence band of titania decreasing the peak intensity. The stability of the photogenerated electrons is expected to increase the photoactivity of doped titania.
For M-TiO2 catalysts, the peak intensity was minimum for Cu-TiO2 and maximum for Ni-TiO2. This suggests that non-radiative trapping of photogenerated electrons was maximum for Cu doped titania and minimum in presence of nickel. The relative trapping efficiency of transition metals is expected to depend on ease of accepting electrons, on other words on reduction potential. Hence the transition metal with more positive reduction potential with respect to that of titanium will have higher ability of trapping electrons. The reduction potentials of the transition metals are summarized in Table 4.8 (Nakamura et al., 2007; Litter, 1999; Slamet, 2005; Rajeshwar et al., 2002).
Table 4.8. Reduction potential values of the transition metals
All the transition metals have positive reduction potential values with respect to that of titanium and hence all are expected to show some trapping efficiency. The copper with positive reduction potentials for all electronic transitions is expected to show the highest non-radiative electron trapping efficiency and hence the lowest peak intensity was observed for Cu-TiO2 sample. Similarly nickel with lowest reduction potential is expected to be least efficient for electron trapping.
Reduction reactions Reduction potential (V)
Ti4+/Ti3+ -0.670
Fe2+/Fe0 -0.440
Ni2+/Ni0 -0.257
Cu2+/Cu+ 0.159
Cu2+/Cu0 0.340
Cu+/Cu0 0.520
Fe3+/Fe2+ 0.771
76 Photocatalytic activity studies
The photocatalytic hydrogen production over transition metal doped TiO2 is shown in Fig.4.23. Under the experimental conditions, the hydrogen production over unmodified titania, having surface area of 84 m2/g and mainly anatase phase, was 96 µmol/g/h from water-methanol solution. In comparison, all the transition metal doped titania samples showed higher activity. The photocatalytic hydrogen production was highest for Cu-TiO2 (938 µmol/g/h). For Fe-TiO2 and Ni-TiO2, the hydrogen production was 269 and 250 µmol/g/h, respectively. For Cu-TiO2, the highest efficiency for trapping the photogenerated electrons contributed to higher availability of electrons for reduction of hydrogen at conduction band and resulted in higher activity. The higher activity for Cu-TiO2 can be also attributed to higher surface area and significant absorption of visible light.
Fig.4.23. Hydrogen production for M-TiO2 and M-1N-TiO2 (M = Cu, Fe and Ni) For Ni-TiO2, lowest activity can be attributed its highest rate of recombination photogenerated electrons as observed in PL spectra as well as to lowest absorption of visible light. The apparent quantum efficiency of Cu-TiO2, Fe-TiO2 and Ni-TiO2 were determined to be 18.7, 5.3 and 5.0 % respectively.
0 200 400 600 800 1000 1200
Ni-1N-TiO2 Fe-1N-TiO2
Cu-1N-TiO2 Ni-TiO2
Fe-TiO2
Hydrogen production (mol/g/h)
Catalyst type
Cu-TiO2
0.0 0.2 0.4 0.6 0.8 1.0 0
100 200 300 400 500
Adsorbed volume (cm3 g-1 )
Relative pressure, (P/Po)
Cu-1N-TiO2 Fe-1N-TiO2 Ni-1N-TiO2
0 20 40 60 80 100 120 140 160 180 200 0.000
0.005 0.010 0.015 0.020 0.025 0.030 0.035
Pore diameter (nm)
Pore volume (cc/g)