To understand the chemical compositions and chemical states of vanadium ions in the bulk lattice and surface sites of the doped TiO2, samples were characterized using ICP-MS, SIMS and XPS. Table 4-1 lists the bulk and surface V/Ti atomic ratios of bulk doped materials. Because the vanadium solutions were diluted by 2-isopropanol, the solution creates the errors between added V/Ti ratios and the measured bulk ratios. The bulk V/Ti ratios ranged between 4.41×10-5and 1.22×10-2which were similar to the added ratios (ranged between 5.00×10-5 and 1.00×10-2). However, the surface V/Ti ratios ranging 3.03×10-5-1.67×10-2 were slightly larger than the bulk ratios. These results indicate that almost vanadium ions were successfully doped into TiO2in the sol-gel process. In addition, higher amounts of the doped vanadium ions were accumulated in the surface lattice.
Table 4-1 The bulk and surface V-to-Ti atomic ratios of bulk doped TiO2.
Added V/Ti ratios Bulk V/Ti ratiosa Surface V/Ti ratiosb Sample name
5.00×10-5 4.41×10-5 7.26×10-5 VT 4.41×10-5
1.00×10-4 1.34×10-4 2.36×10-4 VT 1.34×10-4
5.00×10-4 5.09×10-4 6.72×10-4 VT 5.09×10-4
1.00×10-3 1.27×10-3 1.84×10-3 VT 1.27×10-3
1.00×10-2 1.22×10-2 1.67×10-2 VT 1.22×10-2
adetermined by ICP-MS,bdetermined by SIMS.
Table 4-2 lists the bulk and surface V/Ti atomic ratios for the surface doped materials with increasing contents of vanadium ions. The surface doped TiO2 contained bulk and surface V/Ti ratios of 1.73×10-3 - 1.10×10-2 and1.54×10-2 - 2.97×10-1, respectively. Since the surface doped samples were prepared by coating a thin V/TiO2 film on the TiO2particles, the bulk V/Ti ratios were much smaller than the surface ones. Similar to the bulk doped TiO2, the surface V/Ti ratios were 1.5 - 2.7 times larger than the added V/Ti ratios of the surface coating (1.00×10-2 - 2.00×10-1), indicating the migration of the vanadium ions from
where Tf represents the melting temperature of oxides. The Tf of TiO2 is 2190 K, thus its Tammann temperature is 780-1118 K.16, 58 However, the enhanced mobility of V5+ is associated with its lower Tammann temperature of V2O5 (i.e. 209 °C) compare to TiO2. 66 So, some vanadium ions migrated from bulk lattice toward to surface lattice of TiO2.
Herein, the bulk chemical compositions determined by ICP-MS were used to name all the doped samples. For example, the bulk doped TiO2with total V/Ti ratio of 4.41×10-5was called VT 4.41×10-5. For surface doped TiO2, the sample with total V/Ti of 1.10×10-2 was named as SVT 1.10×10-2. In addition, the sample which was coated pure V2O5on TiO2was named SVTP. Its total bulk V/Ti ratio was 4.74×10-3, as shown in Appendix F.
Table 4-2 The bulk and surface V-to-Ti atomic ratios of surface doped TiO2. Added V/Ti ratios of
the coating layer
Bulk V/Ti ratiosa Surface V/Ti ratiosb Sample name
2.00×10-1 1.10×10-2 2.97×10-1 SVT 1.10×10-2
1.00×10-1 6.40×10-3 2.03×10-1 SVT 6.40×10-3
5.00×10-2 3.47×10-3 1.37×10-1 SVT 3.47×10-3
1.00×10-2 1.73×10-3 1.54×10-2 SVT 1.73×10-3
adetermined by ICP-MS,bdetermined by SIMS.
The chemical states of the vanadium ions in the bulk and the surface lattices were determined using ESCA (see Appendix D). The vanadium ions at the surface sites were mainly V5+form, while V4+were observed in the bulk lattices. The reduction of V5+ to V4+
was attributed to thermal-induced dehydroxylation which preliminarily led to the formation of Ti3+sites followed by the consecutive electron transfer from the Ti3+ to V5+sites.67-69 These processes can be expressed schematically by the following set of reactions.
)
4-2 Morphology
Figure 4-1 shows the SEM images of as-dried and calcined TiO2. The surface of the as-dried TiO2was smooth. However, the particles were composed of small grains with sizes smaller than 100 nm after calcination at 300 °C. In general, the particles are agglomerated and basically irregular in shape with a substantial variation in particle sizes. The results indicate that degree of agglomeration tended to increase with increasing calcination temperature.70 Thus, polycrystalline structure and agglomerated phenomena occurred during calcination at temperature of 300 °C. Figure 4-2 shows the SEM image of the bulk doped TiO2 calcined at 300C. Similar to the pure TiO2, the bulk doped TiO2 was also consisted by small grains which was smaller than 100 nm. Figure 4-3 shows the SEM images of surface doped TiO2 particles and the cross sectional view of its V-doped TiO2 coating. The surface coating film contained a thickness of around 50 nm. Different from the bulk doped TiO2, the surface doped TiO2 was smooth due to rapid hydrolysis while surface sol-gel processes did not add HCl. In addition, the SVTP 4.74×10-3 coated a thin film of V2O5 on TiO2 also exhibited smooth morphology (see Appendix F). These findings suggest that the thin film prepared by surface sol-gel was uniform which resulted from lack of HCl.
Figure 4-1 The morphology of pure TiO2calcined at (a) 150 °C for 3 h and (b) 150 and 300
°C for 3 h.
Figure 4-2 The SEM images of bulk doped materials
Figure 4-3 The SEM images of (a) surface doped materials and (b) cross sectional view of a V-doped TiO2film prepared by the surface sol-gel method.
4-3 Microstructures
To examine the effect of doping sites on the crystalline structure, grain size, d-spacing and specific surface area, all samples were analyzed by XRD and BET. Figure 4-4 displays the crystalline structures of pure and bulk doped TiO2. The peaks of the anatase (101) and rutile (110) profiles centered at 25.4 and 27.5°2θpositions, respectively. It indicated the coexistence of anatase and rutile phases in the pure TiO2and the weight ratio of rutile was 31.1wt% which error bar is 5 wt%. The weight ratio of rutile (i.e. 21.3-26.5 wt%) of bulk doped materials slightly decreased while V/Ti atomic ratio was lower than 1.27×10-3. Besides, the rutile phase was not detected while the V/Ti atomic ratio was higher than 1.27×10-3, indicating that the presence of V4+/V5+inhibits the anatase-to-rutile transformation.
The anatase-rutile transformation was restrained by formation of V2O5phase occurred at high vanadium ions and low calcination temperature.71 No diffraction peaks corresponding to vanadium oxide were observed in the XRD pattern. Therefore, the vanadium ions were either highly dispersed in the TiO2 matrix or formed as tiny vanadia crystallites having the size beyond the detection capacity of the powder X-ray diffraction technique (less than 5 nm).
Table 4-3 lists the d-spacing, crystallite sizes and weight ratios of rutile phases of the bulk doped TiO2. The d-spacing of (101)aof the bulk doped TiO2were 351 pm. This value was similar to that of pure TiO2 (350 pm) and the d-spacing was slightly increased from 530 to 353 pm even under heavily doping (See Appendix E). Since the ionic radius of V4+/V5+ is 72/68 pm which is closed to that of Ti4+ (74.5 pm),72 the vanadium ions are doped into the TiO2lattice by substituting Ti4+ions.
The average crystallite sizes of pure TiO2, estimated from the broadening of the anatase (101) diffraction peak, are 6.1 nm which error bar is 0.6 nm. Compare with pure TiO2, the crystallite sizes (i.e. 6.0-6.5 nm) of bulk doped materials had no obviously difference. The results show the concentration of vanadium-ion (V/Ti < 1.00×10-2) is too low to affect the crystallite size. However, when V/Ti atomic ratio increased to 2.00×10-1, the crystallite size of anatase decreased from 6.1 to 5.2 nm, as shown in Appendix E. The inhibition of the growth of crystallite sizes was resulted from increasing surface energy and surface stress caused by lattice vanadium ions.58, 67 Figure 4-5 schematically illustrates the inhibited growth of crystallite sizes of bulk doped TiO2.
20 30 40 50 60 70 80
Figure 4-4 The XRD patterns of bulk doped TiO2at different vanadium ion concentrations.
Table 4-3 The crystallite sizes, d-spacing of materials and weight ratio of rutile phase of bulk doped TiO2.
Figure 4-5 The effect of bulk defect on growth of crystal.
To analyze the surface microstructure of the V-doped TiO2, GI-XRD was used to examine the surface crystalline properties of the samples. Figure 4-6 shows the GI-XRD patterns of bulk doped TiO2. The two peaks centered at 25.4 and 19.2°2θ positionswere denoted to anatase and V2O5, respectively. In addition, the relative intensity of V2O5
increased with increasing vanadium ions. The data indicate parts of vanadium ions transferred toward surface of matrix and agglomerated to perform V2O5 at 300 ºC, while the chemical state of vanadium ions was V5+.
20 30 40 50 60 70 80
Figure 4-6 The GI-XRD patterns of bulk doping materials.
Figure 4-7 shows the XRD patterns of surface doped TiO2. Table 4-4 lists the d-spacing, crystal sizes and weight ratios of rutile phases of the surface doped TiO2. The crystallite sizes of anatase TiO2 were in the range of 6.0-6.5 nm, while their d-spacing of anatase (101) profile were in the range of 350-352 pm. In addition, the weight ratio of rutile were not detected except SVT 1.73×10-3 since these characters were similar to those of pure TiO2, surface doped V5+ ions had little effects on the bulk microstructures of TiO2. Figure 4-8 displays the V2O5/TiO2 thin films coated on glass which were prepared by surface sol-gel.
In contrast to bulk doped TiO2, V2O5 crystallites were observed only on the surface of substances. This phenomenon revealed that the TiO2was coated with V2O5 as a core-shell structure in the surface doped TiO2samples. Therefore, the chemical status of vanadium in bulk doped TiO2co-existed both V4+and V5+. The chemical status is an important evidence to deduce whether vanadium was located in the TiO2 octahedral lattice. The V4+ was possible in the octahedral lattice of TiO2, while V5+ may be V2O5 highly dispersed within crystalline of TiO2. In addition, both the intensity and crystal sizes of V2O5 (i.e. 11.2 nm) did not alter with increasing vanadium ions.
20 30 40 50 60 70 80
Figure 4-7 The XRD patterns of surface doped materials at different vanadium ions concentration.
Table 4-4 The crystallite sizes of materials and weight ratio of rutile phase of surface doped TiO2
20 30 40 50 60 70 80
V
2O
5
SVT 1.73*10
-3SVT 3.47*10
-3SVT 6.40*10
-3In te n si ty (A .U .)
Degree (2 )
SVT 1.10*10
-2Figure 4-8 The GI-XRD patterns of surface doped materials.
Figure 4-9 illustrates the microstructures of bulk and surface doped TiO2. In the bulk doped TiO2, vanadium ions were subsitutionally doped in the anatase lattice while few V2O5
crystals were existed in the surface layers. However, TiO2/V2O5 core-shell structures were formed in the surface doped TiO2. The specific surface area of bulk and surface doped TiO2
were listed in Appendix C. The specific surface areas of bulk doped materials were ranged between 99 and 110 m2/g, while the surface doped materials exhibited their specific surface areas of 99-105 m2/g. Either doping TiO2 with V4+/V5+ ions in the bulk lattice or on the surface sites had little effects on the specific surface areas because the as-dried oxides are usually amorphous, they must be calcined at high temperature for crystallization.
Unfortunately, the decreasing crystallite size increased the surface area of the photocatalysts.
70, 73
Figure 4-9 The microstructures of bulk and surface doped TiO2.
4-4 UV-Visible absorption
In order to elucidate the optical properties for photocatalysts, UV-vis diffuse reflectance spectroscopy (DRS) was applied to study the bonding information of the V-doped TiO2. Figure 4-10 shows the optical absorbance of pure TiO2 from wavelength of 900 to 200 nm.
The absorption edge was at 405 nm, corresponding to 3.1 eV of the bandgap energy.
Below 405 nm, there were two bands separated at 263 nm. One broad band was ranged between 200-300 nm and centered at 233 nm which was denoted to upper CB.74, 75 The upper band centered at 233 nm was assigned to a charge-transfer transition between the oxygen ligands and a central Ti4+ion with a tetrahedral coordination (4-fold).74, 75 The other band centered at 353 nm which was denoted to lower CB.
200 300 400 500 600 700 800 900
Figure 4-10 UV-Vis DRS spectra of TiO2. The red solid line: Upper conduction band.
The blue dash line: Lower conduction band.
Figure 4-11 shows the optical absorbance of bulk doped TiO2 with various contents of vanadium-ion doping. The spectra of bulk doped materials showed similar absorption behavior at an absorption edge of around 405 nm (3.1 eV), except VT 1.22×10-2 which exhibited the absorption edge of 426 nm (2.9 eV). Table 4-5 lists the band gaps of the bulk doped TiO2 containing varies V/Ti ratios. The bulk doped TiO2with vanadium ions, which was lower 1×10-2, showed a broad adsorption peak ranged 250-320 nm and centered at 289 nm, which was denoted to V5+.76 The V5+ broad band was occurred between upper and lower CB. So, the V5+(3d) bands lied in the CB edge of TiO2, and the intensity of V5+peak increased with raising vanadium ions. However, when V/Ti was larger than 1 mol %, the d-d transition of V4+ occurred. The results show a broad adsorption from 779 nm in the inside graph of Figure 4-11. So, the V5+ ions reduced to V4+ under higher vanadium concentration.
Because a spontaneous reduction of V5+ to V4+ occurred at the vanadium oxide and titanium oxide interface during calcination at temperatures above 450 °C,68, 77 the chemical states of vanadium ions of bulk doped materials was V5+ mainly under lower vanadium concentration (V/Ti < 1 mol %) and lower calcination temperature ( 300 °C < 450 °C). In
addition, the V4+ reduced by V5+ performed extra band located above VB 1 eV when V/Ti was larger than 1 mol %, as shown in Figure 4-12.29, 78 In this study, Figure 4-13 shows the different electronic structure with high and low vanadium contents.
200 300 400 500 600
K u b elk a -Mu n k (A .U. )
K u b e lk a -M u n k (A .U .)
Wavelength (nm)
Wavelength (nm)
289 nm 343 nm 233 nm
VT 4.41*10 -5 VT 1.34*10 -4 VT 5.09*10 -4
TiO 2 VT 1.73*10 -3
200 300 400 500 600 700 800 900 d-d transitions of V
4+(779 nm) red shift
TiO
2VT 1.22*10
-2Figure 4-11 The UV-visible absorption spectra of all bulk doped materials at different vanadium ions concentration, except VT 1.22×10-2which is shown in inside graph.
Table 4-5 The band gap energy of bulk doped TiO2. Bulk doped TiO2
Materials Band gap (eV)
TiO2 3.1
VT 4.41×10-5 3.1
VT 1.34×10-4 3.1
VT 5.09×10-4 3.1
VT 1.27×10-3 3.1
VT 1.22×10-2 2.9
Figure 4-12 Energy levels of impurity ions in rutile.29
(V
4+)
(V
3+)
(V
2+)
Figure 4-13 The concept of electronic structure for bulk doping materials. (a) low vanadium ions (< 1 mole%); (b) high vanadium ions (i.e. VT 1.22×10-2).
Figure 4-14 displays the optical absorbance of surface doped TiO2 from wavelength of 900 to 200 nm. The similar absorption behaviors of surface doped materials at an absorption edge of around 405 nm (3.1 eV) are shown in Figure 4-14. Table 4-6 lists the band gap of surface doped TiO2 at various vanadium ions concentration. The band gap surface doped TiO2was around 3.1 eV.
The results show that vanadium ions doped on the surface of TiO2had no obvious effect on the electronic structure of TiO2. Clearly, the findings indicate that the bulk doping sites had more obvious effect than surface doping site on electronic structure of TiO2. Besides, Figure 4-15 illustrates the scheme of electronic structure of TiO2/V2O5 composite semiconductors. It refers that the CB of V2O5was lower than TiO2, because d-orbital energy of the highest occupied atomic d-orbital of vanadium was lower than that of titanium.79, 80 Therefore, to summarize the salient features of the analysis, the image electronic structure of bulk and surface doped TiO2were illustrated in Figure 4-13 and Figure 4-16 respectively.
Table 4-6 The band gap energy of surface doped TiO2 Surface doped TiO2
Materials Band gap (eV)
SVT 1.73×10-3 3.1
SVT 3.47×10-3 3.1
SVT 6.40×10-3 3.1
SVT 1.10×10-2 3.1
SVTP 4.74×10-3 3.1
200 300 400 500 600
Upper CB Lower CB
TiO
2SVT 1.73*10
-3SVT 3.37*10
-3SVT 6.40*10
-3SVT 1.10*10
-2K u b el k a -M u n k (A .U .)
Wavelength (nm)
Figure 4-14 The UV-visible absorption spectra of the surface doped materials at different vanadium ions concentrations.
Figure 4-15 Schematic band energy diagram for the TiO2/V2O5composite semiconductor.80
Figure 4-16 The electronic structure of surface doping materials.
4-5 Photocatalytic activity
The photocatalytic activities of vanadium ions doped TiO2 was examined by the decoloration of 0.01 mM RhB monitored at 553 nm. Figure 4-17 displays the photocatalytic activities of bulk doped TiO2 irradiated with UV light at 365 and 305 nm, respectively. In the absence of a photocatalyst, RhB was stable when irradiated with UV light at 365 and 305 nm. The photodecomposition of RhB followed pseudo-first-order kinetics with 305 nm irradiation. The degradation of RhB in the presence of the prepared nanocrystals indicated each of the bulk doped photocatalysts exhibited lower photoactivity. Since all bulk doped TiO2 exhibit anatase form and similar band gaps (3.1-2.9 eV); it is suggested that the extra energy band caused by vanadium ions had negative influence on the photoactivities, because V4+reduced by V5+ acted as recombination center, as shown in Figure 4-18. Moreover, the decreased photocatalytic activities were caused by increasing vanadium-ion doping. In addition, the tendency of photodecomposition of RhB with 365 nm irradiation was similar to 305 nm irradiation. Figure 4-18 displays the dependence of photocatalytic rate constants of bulk doped TiO2 on the V/Ti ratios under irradiation of UV light at 365 and 305 nm.
According to Langmuir-Hinshelwood (LH) kinetics, the photodecomposition of RhB was followed zero-order kinetics under 365 nm UV irradiation and that was followed pseudo-order kinetics with 305 nm irradiation. The difference was presumably due to fewer amounts of charge carriers were generated under irradiation of the UV light with higher wavelength. Thus, the effective concentration of RhB was augmented and zero-order kinetics was followed under this situation.
Table 4-7 lists the degradation rate constant (min-1) of bulk doped samples with UV light at 365 and 305 nm. The VT 4.41×10-5 (k = 5.60×10-2 min-1) exhibited the highest rate of decomposition of RhB with UV at 305 nm, followed by, TiO2 (k = 5.20×10-2 min-1), VT 1.34×10-4 (k = 5.10×10-2 min-1), VT 5.09×10-4 (k = 4.00×10-2 min-1), VT 1.27×10-3 (k = 3.30×10-2 min-1) and VT 1.22×10-2 (k = 1.50×10-2 min-1). Under 365 UV irradiation, the samples with bulk doped ratios lower than 5.09×10-4 exhibited similar rate constants (k = 1.33×10-2 mM/min) for decomposition of RhB followed by VT 1.27×10-3 (k = 1.00×10-2 mM/min) and VT 1.22×10-2(k = 6.00×10-3mM/min).
0 10 20 30 40 50 60
Figure 4-17 The decoloration of 0.01 mM RhB by pure and bulk doped TiO2under (a) 305 nm and (b) 365 nm UV irradiation.
0
(a) V/Ti (surface molar ratio)
Bulk doped materials
(b) V/Ti (surface molar ratio)
Bulk doped materials
V/Ti (Bulk molar ratio)
R at e co n st an ts (m M /m in )
365 nm
Figure 4-18 The rate constants of bulk doped materials at various vanadium ions concentration compared with pure TiO2under (a) 305 nm UV and (b) 365 nm UV irradiation.
Table 4-7 The rate constants of bulk doped TiO2 at various vanadium concentration compared with pure TiO2under 365 and 305 nm UV irradiation.
Bulk doped TiO2
Sample name Surface V/Ti ratioa
Rate constants at 365 nm UV irradiation (mM/min)
Rate constants at 365 nm UV irradiation (min-1)
TiO2 -b 1.33×10-2± 3.33×10-4 5.20×10-2
adetermined by SIMS.bnot available.
For pure TiO2with 305 nm irradiation, the rate constant was 5.20×10-2min-1. When the V/Ti ratio increased to 1.34×10-4, the photoactivity increased to 6.10×10-2 min-1 which was similar to pure TiO2. The photo-electrons were trapped in bulk-V5+sites which position was lower than upper CB after 305 nm UV irradiation while the lots of generated holes performed at the same time. Unfortunately, it was hard to confirm the photo-holes migrated to surface and reacted with donors without recombined with electrons. Moreover, the photocatalytic activities of bulk doped materials with low vanadium-ion doping were similar to pure TiO2. Nevertheless, when the bulk V/Ti higher than 1.00 atomic percent, the photoactivity decreased from 6.10×10-2 to 1.50×10-2 min-1. Because parts of generated holes were trapped by V4+
ions reduced from V5+, the trapped holes could not react with donors. Moreover, the substitution V4+ in the lattice of quantum-size TiO2 acted primarily as a charge-carrier recombination center that with a net reduction: V3++ V5+→2 V4+ when the electrons/holes were hard to migrate to surface and react with acceptors/donors, as shown in Figure 4-19.14
Figure 4-19 The concept of higher content V4+at substitutional site in Q-size TiO2lattice.14
Figure 4-20 displays photocatalytic activity of surface doped TiO2 irradiated with UV light at 365 and 305 nm. The photodecomposition of RhB was followed pseudo-first-order kinetics with 305 nm irradiation and it was followed zero-order kinetics with 365 nm irradiation. The tendency of photoactivities with 305 nm was similar to 365 nm irradiation.
Therefore, the mechanism of photocatalysis at 305 nm irradiation was discussed since all surface doped TiO2 exhibited anatase form and similar band gaps (3.1 eV); it suggested that the surface structural properties had a greater influence on the photoactivities rather than the bulk ones do.
Figure 4-21 displays the dependence of photocatalytic rate constants of surface doped TiO2on the V/Ti ratios under irradiation of UV light at 365 and 305 nm. Table 4-8 lists the dependence of photocatalytic rate constants of the surface doped TiO2irradiated with 305 and 365 nm UV light on the various V/Ti ratios. And the SVT 1.10×10-2sample (k = 9.80×10-2 min-1) exhibited the highest rate of decomposition of RhB with 305 nm irradiation, followed by SVT 6.40×10-3 (k = 6.30×10-2 min-1), SVT 3.47×10-3 (k = 5.90×10-2 min-1), TiO2 (k = 5.20×10-2 min-1), SVT 5.09×10-4 (k = 4.60×10-2 min-1). Under 365 UV irradiation, the SVT 1.10×10-2 sample (k = 1.80×10-2 mM/min) exhibited the highest rate of decomposition of RhB, followed by SVT 6.40×10-3(k = 1.40×10-2mM/min), TiO2(k = 1.33×10-2 mM/min), SVT 3.47×10-3(k = 1.25×10-2mM/min), SVT 5.09×10-4(k = 1.07×10-2mM/min).
0 5 10 15 20 25 30 35 40 45
Figure 4-20 The decoloration of 0.01 mM RhB by surface doping materials at various
Figure 4-20 The decoloration of 0.01 mM RhB by surface doping materials at various