MATERIALS AND METHODS

Figure 3-1 shows the flow chart of the experimental design in this study. Catalysts are synthesized with sol-gel and surface sol-gel methods.

3-1 Materials

Titanium isopropoxide (TTIP, Acros, 98 %+) and vanadium (Ⅴ) oxytriisopropoxide (VTIP, Aldrich, 99 %+) were used as the precursors of titania and vanadium, respectively.

2-propanol (C3H7OH, J.Backer, 100 %) was used as solvent to dissolve titanium isopropoxide and vanadium (Ⅴ) triisopropoxide. Rohdamine B (RhB, C₂₈H₃₁N₂O₃Cl, Sigma Aldrich, Dye content 95 %) was used as the target compound for photocatalysis and its structure is shown in Figure 3-2. Hydrogen acid (HCl, Crown, 35 ~ 37 %) was used to adjust the pH of sol solution to slow down the hydrolysis. Filter membrane (Critical, 47 mm in diameter, 0.2 μm in pore size) was used as a support for the TiO2 during its surface coating process.

5,5-dimethyl-1-pyrroline N-oxide (DMPO, Aldrich, 97%, d= 1.05 g/mL)wasused asa•OH trapped agent. The original DMPO solution was stored at –25 °C. In addition, DMPO diluted by DI water were decay, so the solution need to use immediately for least 2 two days stored at 4 °C.

Figure 3-1 Flow chart of experimental design in this study.

Figure 3-2 The chemical structure of RhB.

3-2 Preparation of bulk doped TiO

via sol-gel process

Figure 3-3 shows the preparation procedure for bulk doped TiO2. Firstly, TTIP and VTIP were dissolved in 70 mL isopropanol in sample vials (110 mL) to reach various V/Ti atomic ratios (1×10^-5 ~ 1×10^-2). Then, 2 mL hydrogen chlorate acid (24.5%, HCl) was injected into the mixtures at 4°C with stirring at 250 rpm. The solutions were maintained at this temperature for 9 h to complete the hydrolysis of TTIP and VTIP. Afterward, the solutions underwent gelation at room temperature for 4 h. The doped TiO2 powders were obtained through evaporating solvent at 100°C for 2 d followed by 150°C for 3 h. The solids were then calcined under air at 300 °C for 3 h. The resulting xerogels were called VT.

Figure 3-3 The synthetic process of bulk doped materials. (a) the flow charts of the synthesis and (b) the recipes of precursor solutions.

3-3 Preparation of surface doped TiO

via surface sol-gel process

Figure 3-4 shows the preparation procedure for surface doped TiO2. Surface doped materials were prepared via coating a thin V-doped TiO2layer onto TiO2using surface sol-gel process. Figure 3-5 shows the carton and photograph of surface sol-gel system. Moreover, the dried TiO2 powders were obtained through evaporating solvent at 100°C for 2 d followed by 150°C for 3 h. The dried TiO2 powders were stood between Teflon and filter paper, which were immersed in shallow container filled with solution D. And the solution D at different TTIP/VTIP atomic ratios was prepared as following: Firstly, TTIP and VTIP were dissolved in 35 mL isopropanol to reach varies VTIP/TTIP atomic ratios (1×10^-2~ 2×10^-1).

Then, 1 mL of above mixing solution (solution C) was injected into 39 mL isopropanol for dilution, and the diluted solutions were named solution D, as shown in Figure 3-4.

The dried TiO₂ powders were immersed in solution D about 10 minutes. Then the dried TiO₂ were raised by hand and then the powders were separated from diluted precursor by gravity. Afterward, the solids were dried at 100 °C for few minutes and then were calcined at 300 °C for 3 h. The resulting xerogels obtained were named SVT. In addition, the pure V₂O₅ coated on the surface of TiO₂ were named SVTP, which was shown in Appendix F. To summary the structures of bulk doped and surface doped TiO₂, Figure 3-6 shows the three type materials in this study.

Figure 3-4 The synthetic process of surface doped materials. (a) the flow charts of the synthesis of surface doped materials, (b) the recipes of precursor solutions, and (c) the surface sol-gel processes for coating.

Figure 3-5 A design chart and a photograph of surface sol-gel system.

Figure 3-6 The geometric structure of pure, bulk and surface doped TiO2.

3-4 Characterization

3-4-1 X-ray photoelectron spectroscopy (XPS)

XPS measurements were performed with an ESCA PHI 1600 spectrometer using an Al Kα radiation (1486.6 eV). Thephotoelectronswerecollected into theanalyzerwith a23.5 eV pass energy. The collection step size in wide range scan and high-resolution analysis are 1.0 eV and 0.1 eV, respectively. All analytical process was controlled under ultrahigh vacuum at the pressure below 1.4 ×10^-9Torr. In addition, bulk chemical compositions were detected after etching by Ar ion for 60 seconds. At low V loading, a charging effect occurs and was corrected using the C 1s peak at 284.8 eV as a reference. For advanced qualification and quantification of each element, curve fitting of XPS spectra was performed on program. The atomic ratio was calculated from the integrated peak areas normalized to sensitive factors. The equation for atomic ratio calculation is shown:

2

Where n denotes the atomic numbers, I is the intensity of species on XPS spectra, A is the peak area, ASF stands for the atomic sensitive factor of element and Arabic number represents

elemental types.

3-4-2 Time-of-Flight Secondary Ion Mass Spectrometer (TOF-SIMS)

SIMS measurements were performed with a TOF-SIMS Ⅳ (ION-TOF, Munich, Germany) spectrometer. The TOF-SIMS spectra were recorded at 25 °C in positive detection modes. The primary ion source was a pulsed Ga⁺source (pulsing current 1.0 pA, pulse width 30 ns) operated at 25 keV. An area of 100×100 μm², a sputter time of 120 s, a data acquisition time of 150 s and charge compensation by applying low energy electrons (~

30 eV) from a pulsed flood gun were used for all measurements. The pressure of the main chamber was kept around 10^-9 mbar. The m/z of mass spectra in the positive mode was ranged from 40 to 60.

The surface atomic ratio was calculated from the intensity of secondary ions which are normalized to relative sensitivity factor. A relative sensitivity factor (RSF) is a conversion factor from secondary ion intensity to atom density. The RSF is defined by sub-equation⁶²

I RSF I

m





Where ρ_i is the impurity atom density in atoms/cm³, I_i is the impurity isotope (V, m/z=47.9) secondary ion intensity in counts/s, Im is the matrix isotope (Ti, m/z= 50.8) secondary ion intensity in counts/s, and RSF has unit of atoms/cm³.

3-4-3 Scanning electronic microscopy (SEM)

Scanning electronic microscopy (SEM, Hitachi, S-4700, Type II) was used to observe the morphology of the doped TiO2 under an accelerating voltage of 25 KV and a pressure of 3×10^-6Pa. The samples for the SEM observation were prepared by suspending the powders in 15 mL acetone solution via ultrasonic vibration for 20 minutes. The suspension was then directly dropped on the glass and dried at 100°C. To prevent charge accumulation, the samples were pre-coated with a Au film by Ion coater (Eiko IB-2) for 3 minutes which

3-4-4 X-ray diffractometry

Powder X-ray diffraction patterns of the samples after calcination were recorded with a computer controlled X-ray powder diffractometer (XRPD, MAC Sience, MXP18) using Cu Kα radiation and operating ataccelerating voltageof30 kV and an emission currentof20 mA.

Thescanning 2θ rangeisfrom 10°to 80°atasampling width of0.02°and scanning speed is4

°/min. If a sample only contains anatase and rutile, the weight ratio of rutile phase (WR) can be calculated from the following equation:³³

R

where A_A is intensity of anatase (101) peak and A_R is intensity of rutile (110) peak. The crystalline size (D)ofallsampleswascalculated from Scherrer’sequation:⁶³ width of the peak (full width at half maximum, FWHM) after correcting for instrumental peak broadening (βexpressed in radians), θis the Bragg angle and K is the Scherrer constant.

According to Bragg’slaw,thed-spacing could be calculated by this law, and geometric figure is shown in Figure 3-11.



 sin 2

d  n

Where d is the d-spacing (nm), λis the wavelength of incident X-ray, θis the Bragg angle and n= 1.

To investigate the surface structures of the samples, the V₂O₅/TiO₂ films were coated onto glasses and dried at 100°C for few minutes. The coated film were analyzed by a grazing incident X-ray diffractometer (GI-XRD, Rigaku，RU-H3R), which use Cu Kα radiation with incident angle of 1° and operate at accelerating voltage of 60 kV and an

emission currentof300 mA. Thescanning 2θ rangeisfrom 15°to 80°ata sampling width of 0.02° and scanning speed is 4 °/min.

3-4-5 UV/Vis diffuse reflectance spectroscopy (UV-Vis DRS)

The UV-vis diffused reflectance spectra wre recorded on a U-3010 Hitachi spectrometer with an integrating sphere reflctance accessory. The spectra were recorded from 900 to 200 nm at a scanning rate of 300 nm/min. Aluminum oxide, which was considered to exhibit total reflections, was used to be the reference. The spectra were transformed into absorptions according to Kubelka-Munk equation shown in equation 3-6.⁶⁴

S

Where k is absorption coefficient, S is scattering coefficient and R represents %R reflectance

3-4-6 Inductively Coupled Plasma Mass Spectromstry (ICP-MS)

ICP-MS (Perkin Elmer, SCIEX ELAN 5000) was used to analyze bulk chemical compositions for V/Ti weight ratio of all samples. All solid of samples were digested with acid solution coupled with microwave.

3-4-7 Specific surface area

The Brunauer, Emmett, and Teller (BET) surface area of catalysts were measured by N2

physisorption using a TriStar 3000 gas adsorption analyzer. The BET model was used to estimate the surface area of the samples according to the N₂adsorption data. For providing sufficient surface area for model calculation, over 0.2 g of powders was used for analysis.

Because the calcination temperature of V-doped materials was 300 °C, as-prepared sample was degassed at 120 °C for 6 h.

3-4-8 Electron paramagnetic resonance (EPR)

positioned at a fixed distance from a sample cavity. In addition, Figure 3-7 shows the distribution of the UV light system (Moritex, MUV-250U-L). The measurements were carried out at 77 K either in the dark or under irradiation. The instrumental conditions were set at a center field of 3400 G and a sweep width of 200.0 G. The microwave frequency was 9.50 GHz and the power was 8.0 mW.

The EPR spin trapping experiments using DMPO were performed at room temperature.

The 0.03 M DMPO solution was prepared by adding 0.0345 mL DMPO into 10 mL DI water, and it was stored at 4°C. Each sample was aerated with 30 min O2 before analysis. After addition of 1 mL of 0.03 M DMPO into 10 mL catalyst suspension (1g/l), the mixtures were shaken by hand to reach a homogeneous condition. Subsequently, the samples were delivered to a quartz capillary tube and analyzed the spin-trapped adducts under irradiation of UV light at room temperature. The settings for the EPR spectrometer were center field = 3480.0 G;

sweep width = 200.0 G; microwave frequency 9.77 GHz; modulation frequency 50.0 kHz and power 10 mW. To minimize measurement errors, the same quartz capillary tube was used throughout the EPR measurements.

Figure 3-7 The UV lamp spectral distribution.⁶⁵

3-4-9 Transmission Electron Microscope (TEM)

The particle size and shape of nanocrystals were examined by a transmission electronic microscopy (TEM, JEM 1200) at an accelerating voltage of 120 KV. The specimen was prepared by dispersing of powders into acetone with ultrasonic vibration. The colloid was dropped on a holey carbon film supported on a Cu grid (Ted Pella, Inc., 200 meshes). TEM images are displayed in Appendix I.

3-5 Photocatalytic of RhB decomposition

Rhodamine B (RhB) was selected as the target compound to test the photocatalytic activity of the doped TiO₂. Figure 3-8 shows the UV-Vis spectrum of 0.01 mM RhB at 400-700 nm. The most intensive absorption peak appeared at 553 nm. The degradation of the RhB was monitored according to the decreasing intensity of this characteristic peak. The catalysts (20.0 mg) were dispersed ultrasonically into 20 mL of RhB solutions at concentration of 0.01 mM in a fused-silica tube. Prior to irradiation, the suspension was purged with O₂ in the dark with stirring for 30 minutes for equilibrium of adsorption and desorption of RhB and saturation of the solution with O2. The purging was continued during photocatalysis. The photocatalysis was carried out under irradiation of 8 UV lamps each of (8 W) at 365/305 nm. Figure 3-9 displays the cartoon diagram and photographs of the photocatalytic system.

400 450 500 550 600 650 700

0.00

Figure 3- 8 The UV-Vis spectrum of 0.01 mM RhB.

Figure 3-9 A design chart for photocatalysis reactor and A photograph of photocatalytic reactor.

Chapter 4 Results and discussion