1. Effect of method of synthesis and different metal doping on the photocatalytic activity of titania Department of Chemistry Anna University Chennai P. Vijayan

2.  In the year 1972, Fujishima and Honda discovered the photocatalytic splitting of water on titania semiconductor electrode. Later on Sato and White decomposed water on platinum loaded TiO 2 powder.  After that researchers concentrated towards the usage of semiconductor photocatalysis for environmental protection.  Photocatalysis has been exploited for various environmental process such as deodorization, water purification, air purification, sterilization and soil proof.  Many researchers have put their effort on degradation and mineralization of variety of toxic substances by photocatalytic oxidation. Carbon dioxide has been reduced efficiently with water by photocatalysis.  Different types of semiconductors were used as photocatalyst. Among these titania is the widely used photocatalyst due to its strong oxidizing power, stable at different pH and favorable band gap energy. Photocatalysis

3. 3 Photocatalysts and their band gap energy Semiconductor Valence band Conductance band Band gap (eV) Band gap wavelength (nm) SnO 2 +4.1+0.33.8318 ZnO+3.0-0.23.2390 ZnS+1.4-2.33.7336 CdS+2.1-0.42.5497 CdSe+1.6-0.11.7730 TiO 2 +3.1-0.13.0380 GaP+1.32.3540

4. 4  Even though titania is the widely used semiconductor, it has some disadvantage like low surface area, fast recombination and wavelength maximum lies in UV region.  Commercial titania P-25 has average particle size of 40-50nm. Since larger particles increases the specific surface area of the catalyst, reduction of particle size become essential.  Recombination are of two types (i) surface recombination (ii) volume recombination.  Surface recombination can be avoided by doping some foreign material like metal, non-metals and coupling titania with another semiconductor. Volume recombination can be reduced by preparing titania in nanoscale. Wavelength of maximum can also increased to higher wavelength by doping metals or non-metals.  It is well known that wavelength and surface area are mainly depend on particle size.  It is well known that wavelength and surface area are mainly depend on particle size. Altering the size of the particle alters the degree of confinement of the electrons, and affects the electronic structure of the solid, in particular ‘band edges’, which are tunable with particle size.  From the above fact that the synthesis of titania with controlled nanometer dimension and doped with altervalent ion is essential.

5. 5 Size Dependence of Light Absorption  Ismat shah etal studied the effect of particle size for the degradation of 2-chlorophenol using titania synthesized by metallo organic chemical vapour deposition method with different particle size.  Particle size decrease from 29 to 17 nm showed red shift and further decrease i.e less than 17nm leads to blue shift.  Titania with 17nm have shown remarkable catalytic activity towards the degradation of 2-chlorophenol than the catalysts having 23nm and 12nm.  Comparison of band gaps B 17nm < B 12nm < B 23nm Appl.Catal.B:Environ, Vol. 68, 2006, Pages. 1-11

6. 6 Methods used  Sol-gel  Liquid phase deposition  Pulse laser deposition  Chemical vapour deposition  Magnetic sputtering  Direct hydrothermal method

7. 7 Sol-gel  Generation of a dispersion of colloidal particles suspended in Brownian motion within a fluid matrix.  Colloids are suspension of particles of linear dimensions between 1nm and 1  m. The colloidal suspensions can subsequently convert to viscous gels and then to solid materials  Sol-gel preparation leads to the greatest possible homogeneous distribution of the dopant ion in the host matrix.  Products have high purity and homogeneity, ease of processing and composition control Sol-gel synthesis involving following steps  Ageing  Gelation  Drying  Densification

8. 8 Hydrothermal treatment  The hydrothermal process including aqueous solvents as reaction medium is eco-friendly since it is carried out in a closed system and the contents can be recovered and reused after cooling down to room temperature.  The equipment and processing required are simpler and reaction is low energy consumption,  By controlling hydrothermal temperature and duration of the treatment, various crystalline products with different composition, structure and morphology could be obtained.  Fine particle size can be obtained with more uniform distribution and high dispersion either in polar and nonpolar solvents  In this way the energy band structure becomes discrete and titania nanoparticles exhibit improved optical and photocatalytic properties.

9. 9 Sol-gel +hydrothermal  Materials can be obtained with high purity  Particle size can be reduced and controlled  Highly crystalline material can be obtained  Material with higher light harvesting character  Phase formation can be decided before the heat treatment i.e, calcination  Eco-friendly

10. 102,4,6-Trichlorophenol  Chlorophenols are deadly toxic compounds, present in wastewater which mainly arise from chemical intermediates or by-products in petrochemical, paper making, plastic, pesticidal and water disinfection  PCDD/PCDF are the most toxic chlorinated substances which are 1000 times poisonous than arsenic. Due to handling problem of these chemicals in the laboratory, the model pollutant such as chlorophenol and chlorobenzene which are having structural similarities with them are taken  19 different chlorophenols have been listed by the US-EPA as priority pollutants  Among these 2,4,6-trichlorophenol is one of the most vulnerable water pollutants, which causes serious damage to the vital organs of human beings. Thus, the removal of the chlorophenols from the waste water is highly imperative.  In this work, we have used a modified sol-gel followed by hydrothermal treatment to synthesize metals (Fe, Cr, Co and Zr ) doped titania and used them for the oxidative degradation of 2,4,6-TCP.

11. 11 Experimental  Pure titania was synthesized by sol-gel, hydrothermal and combination of these two methods  Parameters affecting synthesis such as Water : Ethanol ratio, hydrothermal temperature and duration were optimized for effective synthesis.  Doped titania were synthesized by sol-gel + hydrothermal method with various weight percentage of loading (0.3, 0.5, 0.7 and 1wt%). Reactor Reactor  Slurry photoreactor  8x8w lamp  Emits 365nm wave length

12. 12 Procedure adopted------Sol-gel

13. 13 Procedure adopted- --- hydrothermal

14. 14 Procedure adopted----sol-gel + hydrothermal

15. 15 Effect of synthesis parameters

16. 16 XRD pattern of (a) sol-gel (b) hydrothermal and (c ) sol-gel + hydrothermal synthesized titania  In sol-gel, hydrothermal and combination of sol-gel and hydrothermal methods, only anatase phase is formed.  Particle size and Crystallinity of titania with different method of synthesis follows the order sol- gel< sol-gel + hydrothermal < hydrothermal.  Most of the particle are in same size in both the hydrothermal and sol-gel + hydrothermal methods  In sol-gel restriction of particle size within a range is not possible. Particles are in wide range of sizes.

17. 17 Histogram of (a) sol-gel (b) hydrothermal and (c) sol-gel + hydrothermal (a)(b) (c)

18. 18 UV-Vis DRS spectra of (a) sol-gel (b) hydrothermal and (c )sol-gel + hydrothermal synthesized titania  Titania synthesized by sol-gel (11nm), hydrothermal (24 nm) and combination of these two methods (17nm) show different electrical and optical properties  Titania synthesized by combination of sol-gel followed by hydrothermal show red shift while titania synthesized by sol-gel and hydrothermal separately show blue shift  From the above observation it can be concluded that the particle size greatly influenced on the electrical and optical properties of the catalysts

19. 19 XRD pattern of metals (Fe, Cr, Co and Zr) doped titania  Metals doping does not alter the phase formation  Metal doping reduces the crystallite growth  There is no peak observed for the formation of metal oxide  Particle size was calculated using scherrer equation. Among the metals doped, cobalt doped titania having the smaller size.

20. 20 UV-Visible DRS of metals doped titania  Cr, Fe and Co doping shifted the wavelength maximum towards visible region. Whereas zirconium showed blue shift when compared with bare titania.  The band gap energies of Co, Cr, Fe and Zr are 2.85, 2.92, 2.95 and 3.18 eV respectively.  Among the different metal doping, cobalt doped titania shows an extra absorption at 500-700nm.

21. 21 EPR spectral analysis of metals doped titania (A)(B) (C)  Spectra shows the EPR signal of (A) Fe/TiO 2 (B) Co/TiO 2 and (C) Cr/TiO 2 catalyst respectively  All the spectra show only one signal corresponding to (A) Fe 3+, (B) Co 2+ and (C) Cr 3+ in anatase matrix respectively.

22. 22 Transmission Electron Microscopy

23. 23 Extent of degradation  Among the different metal ion doped, cobalt doped titania which is having smaller particle size and increased wavelength absorption showed higher activity  Iron and chromium doped titania are almost same in catalytic activity. Among these two, iron doped titania shows better activity.  Zirconium doped titania showed poor catalytic activity when compared with others. Wavelength maximum shifts to blue line of the spectrum as zirconium concentration is increased may be the reason for this.

24. 24 Extent of mineralization  Mineralization profile follows the same order with degradation profile  Rate of Mineralization follows the order: Cobalt > Iron > Chromium > Zirconium

25. 25 Conclusion  Sol-gel method can be used for the preparation of nanosized titania with high purity but size restriction is not possible anywhere in the preparation procedure. Sol-gel method does not produce phase purity  Titania synthesized by hydrothermal method have more uniform distribution in particle size. High purity and reduced particle size is not possible in hydrothermal methods.  Titania synthesized by combination of these two methods have the advantages of sol-gel and hydrothermal methods and overcame the disadvantages.  Catalysts synthesized by combination methods have lower band gap energy and controlled particle size ( 15-20 nm) than sol-gel and hydrothermal methods.  These observations conclude that titania synthesized by the combination method is the opt one for photocatalytic activity.

26. 26  Iron, cobalt and chromium doping shifts the wavelength absorption maximum in to visible region whereas zirconium doping shifts towards UV region.  From the XRD patterns of metal doped titania, metal doping doesn’t affect the phase formation of core species i.e titania. But restricted the crystallite growth and thereby reduced the particle size.  From the EPR spectra, all the dopant ion present in anatase matrix and not in the surface is confirmed.  From the TEM histogram, the uniform distribution of particle size acquired by combination methods is confirmed. From the TEM picture, particle size of all the doped catalyst is ranging between 15-20nm is observed.  Cobalt doped titania have smaller particle size and higher wavelength of absorption than the other doped titania.  Above synthesized catalysts was tested for the photocatalytic oxidative degradation of 2, 4, 6- Trichlorophenol in aqueous suspension.

27. 27  From the degradation and mineralization profiles, cobalt shows higher catalytic activity than other doped catalysts.  Iron and chromium doped titania shows almost similar activity profile, but iron doping shows higher catalytic activity.  Zirconium doped titania which showed blue shift as the metal content increased shows poor catalytic activity.  On comparison with other dopant ion, cobalt shows higher catalytic activity even at higher concentration (1 wt%).

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