1. Semiconductor Metal Oxide Nanoparticles for Visible Light Photocatalysis NSF NIRT Grant No. 0210284 University of Delaware S. Ismat Shah Materials Science and Engineering Physics and Astronomy C.P. HuangCivil and Environmental Engineering J. G. ChenChemical Engineering D. DorenChemistry and Biochemistry M. BarteauChemical Engineering http://www.physics.udel.edu/~ismat/NIRT.htm

2. Students and Post-Docs M. Barakat:Materials Science and Engineering (Post-Doc)M. Barakat:Materials Science and Engineering (Post-Doc) S. Rayko: Chemical Engineering (Post-Doc)S. Rayko: Chemical Engineering (Post-Doc) S. Lin: Graduate Student, Civil and Environmental Engin.S. Lin: Graduate Student, Civil and Environmental Engin. Y. Wang: Graduate Student, Chemistry and BiochemistryY. Wang: Graduate Student, Chemistry and Biochemistry S. Chan: Graduate Student, Chemical EngineeringS. Chan: Graduate Student, Chemical Engineering J. McCormick: Graduate Student, Chemical EngineeringJ. McCormick: Graduate Student, Chemical Engineering W. Li: Graduate Student, Materials Science and Engin.W. Li: Graduate Student, Materials Science and Engin. S. Buzby: Graduate Student, Materials Science and Engin.S. Buzby: Graduate Student, Materials Science and Engin. Greg Hayes: Undergraduate Student, Mechanical Engin.Greg Hayes: Undergraduate Student, Mechanical Engin. Holly Sheaffers: Undergraduate Student, Chemical Engin.Holly Sheaffers: Undergraduate Student, Chemical Engin.

3. Objectives To develop an understanding of the chemical and photochemical properties of pure and modified TiO 2 in nanostructure form. Modification involves the selective decoration and doping of nanoparticle surfaces. To utilize unique physical and chemical vapor deposition processes to obtain TiO 2 nanoparticles. visible light photocatalysis.To modify TiO 2 nanoparticles to induce visible light photocatalysis. To characterize the nanoparticles for structural, chemical and optoelectronic properties. To utilize first-principles calculations to acquire an atomistic understanding of nanoparticle properties.

4. TiO 2 TiO 2 is desirable for photocatalysis due to its inertness, stability, and low cost. It is also self regenerating and recyclable. Its redox potential of the H 2 O/*OH couple (-2.8 eV) lies within the band gap. However, its large band gap (Eg=3.2 eV) only allows absorption the UV of solar spectrum. An absorber in the visible range is desired. Absorption in the visible range can be improved by dye sensitization, doping, particle size modification, and surface modification by noble metals.

5. Why nano-TiO 2 ? Considerations: –Volumetric Recombination –Surface Recombination –Quantum Confinement effects h+h+ e-e- Oxidation Reaction Shallow e - traps Shallow h + traps Deep e - traps Deep h - traps Volume RecombinationSurface Recombination Reduction Reaction h

6. Methodology Study Size Effects Study Doping Effects Characterize Photocatalytic Properties

7. Schematic of MOCVD System for TiO 2 Synthesis

8. Split Cathode Magnetron

9. (b) bright field image TEM Characterization of TiO 2 Nanoparticles The structure of all as-grown samples is anatase. The particle sizes from TEM range between 15 and 25 nm. (a) dark field image(c) diffraction patterns (d) lattice image 20nm (d) Lattice Image

10. XRD of TiO 2 Nanoparticles as a Function of Deposition Temperature

11. TiO 2 Phase Transformation: Effect of Particle size 20nm XRD patterns from as-deposited samples and samples annealed at 700, 750, and 800 o C. The phase compositions were calculated based on formula Particle sizes were calculated. A. A. Gribb and J. F. Banfield, Am. Mineral. 82, 717 (1997). (*)

12. Activation Energy Calculation A R =A 0 Exp(-E a /KT), A 0 =0.884A A +A R E a is anatase to rutile transformation activation energy. The activation energy decreases with the particle size and 12-nm sample has the lowest activation energy of 180.28 kJ/mol. Bulk TiO 2 has activation energy of 450 kJ/mol.(*) (*) H. Zhang and J. F. Banfield, Am. Mineral. 84, 528 (1999).

13. The Effect of Dopants on Photocatalytic Kinetics: Degradation of 2-chlorophenol TiO 2 = 10 mg, C 0(2-CP) = 50 mg/L, Volume= 1 L, pH = 9.5, Temperature = 22 o C, P uv lamp = 100 Watts.

14. Apparent Quantum Yields for Doped and Undoped TiO 2 Nanoparticles

15. Ionic Radii of the Dopants

16. Band Gap Calculation from Light Absorption a: Nd=0% b: Nd=0.6% c: Nd=1% d: Nd=1.5%

17. O 1s T 2g EgEg A 1g T 1u -NEXAFS reveals LUMO and HOMO states (related to Eg) of TiO 2 are modified Review on NEXAFS: Chen, Monograph in Surface Science Reports, Vol. 30 (1997) Characterizing TiO 2 Nanoparticles Using Near-Edge X-ray Absorption Fine Structure (NEXAFS)

18. Theoretical Calculation of Band Gap (d) lattice image 20nm Density functional theory calculations using the generalized gradient approximation with the linearized augmented plane wave method are used to interpret the band gap narrowing. Some electronic states are introduced into the band gap of TiO 2 by substitutional Nd 4f electrons, to form the new LUMO band. The absorption edge transition for the doped material can be from O 2p to Nd 4f instead of Ti 3d, as in pure TiO 2.

19. Short Term Program Optimization of the doping concentration Combined nanosize and doping effects Nd: Substitutional or interstitial? NEXAFS and EXAFS analyses. Theoretical calculations of bandgap variation with the doping type and concentrations. Degradation kinetics, intermediates, etc.

20. Long Term Program Photocatalysis with visible light Anion doping: C,O,N Surface decoration with Pt-group metals nanoparticles for charge transfer enhancement DLTS characterization for dopant level Transient absorption spectroscopy to study the carrier life time in nanoparticles.

21. Outreach Activities Vacuum on wheels: A demonstration unit for area middle schools showing the affects and uses of vacuum. Nanotechnology and Society: A lecture series being developed for local school and junior colleges. Minority recruitment activities for participation in the NIRT program. Visit our web site: http://www.physics.udel.edu/~ismat/NIRT.htm

22. XRD Result Only anatase phase is detected for all (0.6%, 1%, and 1.5% Nd) doped and undoped samples. These diffraction patterns are from 1% Nd doped TiO 2. Part III: Structural, Optical, Photocatalytic Properties of Nd 3+ Doped TiO 2 Nanoparticles

23. Visible Light Photocatalysis of TiO 2 Nanoparticles Part III: Structural, Optical, Photocatalytic Properties of Nd 3+ Doped TiO 2 Nanoparticles Degradation of 2-chlorophenol: TiO 2 = 5 mg, C 0(2-CP) =20 mg/L, Volume=0.5 L, pH = 9.5, Temperature = 22 o C, P Visible Lamp = 100 Watts.

24. Conclusions  Doped and undoped TiO 2 nanoparticles were synthesized by MOCVD method.  The effect of growth temperature on particle size and size distribution was investigated. Results showed that particles deposited at 600 o C had the smallest size and narrowest size distribution.  Some transition metal ions were selected to study the dopant effect on the photocatalytic efficiency and Nd 3+ was found to have the highest enhancement.  The absorption range of TiO 2 nanoparticles was extended into visible light region by Nd doping.  The positions of Nd in the TiO 2 lattice are being studied.  Measurements of electric current and photocatalysis under irradiation of visible light are being carried out.

25. Acknowledgements We would like to thank NSF - NIRT for financial support of this project.

26. 350 o C 700 o C 600 o C 500 o C TEM bright field images, diffraction patterns and particle size distributions of undoped TiO 2 nanoparticles as a function of the growth temperature. The doped TiO 2 has the similar results. TEM Results Part I: Structure and Size Distribution of TiO 2 Nanoparticles

27. DLS Study of TiO 2 Particle Size Distribution Part I: Structure and Size Distribution of TiO 2 Nanoparticles

28. Effect of Growth Temperature on Size of TiO 2 Part I: Structure and Size Distribution of TiO 2 Nanoparticles

29. Size Dependence of Structural, Optical, and Photocatalytical Properties of TiO 2 Nanoparticles W. Li 1, C. Ni 1, H. Lin 3, C.P. Huang 3, S. Ismat Shah 1,2 1. Department of Materials Science and Engineering 2. Department of Physics and Astronomy 3. Department of Civil and Environmental Engineering University of Delaware, Newark, DE 19716.

30. Motivation Anatase TiO 2 is desirable for photocatalysis due to its inertness, stability, and low cost. It is also self regenerating and recyclable. Its redox potential of the H 2 O/*OH couple (-2.8 eV) lies within the band gap. It is crucial to 1. design and controllably manipulate TiO 2 phase types and concentrations for more efficient photocatalysis. 2. determine the optimal size for highest photoreactivity. So, we would like to study the effect of particle size on the phase thermal stability, optical, and photoreactivity of TiO 2 nanoparticles.

31. Schematic of MOCVD System for TiO 2 Synthesis Temperature profile 20c m Chemical reaction in the chamber Ti[OCH(CH 3 ) 2 ] 4 + 18O 2 →TiO 2 + 12CO 2 +14H 2 O

32. Experimental Conditions (1) Carrier gas Ar: 3 sccm. Reactant gas O 2 : 10 Torr. 20, 25, and 35 sccm flow rates of O 2 were used to obtain different size of TiO 2 nanoparticles. Ti precursor: Titanium Tetraisopropoxide Ti[OCH(CH 3 ) 2 ] 4 (TTIP). TTIP bath temperature=220 o C (B.P.=232 o C) Growth temperature: 600 o C.

33. Experimental Conditions (2) Annealing conditions: Isochronal annealings were carried out with temperatures 700, 750, and 800 o C for 1 hr in the air.

34. X-ray Diffraction Patterns for TiO 2 with Different Particle Sizes Effect of O 2 gas flow rate on particle size. All peaks belong to the anatase phase and no other phase is detected within the X-ray detection limit The measured average particle sizes were 12 ±2, 17 ±2, and 23 ±2 nm for the three samples.

35. Samples 12 nm 17 nm 23 nm P25 Surface Areas (±5 m 2 /g) 125956560 BET Surface Area Measurements

36. Transmission Electron Microscopy Study of TiO 2 Phase Transformation (1) As-deposited700 o C800 o C TEM diffraction patterns for annealed and as-deposited 12-nm sample.

37. Transmission Electron Microscopy Study of TiO 2 Phase Transformation (2) As-deposited700 o C 800 o C TEM bright field images for annealed and as-deposited 12-nm sample.

38. X-ray Diffraction Study of TiO 2 Phase Transformation (1) (d) lattice image 20nm XRD patterns from as-deposited samples and samples annealed at 700, 750, and 800 o C. The phase compositions were calculated based on formula Particle sizes were calculated. A. A. Gribb and J. F. Banfield, Am. Mineral. 82, 717 (1997). (*)

39. Activation Energy Calculation A R =A 0 Exp(-E a /KT), A 0 =0.884A A +A R E a is anatase to rutile transformation activation energy. The activation energy decreases with the particle size and 12-nm sample has the lowest activation energy of 180.28 kJ/mol. Bulk TiO 2 has activation energy of 450 kJ/mol.(*) (*) H. Zhang and J. F. Banfield, Am. Mineral. 84, 528 (1999).

40. Mechanism of Phase Transformation  Interface boundary atomic migration is the primary source for phase growth. This has been previously reported by other researchers. [A, B]  TiO 2 nanoparticles have smaller activation energy. It is easier to overcome the energy barrier to new phase. Smaller particles have lower activation energy. References: [A] T. C. Chou and T. G. Nieh, Thin Solid Films 221, 89 (1992). [B] P. I. Gouma, P. K. Dutta, and M. J. Mills, NanoStruct. Mater. 11, 1231 (1999).

41. Size Dependence of Light Absorption 17 nm sample has the largest red shift. Comparison of band gaps B 17nm < B 12nm < B 23nm

42. Size Dependence of Photoreactivity Photodegradation of 2-chlorophenol solutions with different size samples. 17 nm sample has the highest photoreactivity.

43. The Optimal Size  17 nm sample has the highest photoreactivity compared with 12 nm and 23 nm samples.  The optimal size is determined by several aspects of TiO 2 including surface area, light absorption efficiency, and charge carrier recombination rate.

44. Conclusions  TiO 2 nanoparticles with different sizes were synthesized by MOCVD.  The particle size role in the anatase to rutile phase transformation was studied. The activation energies for particles were calculated to be 180.28, 236.38, and 298.85 kJ/mol for 12, 17, and 23 nm samples, respectively.  The 17 nm sample had the smallest band gap and highest photoreactivity compared with the other samples.

45. Acknowledgements We would like to thank NSF - NIRT for the funding of this project.