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Semiconductor Metal Oxide Nanoparticles for Visible Light Photocatalysis

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. Huang Civil and Environmental Engineering J. G. Chen Chemical Engineering

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Semiconductor Metal Oxide Nanoparticles for Visible Light Photocatalysis

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  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. Huang Civil and Environmental Engineering J. G. Chen Chemical Engineering D. Doren Chemistry and Biochemistry M. Barteau Chemical Engineering http://www.physics.udel.edu/~ismat/NIRT.htm

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

  3. Objectives • To develop an understanding of the chemical and photochemical properties of pure and modified TiO2 in nanostructure form. Modification involves the selective decoration and doping of nanoparticle surfaces. • To utilize unique physical and chemical vapor deposition processes to obtain TiO2 nanoparticles. • To modify TiO2 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. TiO2 • TiO2 is desirable for photocatalysis due to its inertness, stability, and low cost. It is also self regenerating and recyclable. Its redox potential of the H2O/*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. e- Shallow e- traps Deep e- traps Surface Recombination Volume Recombination Deep h- traps Shallow h+ traps h+ Oxidation Reaction Why nano-TiO2? • Considerations: • Volumetric Recombination • Surface Recombination • Quantum Confinement effects Reduction Reaction hn

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

  7. Schematic of MOCVD System for TiO2 Synthesis

  8. Split Cathode Magnetron

  9. TEM Characterization of TiO2 Nanoparticles 20nm (a) dark field image (b) bright field image (c) diffraction patterns • The structure of all as-grown samples is anatase. • The particle sizes from TEM range between 15 and 25 nm. (d) lattice image (d) Lattice Image

  10. XRD of TiO2 Nanoparticles as a Function of Deposition Temperature

  11. TiO2 Phase Transformation: Effect of Particle size XRD patterns from as-deposited samples and samples annealed at 700, 750, and 800 oC. The phase compositions were calculated based on formula Particle sizes were calculated. A. A. Gribb and J. F. Banfield, Am. Mineral. 82, 717 (1997). 20nm (*)

  12. Activation Energy Calculation AR=A0Exp(-Ea/KT), A0=0.884AA+AR Ea 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 TiO2 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 TiO2 = 10 mg, C0(2-CP) = 50 mg/L, Volume= 1 L, pH = 9.5, Temperature = 22 oC, P uv lamp = 100 Watts.

  14. Apparent Quantum Yields for Doped and UndopedTiO2 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. T1u A1g Eg T2g O 1s Characterizing TiO2 Nanoparticles Using Near-Edge X-ray Absorption Fine Structure (NEXAFS) -NEXAFS reveals LUMO and HOMO states (related to Eg) of TiO2 are modified Review on NEXAFS: Chen, Monograph in Surface Science Reports, Vol. 30 (1997)

  18. Theoretical Calculation of Band Gap 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 TiO2 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 TiO2. 20nm (d) lattice image

  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. Part III: Structural, Optical, Photocatalytic Properties of Nd3+ Doped TiO2 Nanoparticles 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 TiO2.

  23. Part III: Structural, Optical, Photocatalytic Properties of Nd3+ Doped TiO2 Nanoparticles Visible Light Photocatalysis ofTiO2 Nanoparticles Degradation of 2-chlorophenol: TiO2 = 5 mg, C0(2-CP)=20 mg/L, Volume=0.5 L, pH = 9.5, Temperature = 22 oC, PVisible Lamp = 100 Watts.

  24. Conclusions • Doped and undoped TiO2 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 oC had the smallest size and narrowest size distribution. • Some transition metal ions were selected to study the dopant effect on the photocatalytic efficiency and Nd3+ was found to have the highest enhancement. • The absorption range of TiO2 nanoparticles was extended into visible light region by Nd doping. • The positions of Nd in the TiO2 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. 700 oC 600 oC 500 oC 350 oC Part I: Structure and Size Distribution of TiO2 Nanoparticles TEM Results TEM bright field images, diffraction patterns and particle size distributions of undoped TiO2 nanoparticles as a function of the growth temperature. The doped TiO2 has the similar results.

  27. Part I: Structure and Size Distribution of TiO2 Nanoparticles DLS Study of TiO2 Particle Size Distribution

  28. Part I: Structure and Size Distribution of TiO2 Nanoparticles Effect of Growth Temperature on Size of TiO2

  29. Size Dependence of Structural, Optical, and Photocatalytical Properties of TiO2 Nanoparticles W. Li1, C. Ni1, H. Lin3, C.P. Huang3, S. Ismat Shah1,2 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 TiO2 is desirable for photocatalysis due to its inertness, stability, and low cost. It is also self regenerating and recyclable. Its redox potential of the H2O/*OH couple (-2.8 eV) lies within the band gap. • It is crucial to 1. design and controllably manipulate TiO2 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 TiO2 nanoparticles.

  31. 20cm Temperature profile Schematic of MOCVD System for TiO2 Synthesis Chemical reaction in the chamber Ti[OCH(CH3)2]4 + 18O2 →TiO2 + 12CO2 +14H2O

  32. Carrier gas Ar: 3 sccm. Reactant gas O2: 10 Torr. 20, 25, and 35 sccm flow rates of O2 were used to obtain different size of TiO2 nanoparticles. Ti precursor: Titanium Tetraisopropoxide Ti[OCH(CH3)2]4 (TTIP). TTIP bath temperature=220 oC (B.P.=232 oC) Growth temperature: 600 oC. Experimental Conditions (1)

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

  34. X-ray Diffraction Patterns for TiO2 with Different Particle Sizes Effect of O2 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 ±2nm for the three samples.

  35. BET Surface Area Measurements

  36. Transmission Electron Microscopy Study of TiO2 Phase Transformation (1) As-deposited 700 oC 800 oC TEM diffraction patterns for annealed and as-deposited 12-nm sample.

  37. Transmission Electron Microscopy Study of TiO2 Phase Transformation (2) 800 oC As-deposited 700 oC TEM bright field images for annealed and as-deposited 12-nm sample.

  38. X-ray Diffraction Study of TiO2 Phase Transformation (1) XRD patterns from as-deposited samples and samples annealed at 700, 750, and 800 oC. The phase compositions were calculated based on formula Particle sizes were calculated. A. A. Gribb and J. F. Banfield, Am. Mineral. 82, 717 (1997). 20nm (*) (d) lattice image

  39. Activation Energy Calculation AR=A0Exp(-Ea/KT), A0=0.884AA+AR Ea 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 TiO2 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] • TiO2 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 B17nm < B12nm < B23nm

  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 TiO2 including surface area, light absorption efficiency, and charge carrier recombination rate.

  44. Conclusions • TiO2 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.

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