Semiconductor Metal Oxide Nanoparticles for Visible Light Photocatalysis

NSF NIRT Grant No. 0210284

University of Delaware

S. Ismat Shah Materials Science and EngineeringPhysics and AstronomyC.P. HuangCivil and Environmental EngineeringJ. G. ChenChemical EngineeringD. DorenChemistry and BiochemistryM. BarteauChemical Engineeringhttp://www.physics.udel.edu/~ismat/NIRT.htm

Students and Post-DocsM. 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 BiochemistryS. Chan: Graduate Student, Chemical EngineeringJ. McCormick: Graduate Student, Chemical EngineeringW. 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.

ObjectivesTo 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.

TiO2TiO2 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.

Why nano-TiO2?Considerations:Volumetric RecombinationSurface RecombinationQuantum Confinement effectsReduction Reactionhn

MethodologyStudy Size EffectsStudy Doping EffectsCharacterize Photocatalytic Properties

Schematic of MOCVD System for TiO2 Synthesis

Split Cathode Magnetron

Sputtering gases inlet

Nanoparticle collector

Split Cathodes

Transformer

Water-cooled copper coil for electromagnet

AC Power Supply

TEM Characterization of TiO2 NanoparticlesThe structure of all as-grown samples is anatase.The particle sizes from TEM range between 15 and 25 nm.(b) bright field image(a) dark field image(c) diffraction patterns(d) lattice image20nm(d) Lattice Image

XRD of TiO2 Nanoparticles as a Function of Deposition Temperature

TiO2 Phase Transformation: Effect of Particle size 20nmXRD 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). (*)

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).

The Effect of Dopants on Photocatalytic Kinetics:

Degradation of 2-chlorophenolTiO2 = 10 mg, C0(2-CP) = 50 mg/L, Volume= 1 L, pH = 9.5, Temperature = 22 oC, P uv lamp = 100 Watts.

Apparent Quantum Yields for Doped and Undoped

TiO2 Nanoparticles

Table I. Estimations of initial photodegradation rate, UV photon flux, and apparent

quantum yield for aqueous solutions of 2CP with doped and undoped TiO2 nanoparticles

in the reactor.

Catalysts

Initial rate

(mol/min)

Photon flux

(Einstein/min)

Quantum yield

TiO2 (Nd3+)

29.2 ( 0.25

4.42

6.61 ( 0.06

TiO2 (Pd3+)

17.6 ( 0.82

4.42

3.98 ( 0.19

TiO2 (Pt4+)

12.9 ( 0.47

4.42

2.92 ( 0.11

TiO2 (Fe3+)

3.74 ( 0.64

4.42

0.84 ( 0.14

Pure TiO2

9.72 ( 0.82

4.42

2.20 ( 0.19

Degussa P25

11.7 ( 0.25

4.42

2.65 ( 0.06

_1062245448.unknown

_1062497726.unknown

_1062245419.unknown

Ionic Radii of the Dopants

Ions

Ti4+

Pt4+

Fe3+

Pd2+

Nd3+

Ionic radii ()

0.605

0.625

0.645

0.86

0.983

Band Gap Calculation from Light Absorptiona: Nd=0%b: Nd=0.6%c: Nd=1%d: Nd=1.5%

]Chart2

3.18

2.83

2.5

2.3

Nd Concentration (at.%)

Band Gap (eV)

]Sheet1

03.18

0.62.83

12.5

1.52.3

]Sheet1

Nd Concentration (at.%)

Band Gap (eV)

]Sheet2

]Sheet3

-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)Characterizing TiO2 Nanoparticles Using Near-Edge X-ray Absorption Fine Structure (NEXAFS)

Theoretical Calculation of Band Gap(d) lattice image20nmDensity 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.

Short Term ProgramOptimization of the doping concentrationCombined nanosize and doping effectsNd: Substitutional or interstitial? NEXAFS and EXAFS analyses.Theoretical calculations of bandgap variation with the doping type and concentrations.Degradation kinetics, intermediates, etc.

Long Term ProgramPhotocatalysis with visible light Anion doping: C,O,NSurface decoration with Pt-group metals nanoparticles for charge transfer enhancementDLTS characterization for dopant level Transient absorption spectroscopy to study the carrier life time in nanoparticles.

Outreach ActivitiesVacuum 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

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

Visible Light Photocatalysis of TiO2 NanoparticlesPart III: Structural, Optical, Photocatalytic Properties of Nd3+ Doped TiO2 NanoparticlesDegradation 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.

Chart1

111

0.95990099010.99950.976

0.94455445540.9980.9689

0.92920792080.9970.9589

0.89257425740.99250.9446

0.85643564360.98550.9252

0.81683168320.9830.9112

TiO2(Nd1%)

No TiO2

TiO2(pure)

Reaction Time (min)

Relative Concentration (C/C0)

Sheet1

0120201191.5011960120001

100.9599009901193.9100.9117493473174.6101.0454081633204.9100.9995199.9100.976

200.9445544554190.8200.8428198433161.4201.0285714286201.6200.998199.6200.9689

300.9292079208187.7300.87154047166.9300.9964285714195.3300.997199.4300.9589

500.8925742574180.3500.8156657963156.250500.9925198.5500.9446

700.8564356436173700.7775456919148.9700.9494897959186.1700.9855197.1700.9252

900.8168316832165900.7347258486140.7900.9005102041176.5900.983196.6900.9112

1200.6767624021129.61300.8295918367162.6

Sheet1

000

000

000

000

000

000

000

0

TiO2(Nd1)

Vis-Only

TiO2(pure)

Reaction Time (min)

Relative Concentration (C/C0)

Sheet2

Sheet3

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.

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

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.TEM ResultsPart I: Structure and Size Distribution of TiO2 Nanoparticles

DLS Study of TiO2 Particle Size DistributionPart I: Structure and Size Distribution of TiO2 Nanoparticles

EMBED Origin50.Graph

_1033135056.bin

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

EMBED Origin50.Graph

_1063551617.bin

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.

MotivationAnatase 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.

Schematic of MOCVD System for TiO2 Synthesis Chemical reaction in the chamberTi[OCH(CH3)2]4 + 18O2 TiO2 + 12CO2 +14H2O

Experimental Conditions (1)

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

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 2 nm for the three samples.

BET Surface Area Measurements

Transmission Electron Microscopy Study of TiO2 Phase Transformation (1)As-deposited700 oC800 oCTEM diffraction patterns for annealed and as-deposited 12-nm sample.

Transmission Electron Microscopy Study of TiO2 Phase Transformation (2)As-deposited700 oC800 oCTEM bright field images for annealed and as-deposited 12-nm sample.

X-ray Diffraction Study of TiO2 Phase Transformation (1) (d) lattice image20nmXRD 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). (*)

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).

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).

Size Dependence of Light Absorption 17 nm sample has the largest red shift. Comparison of band gaps B17nm < B12nm < B23nm

Size Dependence of Photoreactivity

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

Chart2

111

0.9980582520.9848076920.992376112

0.9555339810.9346153850.926483935

0.735922330.5903846150.774913777

0.5584466020.3638461540.65347613

0.3943689320.1850.550009076

0.2155339810.0653846150.371755309

0.0848543690.0128846150.237611182

12 nm

17 nm

23 nm

Reaction Time (min)

Relative Concentration (C/C0)

Sheet1

12 nm17 nm23 nmP25

01111

50.9980582520.9848076920.9923761120.987191561

100.9555339810.9346153850.9264839350.937841401

200.735922330.5903846150.7749137770.787907327

300.5584466020.3638461540.653476130.640045206

400.3943689320.1850.5500090760.525334338

600.2155339810.0653846150.3717553090.169523451

900.0848543690.0128846150.2376111820.033716331

Sheet1

000

000

000

000

000

000

000

000

12 nm

17 nm

23 nm

Reaction Time (min)

Relative Concentration (C/C0)

Sheet2

Sheet3

The Optimal Size17 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.

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.

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