Semiconductor Metal Oxide Nanoparticles
for Visible Light Photocatalysis
NSF NIRT Grant No. 0210284
University of Delaware
S. Ismat Shah
C.P. Huang
J. G. Chen
D. Doren
M. Barteau
Materials Science and Engineering
Physics and Astronomy
Civil and Environmental Engineering
Chemical Engineering
Chemistry and Biochemistry
Chemical Engineering
http://www.physics.udel.edu/~ismat/NIRT.htm
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.
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.
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.
Why nano-TiO2?
• Considerations:
– Volumetric Recombination
– Surface Recombination
– Quantum Confinement effects
Shallow
e-
traps
Reduction
Reaction
eDeep e- traps
Surface Recombination
Volume Recombination
Deep h- traps
Shallow h+ traps
h+
Oxidation
Reaction
hn
Methodology
• Study Size Effects
• Study Doping Effects
• Characterize Photocatalytic Properties
Schematic of MOCVD System for TiO2 Synthesis
Split Cathode Magnetron
Water-cooled
copper coil for
electromagnet
Sputtering
gases inlet
Nanoparticle
collector
Split Cathodes
Transformer
AC Power Supply
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
XRD of TiO2 Nanoparticles as a Function of
Deposition Temperature
20
400
30
40
50
A: Anatase
R: Rutile
A
Intensity (arb. units)
R
300
60
R A
o
A
700 C
R
A
A
A
400
R R
300
o
600 C
200
200
o
500 C
100
100
o
350 C
o
250 C
0
20
0
30
40
2 (degree)
50
60
TiO2 Phase Transformation: Effect of Particle size
Intensity (arb. units)
23 nm (b)
o
800 C
XRD patterns from as-deposited
samples and samples annealed
at 700, 750, and 800 oC.
The phase compositions were
calculated based on formula
o
750 C
o
700 C
20nm
as-deposited
12 nm (a)
R(110)
o
Intensity (arb. units)
800 C
R(101)
R(211)
R(220)
R(111)
WR 
AR
AR

A0 0.884 AA  AR
Particle sizes were calculated.
o
750 C
o
700 C
(*) A. A. Gribb and J. F. Banfield, Am.
Mineral. 82, 717 (1997).
A(101)
as-deposited
A(004)
20
30
40
2 (deg.)
A(200) A(105)
A(211)
50
60
Activation Energy Calculation
0
0.00092
0.00096
0.00100
0.00104
0.00100
0.00104
Ln(AR/A0)
-1
-2
-3
-4
-5
12nm (Ea=180.28kJ/mol)
17nm (Ea=236.38kJ/mol)
23nm (Ea=298.85kJ/mol)
R=0.995
R=0.998
R=0.991
0.00092
0.00096
-1
1/T (K )
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-chlorophenol
0
30
60
90
120
150
1.0
1.0
3+
Relative Concentration (C/Co)
TiO2(Nd )
2+
TiO2(Pd )
0.8
TiO2(Pt
0.8
4+
)
3+
TiO2(Fe )
Pure TiO2
Degussa P25
0.6
0.6
0.4
0.4
0.2
0.2
0.0
0
0.0
30
60
90
Reaction Time (min)
TiO2 = 10 mg, C0(2-CP) = 50 mg/L,
Volume= 1 L, pH = 9.5, Temperature = 22 oC,
P uv lamp = 100 Watts.
120
150
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 TiO 2 nanoparticles
in the reactor.
Catalysts
Initial rate
10 R in (mol/min)
Photon flux
10 Ro ( 365) (Einstein/min)
29.2  0.25
17.6  0.82
12.9  0.47
3.74  0.64
9.72  0.82
11.7  0.25
4.42
4.42
4.42
4.42
4.42
4.42
6
TiO2 (Nd3+)
TiO2 (Pd3+)
TiO2 (Pt4+)
TiO2 (Fe3+)
Pure TiO2
Degussa P25
4
Quantum yield
 10  2 CP  R in Ro ( 365)
2
6.61  0.06
3.98  0.19
2.92  0.11
0.84  0.14
2.20  0.19
2.65  0.06
Ionic Radii of the Dopants
Ions
Ionic radii (Å)
Ti4+
0.605
Pt4+
0.625
Fe3+
0.645
Pd2+
0.86
Nd3+
0.983
Band Gap Calculation from Light Absorption
3.2
4
2.8
3
a
1/2
2.6
(E)
Band Gap (eV)
3
2.4
b c
d
a: Nd=0%
b: Nd=0.6%
c: Nd=1%
d: Nd=1.5%
2
2.2
1
2
0
0.5
1
1.5
Nd Concentration (at.%)
2
5
4
3
Energy (eV)
2
Characterizing TiO2 Nanoparticles Using
INTENSITY (Arb. Units)
Near-Edge X-ray Absorption Fine Structure (NEXAFS)
525
534.25
535.25
532.5
534.5
O K-edge
Eg = 1.75 eV
Nd-doped
20 nmTiO 2
T1u
A1g
Eg
Eg = 2.0 eV
20 nm TiO 2
T2g
531.25
534.0
Eg = 2.75 eV
Bulk TiO2
530
535
540
545
550
555
INCIDENT PHOTON ENERGY (eV)
560
O 1s
-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)
Theoretical Calculation of Band Gap
16
arb. units
12
TiO2 Gap: 2.26 eV
total
O-2p
Ti-3d
(a)
2p
3d
8
4
20nm
0
100-10 -8 -6 -4 -2 0
2
4
6
NdTi7O16 Gap: 1.97 eV
arb. units
80
60
8 10
(b)
total
O-2p
Nd-4f
40
2p 4f
20
0
-10 -8 -6 -4 -2 0
eV
2
(d) lattice image
4
6
8 10
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.
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.
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.
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
Part III: Structural, Optical, Photocatalytic Properties of Nd3+ Doped TiO2 Nanoparticles
XRD Result
20
200
25
30
35
40
45
50
55
60
o
(101)
200
Tsubstrate = 600 C
o
Intensity (arb. units)
Tsolution = 220 C
100
100
(200)
(004)
0
20
25
30
35
40
(105)(211)
45
2 (degree)
50
55
0
60
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.
Part III: Structural, Optical, Photocatalytic Properties of Nd3+ Doped TiO2 Nanoparticles
Visible Light Photocatalysis of
Relative Concentration (C/C0)
TiO2 Nanoparticles
1
0.98
0.96
0.94
0.92
0.9
0.88
TiO2(Nd1%)
0.86
0.84
No TiO2
0.82
TiO2(pure)
0.8
0
10
20
30
40
50
60
70
Reaction Time (min)
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.
80
90
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.
Part I: Structure and Size Distribution of TiO2 Nanoparticles
30
700 oC
Frequency (%)
25
25
20
20
15
15
10
10
5
5
0
40
oC
Frequency (%)
600
TEM Results
30
o
700 C
0
40
o
600 C
35
35
30
30
25
25
20
20
15
15
10
10
5
5
0
25
500 oC
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.
0
o
500 C
Frequency (%)
20
15
10
5
35
o
350 C
30
30
25
25
Frequency (%)
350
0
35
oC
20
20
15
15
10
10
5
5
0
0
16
18
20
22
24
26
28
Particle Size (nm)
30
32
34
Part I: Structure and Size Distribution of TiO2 Nanoparticles
DLS Study of TiO2 Particle Size Distribution
10
100
Number of Particles
100
100
80
80
60
60
40
40
20
20
0
0
10
100
Diameter (nm)
Part I: Structure and Size Distribution of TiO2 Nanoparticles
Effect of Growth Temperature on Size of TiO2
Particle Size (nm)
40
350
400
450
500
550
600
650
700
750
40
35
35
30
30
25
25
20
20
15
15
10
10
5
350
400
450
500
550
600
650
o
Substrate Temperature ( C)
700
750
5
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
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 TiO nanoparticles.
Schematic of MOCVD System for TiO2 Synthesis
20c
m
Temperature profile
Chemical reaction in the chamber
Ti[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
20
30
40
50
Intensity (arb. units)
24.5
Intensity (arb. units)
(101)
23 nm (004)
25.0
60
25.5
26.0
26.5
Effect of O2 gas flow rate on
particle size.
Anatase (101)
23 nm
17 nm
12 nm
24.5 25.0 25.5 26.0 26.5
2(deg.)
(200)
(105) (211)
17 nm
12 nm
20
30
40
2(deg.)
50
60
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
Samples
12
nm
17
nm
23
nm
P25
Surface Areas (±5
m2/g)
125
95
65
60
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.
Transmission Electron Microscopy Study of
TiO2 Phase Transformation (2)
As-deposited
700 oC
800 oC
TEM bright field images
for annealed and as-deposited 12-nm sample.
X-ray Diffraction Study of
TiO2 Phase Transformation (1)
Intensity (arb. units)
23 nm (b)
o
800 C
XRD patterns from as-deposited
samples and samples annealed
at 700, 750, and 800 oC.
The phase compositions were
calculated based on formula
o
750 C
o
700 C
20nm
as-deposited
12 nm (a)
R(110)
o
Intensity (arb. units)
800 C
R(101)
R(211)
R(220)
R(111)
WR 
AR
AR

A0 0.884 AA  AR
Particle sizes were calculated.
o
750 C
o
700 C
(*) A. A. Gribb and J. F. Banfield, Am.
Mineral. 82, 717 (1997).
A(101)
as-deposited
A(004)
20
30
(d) lattice image
40
2 (deg.)
A(200) A(105)
A(211)
50
60
Activation Energy Calculation
0.00092
0.00096
0.00100
0.00104
0.00100
0.00104
0
Ln(AR/A0)
-1
-2
-3
-4
-5
12nm (Ea=180.28kJ/mol)
17nm (Ea=236.38kJ/mol)
23nm (Ea=298.85kJ/mol)
R=0.995
R=0.998
R=0.991
0.00092
0.00096
-1
1/T (K )
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
Absorbance
300
400
500
600
700
4
(a)
3
a1 (12nm)
a2 (17nm)
a3 (23nm)
2
a1
a3
a2
17 nm sample has
the largest red shift.
Comparison of band gaps
B17nm < B12nm < B23nm
1
0
300
400 500 600
Wavelength
5
4
3
700
2
4
4
(b)
3
(E)
1/2
a1
3
a2
a3
2
2
1
0
1
5
4
3
Energy (eV)
2
0
Size Dependence of Photoreactivity
Relative Concentration (C/C0)
1
0.9
12 nm
0.8
17 nm
Photodegradation of 2-chlorophenol
solutions with different size samples.
17 nm sample has
the highest photoreactivity.
23 nm
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
20
40
60
Reaction Time (min)
80
100
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.
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.