Transcript Structural, electronic and optical properties of TiO2

Structural, electronic and optical
properties of TiO2 nanoparticles
Matti Alatalo, Sami Auvinen, Heikki Haario
Lappeenranta University of Technology
Juho Jalava, Ralf Lamminmäki
Sachtleben Pigments
Outline
−
−
−
−
−
Motivation, earlier studies
Methods: Brief description
Ab initio results
Simpler approaches
Outlook
Industrial use of TiO2 nanoparticles
− TiO2 pigments are widely used in the industry:
whiteness, opacity
− Nano-TiO2: Plastics, coatings, cosmetics
− Particle size and shape distribution important for
applications
− These distributions can be solved by measuring the
turbidity spectrum of a dilute solution: A nontrivial
inverse problem
Measurement of turbidity spectrum of
rutile or anatase pigments
Turbidity spectra of sample (normalized to 10 mg/l):
XRDI-S 483.44 21.3.05/06.30
0.25
measured
calculated
calculated and norm
pigment + water
+ dispersing agent (MIPA)
0.2
I  I 0e
Light to the sample
absorbance
    L
0.15
0.1
0.05
200
p1011054
TUOTEKEH.LAB.
weight
0.1204 g
conc.
11.33 mg/l
looseness
0.2 w%
300
400
500
600
700
wavelength, nm
800
900
1000
1100
Calculation of the turbidity
− When the refractive index of a material is known at different
wavelengths, the turbidity can be calculated rigorously, e.g., for
spheroid

a

m
     N (q, a )Cext  q,
−
−
−
−
−
−
−
,
np    

nm    
N is the number of particles,
a is the width of spheroid
q is the length/width
Cext is the extinction coefficient
n is the refractive index
p refers to the particle and
m refers to the medium
Cext-matrix for spheroids as function of
wavelength and crystal size diameter
calculated by the T-matrix method
1.5
1.5
1
Cext
Cext
1
0.5
0.5
0
600
0
600
400
400
1000
200
800
200
vol. eq. crystal
size diameter, nm
1000
600
0
Length/width 1.1
400
wavelength, nm
800
vol. eq. crystal
size diameter, nm
600
0
400
Length/width 2.1
wavelength, nm
Limitations of the T-matrix modeling
Fitting is moderate but the error in numerical results is much larger than
expected.
Turbidity spectra of sample (normalized to 10 mg/l):
XRD: 8 nm
2
measured
calculated
calculated and norm
absorbance
1.5
uvtsmfige8
mitattu
weight
conc.
looseness
spektrin kunto
wl(max)
abs(max)
abs(450 nm)
U/V*100
1
0.5
0
200
400
600
800
wavelength, nm
0.1000 g
10.00 mg/l
-117.3 w%
7 16 0 0 (koko UV VIS IR)
278 nm
1.991
0.062
3191
1000
1200
Limitations of the T-matrix modeling
− The results are not good at particle sizes below 200 nm
and wavelengths below 360 nm
− Quantum size effect?
Methods
− Structures, spectra: Density functional calculations as
implemented in the GPAW code
− Projector augmented wave method in real space grids
− Structures, spectra: Density functional tight binding as
implemented in the Hotbit code
− First attempts (testing of the parametrization)
− T-matrix modeling
− Particle size distributions
Details of the GPAW calculation
− Clusters of the size 18-38 TiO2 units were carved from
anatase/rutile bulk (Smaller ones composed of TiO2
molecules)
− For small particles, anatase is known to be the
ground state structure
− The structures were allowed to relax
− Several different structures per particle size were tested
− Absorption spectra were calculated using time
propagation TDDFT
− Grid parameter h=0.17 for structural relaxations, h=0.3
for the calculation of the absorption spectra
Results: Absorption spectra
Atomic vs. electronic structure
(TiO2)28
•Red: O
•Blue: Ti
Effect of structure on the adsorption
spectra
•A:
•B:
Effect of structure on the adsorption
spectra
•A:
•B:
Contributions of different directions
•Note: Bulk anatase is birefringent
Observations
− Structure plays an important role on the absorption
spectra
− Longest dimension dominates
− Compact structures energetically favorable
Density functional tight binding,
first results
•Green:
•GPAW
•Blue:
•DFTB