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Black Titanium Dioxide: A New
Engineered Nanoparticle for
Photocatalysis.
Peter Y. YU
Department of Physics, University of California
&
Lawrence Berkeley National Laboratory
Berkeley, CA 94720
1
Nov 2012
CUHK
ACKNOWLEDGMENTS
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EXPERIMENTAL COLLABORATORS:
Xiaobo Chen & Nathan A. Oyler University of Missouri - Kansas City, Department of
Chemistry, Kansas City, MO 64110, USA.
Zhi Liu, Matthew A. Marcus, Michael E. Grass, Per-Anders Glans, & Jinghua Guo
Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
Wei-Cheng Wang, Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley,
CA 94720, USA. &
Department of Physics, Tamkang University, Tamsui, Taiwan 250, R.O.C.
Baohua Mao,Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA
94720, USA. &
Institute of Functional Nano & Soft Materials Laboratory, Soochow University, Suzhou, Jiangsu
215123, China
Samuel S. Mao, Department of Mechanical Engineering, University of California at Berkeley,
Berkeley, CA 94720, USA. &
Advanced Energy Technology Department, EETD, Lawrence Berkeley National Laboratory,
Berkeley, CA 94720, USA.
THEORETICAL COLLABORATOR:
Lei Liu, State Key Laboratory of Luminescence and Applications, Changchun Institute of
Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, 3888 Dongnanhu Road,
Changchun, 130033, People’s Republic of China
Nov 2012
CUHK
OUTLINE
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1. MOTIVATION
2. INTRODUCTION TO TiO2
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3. FABRICATION OF BLACK TiO2
4. PROPERTIES OF BLACK TiO2
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3
PHOTOCATALYTIC PROPERTIES
STRUCTURAL & VIBRATIONAL PROPERTIES
ELECTRONIC PROPERTIES
DEFECT PROPERTIES
5. MODELING BLACK TiO2
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CRYSTAL STRUCTURE
ELECTRONIC STRUCTURE
WHY TiO2 IS A GOOD PHOTOCATALYST FOR ENERGY AND ENVIRONMENT
DEFECTS & IMPURITIES IN TiO2
FIRST PRINCIPLE CALCULATION
CLUSTER MODELS
6. CONCLUSIONS
Nov 2012
CUHK
MOTIVATION
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TWO IMPORTANT PROBLEMS FACING
THE WORLD TODAY ARE:
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ONE WAY TO SOLVE BOTH PROBLEMS IS
TO USE SOLAR RADIATION TO
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AIR & WATER POLLUTION
GLOBAL WARMING FROM BURING OF FOSSIL
FUEL
REDUCE WATER POLLUTION
PRODUCE RENEWABLE ENERGY
Nov 2012
CUHK
ONE MATERIAL TO ACHIEVE BOTH GOALS
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PHOTOCATALYST:
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TITANIUM DIOXIDE (TiO2) IS AN IDEAL
PHOTOCATALYST:
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USE LIGHT TO REMOVE ORGANIC POLLUTANTS
FROM WATER
HARVEST SOLAR ENERGY IN THE FORM OF
HYDROGEN AS FUEL FOR FUEL CELL
STORE SOLAR ENERGY AS HYDROGEN FUEL FOR
USE WHEN THERE IS NO SUNLIGHT
INEXPENSIVE (MOST COMMON USE: WHITE PAINT)
CHEMICALLY STABLE.
Nov 2012
CUHK
PROBLEM OF TiO2 FOR HARVESTING
SOLAR ENERGY
BAND GAP OF TiO2 IS ~3.4 eV SO IT ABSORBS ONLY THE
UV PART OF SOLAR SPECTRUM (~5% OF TOTAL ENERGY)
AMO
IDEAL ABSORBER
An Ideal Solar
Absorber
should be
Black!
AM1
6
Nov 2012
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HOW TO DECREASE THE BANDGAP OF
TiO2?
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DOPING WITH
IMPURITIES
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H, N, METAL IONS
INDUCE
INTRINSIC
DEFECTS
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RESULTS:
TiO2 BAND GAP IS REDUCED TO
VISIBLE PRODUCING BLUE, YELLOW OR
DIRTY TiO2. NATURAL CYRSTALS OF RUTILE
AND ANATASE ARE OFTEN COLORED BUT
TRANSPARENT.
O VACANCIES
DECREASE
VALENCE OF Ti
FROM 4 TO 3
Red rutile mined in Switzerland
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Nov 2012
Anastase grown in Lab
CUHK
DISCOVERY OF BLACK TiO2
“Increasing Solar Absorption for Photocatalysis with Black Hydrogenated
Titanium Dioxide Nanocrystals” Xiaobo Chen, Lei Liu, Peter Y. Yu, Samuel
S. Mao.SCIENCE VOL 331 page 746 (2011).
A NEW FORM OF TiO2 ENGINEERED BY HYDROGENATING
ANATASE NANOCRYSTALS UNDER PRESSURE
8
Nov 2012
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WHAT IS BLACK TiO2?
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REST OF TALK WILL DESCRIBE:
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FABRICATION OF BLACK TiO2
PROPERTIES OF BLACK TiO2
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PHOTOCATALYTIC PROPERTIES
STRUCTURAL & VIBRATIONAL PROPERTIES
ELECTRONIC PROPERTIES
DEFECT PROPERTIES
MODELING BLACK TiO2 USING FIRST
PRINCIPLE CALCULATION
Nov 2012
CUHK
CRYSTAL STRUCTURES OF TiO2
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THE COMMON FORMS OF TiO2 ARE
RUTILE, ANATASE & BROOKITE.
RUTILE IS THE MOST STABLE BUT THE
RUTILE & ANATASE STRUCTURES ARE
QUITE SIMILAR.
IN BOTH STRUCTURES THE Ti IS
SURROUNDED BY 6 O ATOMS TO FORM
OCTAHEDRALS. IN ANATASE THE
OCTAHEDRALS SHARE ONLY EDGES. IN
RUTILE THE OCTAHEDRALS SHARE BOTH
EDGES AND CORNERS.
THE POINT GROUP SYMMETRY IS D4h.
THE SPACE GROUP SYMMETRY OF
ANATASE IS: D194h
a=0.3747nm;c=0.9334nm; ANGLE(Ti-OTi)=156o IN ANATASE. IN RUTILE THIS
ANGLE IS REDUCED TO 99o.
STRUCTURE OF ANATASE
Conventional
Primitive Unit Cell
Tetragonal Unit Cell
SMALL CIRCLES: Ti
LARGE CIRCLES: O
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Nov 2012
CUHK
INTRODUCTION TO TiO2:
ELECTRONIC STRUCTURE
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Valence Band of Anatase consists mainly of 3
regions [M.Emori et al. Phy. Rev. B 85, 035129 (2012)].
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–
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Top region (a):O (2pp)
Middle region (b): O (2pp) hybridized with Ti 3d (t2g)
Lowest region (c): O(2ps) hybridized with Ti 3d (eg)
(a)
(b)
(c)
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Nov 2012
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TiO2 AS A PHOTOCATALYST FOR WATER
BREAKING (Xu, Y.; Schoonen, M. A. A. Am. Mineral. 2000, 85, 543.)
Schematic Water
Splitting Cell using
TiO2 as photocatalyst
Calculated energy positions of conduction and valence
band edges at pH = 0 for selected metal oxide
Conduction Band Edge
Valence Band Edge
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The valence band of TiO2 can
be raised by >2eV without
affecting its photocatalytic
ability
Nov 2012
CUHK
GAP REDUCTION BY DEFECT &
IMPURITY LEVELS IN TiO2
DENSITY OF
STATES FROM
FIRST-PRINCIPLE
CALCULATIONS.
CONCLUSION:
ONLY
INTERSTITIAL Ti
PRODUCES A
DEEP LEVEL IN
THE GAP
ITi
DOS
OHSurface
IH2
IH
IO
VO
VTi
Nano
Bulk
-5 -4 -3 -2 -1
0
1
2
3
4
5
6
7
8
Energy (eV)
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Nov 2012
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GAP REDUCTION BY DEFECTS IN
TiO2:Ti3+
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14
THEORY SHOWS
THAT DEFECTS
LIKE O VACANCIES
OR Ti3+ ARE DEEP
DONORS BUT THE
MAXIMUM
REDUCTION OF
LESS THAN 1eV
AND NOT ENOUGH
TO MAKE TiO2
BLACK!
A. SELLONI ET AL.
Nov 2012
CUHK
RECIPE FOR MAKING BLACK TiO2
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STEP 1: MAKE ANATASE NANOCRYSTALS
– Make A Precursor Solution Consisting Of Titanium
Tetraisopropoxide (TTIP), Ethanol, Hydrochloric Acid (HCl),
Deionized Water, And The Organic Template, Pluronic F127, With
Molar Ratios Of TTIP/F127/HCl/H2O/Ethanol At 1:0.005:0.5:15:40.
– Heat Solution At 40 oC For 24 Hours, Evaporate And Dry At 110 oC
For 24 Hours. Calcinated The Dried Powder At 500 oC For 6 Hours
To Remove The Organic Template And Enhance Crystallization Of
TiO2.
STEP 2: HYDROGENATION
– Place In Sample Chamber Of A Hy-energy Pctpro High-pressure
Hydrogen System
– Hydrogenate In A 20.0 Bar H2 Atmosphere At About 200 oC For
Five Days.
Nov 2012
CUHK
BLACK TiO2 AS PHOTOCATALYST
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Panel A shows the methylene
Blue absorption decrease after
exposure to TiO2 catalyst and
simulated solar radiation.
Black TiO2 is more efficient
than white TiO2.
Panel B shows that black TiO2
exhibit no degradation after
repeated cycling.
Panel C shows water
splitting using black TiO2
under simulated solar light.
There is no sign of
degradation again after a
period of 22 days and 100
hours of solar irradiation.
Nov 2012
CUHK
EFFICIENCY OF BLACK TiO2 IN SPLITTING WATER
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1 hour of solar irradiation generated 0.2 ± 0.02 mmol
of H2 using 0.02 g of black TiO2 (10 mmol hour–1 g–1
of photocatalysts).
This H2 production rate is about two orders of magnitude
greater than the yields of most semiconductor photocatalysts
– energy conversion efficiency for solar hydrogen production=
(energy in solar-produced hydrogen)/ (energy of the incident
sunlight) reached 24% for black TiO2 nanocrystals. This is as
good as the best crystalline solar cell!
THESE RESULTS HAVE NOW BEEN REPRODUCED
AROUND THE WORLD.
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Nov 2012
CUHK
QUESTIONS RAISED BY CRITICS
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IS BLACK TiO2 HEAVILY DOPED WITH IMPURITIES LIKE N,
INTRINSIC DEFECTS LIKE O VACANCIES AND Ti3+?
DOES IT STORE THE H DURING FORMATION AND THEN
RELEASE THE H DURING WATER SPLITTING?
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If this is the case then black black TiO2 will gradually become
white after many hours of water splitting.
WHAT IS THE STRUCTURE OF BLACK TiO2 ?
WHAT IS THE ROLE OF HYDROGEN?
WHAT IS THE BAND DISCONTINUITY AT THE INTERFACE
BETWEEN DISORDERED SHELL & THE CRYSTALLINE
CORE?
Nov 2012
CUHK
HRTEM PICTURES OF WHITE & BLACK TiO2
c
Count / a.u.
0.352 nm
WHITE TiO2 IS
CRYSTALLINE.
ANATASE
green line
red line
2
4
6
Distance / nm
d
0.674 nm
0.298 nm
0.296 nm
Count / a.u.
0.352 nm
0.420 nm
green line
red line
0
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Nov 2012
3
6
Distance / nm
BLACK TiO2
HAS A
CRYSTALLINE
ANATASE
CORE AND A
DISORDERED
SHELL
9
CUHK
XRD OF WHITE & BLACK TiO2
X-RAY DIFFRACTION PEAKS
ARE CONSISTENT WITH
CRYSTALLINE TiO2 BEING
ANATASE.
BROADENING OF PEAKS
CONSISTENT WITH AVERAGE
PARTICLE SIZE OF AROUND 8
nm.
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Nov 2012
CUHK
RAMAN SPECTRA OF WHITE & BLACK TiO2
RAMAN PEAKS OF WHITE TiO2 AGREE WITH THOSE OF BULK ANATASE
RAMAN MODE
FREQUENCY(CM-1)
Eg(1)
144
B1g(1)
400
B1g(2)
515
A1g
519
Eg(3)
640
THE ADDITIONAL MODES IN BLACK
TiO2 ARE DUE TO THE DISORDERED
PHASE. THIS DIORDERED PHASE IS
NOT AMORPHOUS IN AGREEMENT
WITH HRTEM.
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Nov 2012
CUHK
FTIR REFLECTANCE SPECTRA OF WHITE & BLACK TiO2
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Both black and
white TiO2 exhibit
OH absorption
bands near the
3400 cm-1 region,
The peaks at
around 3730 cm-1
and the 3640 cm-1
are due to the O-H
stretching mode
and wagging
mode.
b
Reflectance / a.u.

FTIR
white TiO2
black TiO2
4000 3500 3000 2500 2000 1500 1000
Wavenumber / cm
22
Nov 2012
-1
CUHK
500
ABSORPTION OF WHITE & BLACK TiO2: INTER-BAND
TRANSITON
0.8mm
1.25mm
Reflectance / a.u.
THE BAND GAP OF BLACK TiO2 IS REDUCED BY >2 eV
whtie TiO2
black TiO2
500
1000
1500
A
2000
2500
Wavelength / nm
THE ABSORPTION SPECTRUM
OF BLACK TiO2 SHOWS 2
ONSETS SEPARATED BY
ABOUT 1 eV.
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Nov 2012
CUHK
VALENCE BAND EDGES OF WHITE &
BLACK TiO2 FROM X-RAY PHOTOEMISSION
THERE IS A BLUE
SHIFT OF THE
VALENCE BAND
EDGE IN BLACK
TiO2 BY ~2.2 eV .
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Nov 2012
CUHK
SCHEMATIC SUMMARY OF DOS OF WHITE
& BLACK TiO2
THE BAND GAP
REDUCTION IN
BLACK TiO2 IS
DUE MAINLY TO
BLUE-SHIFT OF
THE VALENCE
BAND!
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Nov 2012
CUHK
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Presence of Ti3+ can be
detected by measuring Xray Near Edge Absorption
Spectrum (XANES) at the
Ti-K edge using synchrotron
radiation at the Advanced
Light Source (ALS) of LBNL
The XANES spectra of
BLACK and WHITE TiO2
are essentially the same but
quite different from that of
Ti2O3 showing that any
amount of Ti3+ present is the
same for both kinds of
sample.
Nov 2012
Intensity / a.u.
DEFECTS & IMPURITIES IN BLACK
TiO2: Ti3+
white TiO2
black TiO2
bulk Ti2O3
4970
5000
4980
5100
4990
5200
Energy / eV
CUHK
5300
ENVIRONMENT OF H FROM NMR
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Both black and white TiO2 show a large peak at a chemical due to H bonded to O.
Two additional narrow peaks at chemical shifts of 0.73 ppm and -0.03 ppm.
in black TiO2 suggest that H mainly occupy sites not strongly bonded to
neighboring atoms, such as in interstitial sites or in Ti-H bonds.
c
 Intensity / a.u.
Intensity / a.u.
d
1H NMR
White TiO2
Black TiO2
0
Black - white
20
27
10
0
-10
Chemical Shift / ppm
Nov 2012
-20
20
10
0
-10
Chemical Shift / ppm
CUHK
-20
MODEL CALCULATION: METHOD
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FIRST-PRINCIPLES DENSITY-FUNCTIONAL THEORY (DFT)
DFT CALCULATIONS ARE PERFORMED USING THE PERDEWBURKE-ERNZERHOF (PBE) FUNCTIONAL WITHIN THE
GENERALIZED GRADIENT APPROXIMATION
THE KOHN-SHAM EQUATIONS SOLVED WITH THE PROJECTED
AUGMENTED WAVE METHOD AS IMPLEMENTED IN THE VASP
CODE.
USE 30×30×30 Å3 SUPERCELLS, WHERE THE ATOMIC POSITIONS
ARE RELAXED UNTIL THEIR RESIDUAL FORCES ARE LESS THAN
0.05 eVÅ-1.
THE CUT-OFF ENERGY FOR THE PLANE-WAVE BASIS SET IS 400
eV AND THE BRILLOUIN ZONE IS SAMPLED WITH THE SINGLE –
POINT.
Nov 2012
CUHK
CLUSTER MODELS OF BLACK TiO2
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We start with a
cluster:Ti218O436H70. The
amount of H is higher than
necessary to passivate the
dangling bonds on surface.
It starts with the Anatase
structure. After relaxation only
a small core of Anatase is left.
Ti atoms: grey, O atoms: red
and H atoms:white balls
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Nov 2012
CUHK
DOS OF CLUSTER MODEL
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The conduction
band edge is
essentially not
changed.
The valence band
edge is blueshifted by ~1.2eV.
A mid-gap state at
1.8 eV appeared.
b: CLUSTER MODEL
Model a
600
DOS
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a: ANATASE
NANOCRYSTAL
Model b
400
200
Nov 2012
0
-10 -8 -6 -4 -2 0 2
eV
CUHK
4
6
8 10
RADIAL DISTRIBUTION FUNCTION (RDF)
CRYSTALLINE
Model C
Ti-Ti
Ti-O
white TiO 2
black TiO 2
g(r)
The RDF can be determined
from the extended
absorption fine structure
(EXAFS) in x-ray
absorption spectra.
Experimental spectra are
broadened by size of the
TiO2 nanoparticles. Only
small differences between
white and black tio2.
g
0
2
4
Distance / Angstrom
31
Nov 2012
CUHK
6
CALCULATED RDF FROM CLUSTER
MODEL
appearance of the Ti-H
peak around 0.26 nm. The h
relative large length of this
bond indicates the
weakness of the Ti-H bond
in black TiO2.
Black TiO2 mainly shows
disorder in the Ti-Ti bond
distance.
CLUSTERModel
MODEL
D
Ti-Ti
Ti-O
Ti-H
white TiO2
black TiO2
g(r)
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
0
2
4
Distance / Angstrom
32
Nov 2012
CUHK
6
SUMMARY & CONCLUSIONS
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Black TiO2 is a new form of disordered TiO2 in which
H and nm size both played important roles.
The valence band of black TiO2 is blue-shifted by
more than 2eV from that of white TiO2.
The absorption edge of Black TiO2 matches the solar
spectrum so well that its photocatalytic ability to split
water is enhanced by an order of magnitude.
Black TiO2 has the potential to solve some of the
energy and pollution problems in the world because
its inexpensive and durable.
Nov 2012
CUHK

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