Title: Black Titanium Dioxide: A New Engineered Nanoparticle for Photocatalysis.
1Black Titanium Dioxide A New Engineered
Nanoparticle for Photocatalysis.
- Peter Y. YU
- Department of Physics, University of California
-
- Lawrence Berkeley National Laboratory
- Berkeley, CA 94720
2ACKNOWLEDGMENTS
- 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,
Peoples Republic of China
3OUTLINE
- 1. MOTIVATION
- 2. INTRODUCTION TO TiO2
- CRYSTAL STRUCTURE
- ELECTRONIC STRUCTURE
- WHY TiO2 IS A GOOD PHOTOCATALYST FOR ENERGY AND
ENVIRONMENT DEFECTS IMPURITIES IN TiO2 - 3. FABRICATION OF BLACK TiO2
- 4. PROPERTIES OF BLACK TiO2
- PHOTOCATALYTIC PROPERTIES
- STRUCTURAL VIBRATIONAL PROPERTIES
- ELECTRONIC PROPERTIES
- - DEFECT PROPERTIES
- 5. MODELING BLACK TiO2
- FIRST PRINCIPLE CALCULATION
- CLUSTER MODELS
- 6. CONCLUSIONS
4MOTIVATION
- TWO IMPORTANT PROBLEMS FACING THE WORLD TODAY
ARE - AIR WATER POLLUTION
- GLOBAL WARMING FROM BURING OF FOSSIL FUEL
- ONE WAY TO SOLVE BOTH PROBLEMS IS TO USE SOLAR
RADIATION TO - REDUCE WATER POLLUTION
- PRODUCE RENEWABLE ENERGY
5ONE MATERIAL TO ACHIEVE BOTH GOALS
- PHOTOCATALYST
- 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 - TITANIUM DIOXIDE (TiO2) IS AN IDEAL
PHOTOCATALYST - INEXPENSIVE (MOST COMMON USE WHITE PAINT)
- CHEMICALLY STABLE.
6PROBLEM 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)
An Ideal Solar Absorber should be Black!
AMO
IDEAL ABSORBER
AM1
7HOW TO DECREASE THE BANDGAP OF TiO2?
- DOPING WITH IMPURITIES
- H, N, METAL IONS
- INDUCE INTRINSIC DEFECTS
- O VACANCIES
- DECREASE VALENCE OF Ti FROM 4 TO 3
- 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.
Red rutile mined in Switzerland
Anastase grown in Lab
8DISCOVERY 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
9WHAT IS BLACK TiO2?
- REST OF TALK WILL DESCRIBE
- FABRICATION OF BLACK TiO2
- PROPERTIES OF BLACK TiO2
- PHOTOCATALYTIC PROPERTIES
- STRUCTURAL VIBRATIONAL PROPERTIES
- ELECTRONIC PROPERTIES
- DEFECT PROPERTIES
- MODELING BLACK TiO2 USING FIRST PRINCIPLE
CALCULATION
10CRYSTAL STRUCTURES OF TiO2
- 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 - a0.3747nmc0.9334nm ANGLE(Ti-O-Ti)156o IN
ANATASE. IN RUTILE THIS ANGLE IS REDUCED TO 99o.
STRUCTURE OF ANATASE
Conventional Tetragonal Unit Cell
Primitive Unit Cell
SMALL CIRCLES Ti LARGE CIRCLES O
11INTRODUCTION TO TiO2 ELECTRONIC STRUCTURE
- Valence Band of Anatase consists mainly of 3
regions M.Emori et al. Phy. Rev. B 85, 035129
(2012). - 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)
12TiO2 AS A PHOTOCATALYST FOR WATER BREAKING
(Xu, Y. Schoonen, M. A. A. Am. Mineral. 2000,
85, 543.)
Calculated energy positions of conduction and
valence band edges at pH 0 for selected metal
oxide
Schematic Water Splitting Cell using TiO2 as
photocatalyst
Conduction Band Edge
Valence Band Edge
The valence band of TiO2 can be raised by gt2eV
without affecting its photocatalytic ability
13GAP 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
14GAP REDUCTION BY DEFECTS IN TiO2Ti3
- 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.
15RECIPE FOR MAKING BLACK TiO2
- 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 10.0050.51540. - 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.
16BLACK TiO2 AS PHOTOCATALYST
- 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.
17EFFICIENCY OF BLACK TiO2 IN SPLITTING WATER
- 1 hour of solar irradiation generated 0.2 0.02
mmol of H2 using 0.02 g of black TiO2 (10 mmol
hour1 g1 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.
18QUESTIONS RAISED BY CRITICS
- 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? - 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?
19HRTEM PICTURES OF WHITE BLACK TiO2
WHITE TiO2 IS CRYSTALLINE. ANATASE
BLACK TiO2 HAS A CRYSTALLINE ANATASE CORE AND A
DISORDERED SHELL
20XRD 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.
21RAMAN 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) 4
00 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.
22FTIR REFLECTANCE SPECTRA OF WHITE BLACK TiO2
- 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.
23ABSORPTION OF WHITE BLACK TiO2 INTER-BAND
TRANSITON
- THE BAND GAP OF BLACK TiO2 IS REDUCED BY gt2 eV
0.8mm
1.25mm
THE ABSORPTION SPECTRUM OF BLACK TiO2 SHOWS 2
ONSETS SEPARATED BY ABOUT 1 eV.
24VALENCE 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 .
25SCHEMATIC SUMMARY OF DOS OF WHITE BLACK TiO2
THE BAND GAP REDUCTION IN BLACK TiO2 IS DUE
MAINLY TO BLUE-SHIFT OF THE VALENCE BAND!
26DEFECTS IMPURITIES IN BLACK TiO2 Ti3
- Presence of Ti3 can be detected by measuring
X-ray 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.
27ENVIRONMENT OF H FROM NMR
- 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.
28MODEL CALCULATION METHOD
- FIRST-PRINCIPLES DENSITY-FUNCTIONAL THEORY (DFT)
- DFT CALCULATIONS ARE PERFORMED USING THE
PERDEW-BURKE-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 303030 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.
29CLUSTER MODELS OF BLACK TiO2
- We start with a clusterTi218O436H70. 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 atomswhite
balls
30DOS OF CLUSTER MODEL
a ANATASE NANOCRYSTAL
b CLUSTER MODEL
- The conduction band edge is essentially not
changed. - The valence band edge is blue-shifted by 1.2eV.
A mid-gap state at 1.8 eV appeared.
31RADIAL DISTRIBUTION FUNCTION (RDF)
- 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.
CRYSTALLINE
32CALCULATED RDF FROM CLUSTER MODEL
- appearance of the Ti-H peak around 0.26 nm. The
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.
CLUSTER MODEL
33SUMMARY CONCLUSIONS
- 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.