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Title: Black Titanium Dioxide: A New Engineered Nanoparticle for Photocatalysis.


1
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

2
ACKNOWLEDGMENTS
  • 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

3
OUTLINE
  • 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

4
MOTIVATION
  • 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

5
ONE 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.

6
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)

An Ideal Solar Absorber should be Black!
AMO
IDEAL ABSORBER
AM1
7
HOW 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
8
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
9
WHAT 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

10
CRYSTAL 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
11
INTRODUCTION 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)
12
TiO2 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
13
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
14
GAP 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.
15
RECIPE 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.

16
BLACK 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.

17
EFFICIENCY 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.

18
QUESTIONS 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?

19
HRTEM PICTURES OF WHITE BLACK TiO2
WHITE TiO2 IS CRYSTALLINE. ANATASE
BLACK TiO2 HAS A CRYSTALLINE ANATASE CORE AND A
DISORDERED SHELL
20
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.
21
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) 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.
22
FTIR 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.

23
ABSORPTION 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.
24
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 .
25
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!
26
DEFECTS 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.

27
ENVIRONMENT 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.

28
MODEL 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.

29
CLUSTER 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
30
DOS 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.

31
RADIAL 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
32
CALCULATED 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
33
SUMMARY 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.
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