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John T. Yates, Jr.John T. Yates, Jr.
Surface Science CenterSurface Science Center
Department of ChemistryDepartment of Chemistry
University of PittsburghUniversity of Pittsburgh
Photochemistry on TiOPhotochemistry on TiO22 Semiconductor Surfaces Semiconductor Surfaces –– How How Kinetic Measurements Can Provide Insight Into The Kinetic Measurements Can Provide Insight Into The
Behavior of Charge CarriersBehavior of Charge Carriers
PIREPIRE--ECCI/ICMR Summer Program onECCI/ICMR Summer Program on
Techniques of Surface Science and CatalysisTechniques of Surface Science and CatalysisUniversity of California University of California –– Santa BarbaraSanta Barbara
August 2006August 2006
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1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 20050
200
400
600
800
1000
1200
1400
1600
1800
Num
ber o
f Pub
licat
ions
Year
A. A. FujishimaFujishima and K. Honda, Nature 238 (1972) 37.and K. Honda, Nature 238 (1972) 37.
1972: Water Splitting at n1972: Water Splitting at n--TiOTiO22 ElectrodesElectrodes
www.isiknowledge.com. Search terms = (TiO2OR titanium dioxide AND photo*)
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CB
VB+++
hν
3.0 eV
Upon UV excitation, both electrons and holes are photochemically active
towards adsorbates on the TiO2 surface.
DonorMolecule
Product
Product
AcceptorMolecule
Semiconductor Semiconductor PhotoexcitationPhotoexcitation : TiO: TiO22
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Sacrificial Hole Scavenger ActionSacrificial Hole Scavenger Action
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Solar Spectrum on EarthSolar Spectrum on Earth
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Taken from: Fujishima, Hashimoto, Watanabe, “TiO2-Photocatalysis, Fundamentals and Applications”, BKC Inc. Tokyo, 1999
Motivation? TiO2 Based PhotocatalyticTechnology Works!
Motivation? TiO2 Based PhotocatalyticTechnology Works!
TiO2 Based Systems are Efficient Photocatalysts
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TiOTiO22 as a Photochemical Substrateas a Photochemical Substrate
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2.0nm
5-fold Ti
bridging O vacancy
in-plane O 6-fold Tibridging O
5-fold Ti
[001
]
MezhenneyMezhenney et al. Chemical Physics et al. Chemical Physics Letters, 369 (2003) 152.Letters, 369 (2003) 152.
Atomic Structure of TiOAtomic Structure of TiO22(110) Surface(110) Surface
TiOTiO22(110)(110)--(1x1) surface without oxygen(1x1) surface without oxygenvacancies is vacancies is stochiometricstochiometric (TiO(TiO22))
The surface density of oxygen The surface density of oxygen vacancy sites is regulated by vacancy sites is regulated by annealing conditions (time, annealing conditions (time,
presence of Opresence of O22))
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Heating TiOHeating TiO22 above above ~ 700 ~ 700 ––
800 K 800 K →→ OO--vacancy defectsvacancy defects
(reduced TiO(reduced TiO22))
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Pan, J. M.; Maschhoff, B. L.; Diebold, U.; Madey, T. E. J. Vac. Sci. Technol. A 1992, 10, 2470.
He Ion Scattering Spectroscopy He Ion Scattering Spectroscopy –– Detects Detects 1818OO22 on Vacancy Defect Siteson Vacancy Defect Sites
∼8% defects in surface adsorb 18O2
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Ti: 1s22s22p63s23p63d24s24p0
Ti+4: 1s22s22p63s23p63d04s04p0
R.A. Bartynski, et al.
JVST A10, 2591 (1992)
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XPS Evidence for TiOXPS Evidence for TiO22 Reduction by HeatingReduction by Heating
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A More Recent Look at A More Recent Look at Electronic Structure of Electronic Structure of Oxygen Defects/TiOOxygen Defects/TiO22(110)(110)
STM STM -- STSSTS
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Oxygen Defect StateOxygen Defect State
Spatial distribution of Ti 3d stateSpatial distribution of Ti 3d state
Unoccupied Occupied
Ti5c
T. Minami and M. Kawai T. Minami and M. Kawai (unpublished results)(unpublished results)
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Vijay, A.; Mills, G.; Metiu, H. J. Chem. Phys. 2003, 118, 6536.
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Use of CO2 Adsorption to Detect Surface Defect Sites Produced
by Heating
Use of COUse of CO2 2 Adsorption to Detect Adsorption to Detect Surface Defect Sites Produced Surface Defect Sites Produced
by Heatingby Heating
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Apparatus for Photodesorption Studies from TiO2(110)
°
Measurement Capabilities:Measurement Capabilities:
••Auger Electron SpectroscopyAuger Electron Spectroscopy
••Temperature Programmed Temperature Programmed Desorption (TPD)Desorption (TPD)
••PhotoPhoto--induced Desorption induced Desorption (PD)(PD)
UHV Studies of Photochemical Processes on TiOUHV Studies of Photochemical Processes on TiO22(110)(110)
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Comparison of Experimental and Simulated 13CO2 Desorption Spectra on TiO2(110) –
Fully Oxidized Surface
Comparison of Experimental and Simulated Comparison of Experimental and Simulated 1313COCO22 Desorption Spectra on TiODesorption Spectra on TiO22(110) (110) ––
Fully Oxidized SurfaceFully Oxidized Surface
100 150 200 250 300
Experimental Data Simulated Data
Edes = 48.5 kJ/molEinteraction = -10.0kJ/mol
vο = 4.0 x1013 s-1
β =dT/dt =2.0K/sec
Temperature (K)
dθ/dt = νο (1/β) θ exp (-(Ed+Eintθ)/kT)
Δ PCO2
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100 150 200 250 3000.0
1.0x10-7
2.0x10-7
3.0x10-7
4.0x10-7
5.0x10-7
6.0x10-7
7.0x10-7
8.0x10-7
QM
S S
igna
l (M
AS
S 4
5)
Temperature (K)
100 150 200 250 3000.0
1.0x10-7
2.0x10-7
3.0x10-7
4.0x10-7
5.0x10-7
6.0x10-7
7.0x10-7
8.0x10-7
QM
S S
igna
l (M
AS
S 4
5)
Temperature (K)
100 150 200 250 3000.0
1.0x10-7
2.0x10-7
3.0x10-7
4.0x10-7
5.0x10-7
6.0x10-7
7.0x10-7
8.0x10-7
QM
S S
igna
l (M
AS
S 4
5)
Temperature (K)
100 150 200 250 300
0.0
1.0x10-7
2.0x10-7
3.0x10-7
4.0x10-7
5.0x10-7
6.0x10-7
7.0x10-7
8.0x10-7
QM
S S
igna
l (M
AS
S 4
5)
Temperature (K)
100 150 200 250 3000.0
1.0x10-7
2.0x10-7
3.0x10-7
4.0x10-7
5.0x10-7
6.0x10-7
7.0x10-7
8.0x10-7
QM
S S
igna
l (M
ASS
45)
Temperature (K)
100 150 200 250 3000.0
1.0x10-7
2.0x10-7
3.0x10-7
4.0x10-7
5.0x10-7
6.0x10-7
7.0x10-7
8.0x10-7
QM
S S
igna
l (M
ASS
45)
Temperature (K)
Successive Reduction of TiO2(110) Using CO2 to Detect Vacancy Defect SitesSuccessive Reduction of TiOSuccessive Reduction of TiO22(110) Using CO(110) Using CO22 to Detect Vacancy Defect Sitesto Detect Vacancy Defect Sites
600K anneal
700K anneal
800K anneal
900K anneal
1000K anneal
1100K anneal
defects
defects
defects
defects
T. L. Thompson, O. T. L. Thompson, O. DiwaldDiwald and J. T. Yates, Jr.and J. T. Yates, Jr.J. Phys. Chem. BJ. Phys. Chem. B 107 (2003) 11700.107 (2003) 11700.
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A Recent Controversy About A Recent Controversy About Vacancy Defect Sites on TiOVacancy Defect Sites on TiO22(110)(110)
OxygenOxygen--Mediated Vacancy Mediated Vacancy Diffusion on TiODiffusion on TiO22
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2.0nm
STM Image of TiOSTM Image of TiO22(110) Showing Oxygen (110) Showing Oxygen Vacancy Defect Sites Vacancy Defect Sites
Mezhenny, Maksymovych, Thompson, Diwald, Stahl, Walck and Yates. Chem. Phys. Lett. 369 (2003) 152.
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Schematic of O2 - Mediated Vacancy Diffusion - TiO2(110)
O
Ti
O
1.
2.
3.
OOO
O
O
Ti
Ti
Ti
Ti
Ti
Ti
O
OTi
OOOO
O
O
Ti
Ti
Ti
Ti
Ti
Ti
O
O
Ti
OOOO
O
O
Ti
Ti
Ti
Ti
Ti
Ti
O
O
O
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0 25 50
hν on
ΔIO2
16O18O (34 amu)
18O2 (36 amu)
T = 100 K
2 x 10-8 Amps
t (sec)0 25 50
hν on16O18O (34 amu)
18O2 (36 amu)
T = 180 K
2 x 10-8 Amps
ΔIO2
t (sec)
Isotopomer O2 Photodesorption from TiO2(110) Containing Vacancy Sites
T. Thompson, O. Diwald, J. T. Yates, Jr. Chem. Phys. Lett. 393, 28, 2004.
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100 200 300 400 500
18O2 (36 amu)
16O18O (34 amu)
2 x 10-9 AdT/dt = 4 K/sec
ΔIO2
Tads = 105K
T (K)
Isotopomer O2 Thermal Desorptionfrom TiO2(110) Containing Vacancy Sites
T. Thompson, O. Diwald, J. T. Yates, Jr. Chem. Phys. Lett. 393, 28 (2004).
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SummarySummary
1. O2-Mediated vacancy diffusion on TiO2(110) is observed by STM.
2. The mechanism proposed is incorrect since isotopic exchange does not occur from lattice oxygen to molecular oxygen.
3. Single Question: What other O2-mediated models can be imagined?
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The Controversy Has Been SettledThe Controversy Has Been Settled
•• Bright images were not OBright images were not O22/defect /defect site.site.
•• They were TiThey were Ti--OH groups due to OH groups due to HH22O impurity.O impurity.
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Bikondoa, O.; Pang, C. L.; Ithnin, R.; Muryn, C. A.; Onishi, H.; Thornton, G. Nature Materials 2006, 5, 189.
vac – OH + e TiO + H
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Detailed Studies of ODetailed Studies of O22PhotodesorptionPhotodesorption from TiOfrom TiO22(110)(110)
•• Providing insight into the mechanisms Providing insight into the mechanisms of photonof photon--induced electroninduced electron--hole pair hole pair production and the activation of production and the activation of adsorbed molecules by these charge adsorbed molecules by these charge carriers.carriers.
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a)
b)
Oxygen Vacancies
h+ e-c)
hν Oxygen Photodesorption
--OO
OO
OO
OO
a)
b)
Oxygen Vacancies
h+ e-h+ e-c)
hν Oxygen Photodesorption
--OOOO
OOOO
OOOO
OOOO
Mechanism for Oxygen Mechanism for Oxygen PhotodesorptionPhotodesorption from TiOfrom TiO22(110)(110)
h+ + O2-→ O2↑
•de Lara-Castells, M. P.; Krause, J. L. J. Chem. Phys. 2003, 118, 5098-5105.
OOVACVAC + O+ O22 →→ OO22--
••de Larade Lara--CastellsCastells, M. P.; Krause, J. L. , M. P.; Krause, J. L. J. J. Chem. Phys.Chem. Phys. 2001, 2001, 115115, 4798, 4798--48104810
••de Larade Lara--CastellsCastells, M. P.; Krause, J. L. , M. P.; Krause, J. L. Chem. Phys. Chem. Phys. LettLett.. 2002, 2002, 354354, 483, 483--490.490.
••AnpoAnpo, M.; , M.; CheChe, M.; , M.; FubiniFubini, B.; , B.; GarroneGarrone, E.; , E.; GiamelloGiamello, E.; Paganini, M. , E.; Paganini, M.
C. C. TopTop. Catal. 1999, 8, 189-198.
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G. Lu, A. Linsebigler and J. T. Yates, Jr., "The Adsorption and Photodesorption of Oxygen on the TiO2(110) Surface," J. Chem. Phys. 102, 4657 (1995).
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QMSHg Lamp
andFilters
Photodiode Detector
QuartzReflector
Fhν0.096 Fhν
Ta
TiO2
OO22 PhotodesorptionPhotodesorption from TiOfrom TiO22(110)(110)
0 10 20 30 40-2
0
2
4
6
8
10
12
14
Y
(A x
10-9
)
Time (s)
Fhν = 4.08 x 1014 photons cm-2s-1
Ehν = 3.4 +/- 0.05 eV
T = 110 K
O2
hυ on
Tracy L. Thompson and John T. Yates, Jr. J. Phys. Chem. B, 109 (2005) 18230.
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OO22 PhotodesorptionPhotodesorption from TiOfrom TiO22(110):(110):kk11 FFhhνν
hhνν + TiO+ TiO2 2 →→ e + h e + h kk22
h + T h + T →→ T+ T+ (hole capture by a hole trap)(hole capture by a hole trap)kk33
e + h e + h →→ heat heat (on recombination sites) (on recombination sites) kk44
h + Oh + O22--(a) (a) →→ OO22(g) (g) ↑↑
Rate of Rate of photodesorbingphotodesorbing oxygen scales oxygen scales proportionally with the square root of the incident proportionally with the square root of the incident
light intensity:light intensity:
][][
2
2 2/12/1
32
14 Oh
O Fkk
kkdt
dθ
θυ⎟⎟
⎠
⎞⎜⎜⎝
⎛+
−=
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0.0 5.0x106 1.0x107 1.5x107 2.0x107
0
2
4
6
8
10
12
14
hν = 3.4 +/- 0.05 eVT = 110 K
Flux 1/2hν
(photons1/2cm-1sec-1/2)
Y0 (O )
(A
mps
x 1
0-9)
A
B
F1/2hν
(crit.)2
18O2 Initial Photodesorption Yield – TiO2(110)
-10 0 10 20 30 40 50-2
0
2
4
6
8
10
12
14
Ion
Cur
rent
(36
amu)
Time (s)
Fhν = 4.08 x 1014 cm-2s-1
Y (O2)
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dhν
Fhν
O2- O2
-O2- O2
-O2-O2
-O2-
TiO2
O2-
Hole trap centers (T)and average range of photogenerated holes which will be trapped.
* * * * * * **
Bulk Hole Trapping Sites Within TiOBulk Hole Trapping Sites Within TiO22
After hole trap filling, k2 ceases to contribute
to the charge exchange process.
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Use of Adsorbed Hole Trap Use of Adsorbed Hole Trap Molecules to Increase Trap Molecules to Increase Trap Density Above Bulk DensityDensity Above Bulk Density
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Role of CHRole of CH33OH Surface Hole Traps OH Surface Hole Traps on Oon O22 PhotodesorptionPhotodesorption YieldYield
200 300 400 500 600-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
QM
S Si
gnal
Temperature (K)
TPD of Methanol from TiO2(110)
ab
a
bTiO2
CH 3OH
O 2
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0.0 5.0x106 1.0x107 1.5x107 2.0x107
0
2
4
6
8
10
12
14
θ (CH3OH) = 0.6 ML
θ (CH3OH) = 0.2 ML
Flux1/2hυ
(photons1/2cm-1sec-1/2)
hν = 3.4 +/- 0.05 eVT = 110 K
Y
0 (O )
(A
mps
x 1
0-9)
2
θ (CH3OH) = 0 ML
0 1x106 2x106 3x106 4x106 5x106
0.00
0.25
0.50
0.75
1.00
1.25
θ (CH3OH) = .6 ML
θ (CH3OH) = .2 ML
Flux1/2hυ
(photons1/2cm-1sec-1/2)
Y0 (O
)
(Am
ps x
10-9
) 2
θ (CH3OH) = 0 ML
Effect of CHEffect of CH33OH (a) Hole Traps on OH (a) Hole Traps on Initial OInitial O22 PhotodesorptionPhotodesorption YieldYield
The critical point (where slope changes) is reflective of the filling of the active trap sites.
CH3OH causes break point to move to higher Fhυ.
The critical point (where slope changes) is reflective of the filling of the active trap sites.
CH3OH causes break point to move to higher Fhυ.
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Summary
• Charge carrier dynamics monitored by quantitatively studying a simple model photoreaction – O2 photodesorption.
• F rate law found – caused by second-order e-h recombination kinetics.
• Artificial enhancement of hole trapping by adding CH3OH – a hole trap molecule.
h υ½
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QMSHg Lamp
and Filters
Photodiode Detector
QuartzReflector
Fhν
0.096 Fhν
Ta
TiO2(110)
18O2 Photodesorption from TiO2(110)
0 10 20 30 40
0
2
4
6
8
10
12
14
Y
(A x
10-9
)Time (s)
Fhν = 4.08 x 1014 photons cm-2s-1
Ehν = 3.4 +/- 0.05 eV
T = 110 K
O2
hυ on
18O2
YO (in 0.1s)2
o
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Kinetics of Hole TrappingKinetics of Hole Trapping--As Studied by OAs Studied by O22 PhotodesorptionPhotodesorption
•• Holes are either partially filled or completely filled by theHoles are either partially filled or completely filled by thetime the first point is measured [in time the first point is measured [in ΔΔt (sampling)].t (sampling)].
•• ΔΔt (sampling) = 0.10 secondst (sampling) = 0.10 seconds
•• Therefore, at Therefore, at FFhhνν((critcrit.):.):
•• 0.10 s x 0.10 s x FFhhνν(critical) = # photons needed to saturate holes in (critical) = # photons needed to saturate holes in photon penetration depth.photon penetration depth.
•• Photon penetration depth = ~100Photon penetration depth = ~100ÅÅ x 10x 10--88cm cm ÅÅ--11 = 10= 10--66 cmcm
•• Therefore, density of traps = ~ Therefore, density of traps = ~ FFhhνν((critcrit) x ) x ΔΔtt / 10/ 10--66 cm = ~ 3 x cm = ~ 3 x 10101818 cmcm--33
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ConclusionConclusion-- OO22 PhotodesorptionPhotodesorption Detector of Detector of Hole Trapping PhenomenonHole Trapping Phenomenon
•• First highlyFirst highly--controlled study of charge carrier controlled study of charge carrier trapping effect on TiOtrapping effect on TiO22 single crystal.single crystal.
•• Hole trapping strongly inhibits surface Hole trapping strongly inhibits surface photoreaction.photoreaction.
•• Hole trap density estimated to be ~3x10Hole trap density estimated to be ~3x101818cmcm--33..
•• This hole trap density corresponds to ~3x10This hole trap density corresponds to ~3x10--55
fraction of atomic sites in the crystal bulk.fraction of atomic sites in the crystal bulk.
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•• So far you have seen only kinetic So far you have seen only kinetic
measurements of the initial rate of Omeasurements of the initial rate of O22(a) (a)
photodesorptionphotodesorption –– Y at constant Y at constant θθ
•• Studies of the rate of Studies of the rate of photodesorptionphotodesorption asas
a function of time and the depletion of a function of time and the depletion of
OO22(a) have yielded exciting new results.(a) have yielded exciting new results.
OO22
OO
OO22
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O2 Photodesorption from TiO2(110):k1 Fhν
hν + TiO2 → e + h k2
h + T → T+ (hole capture by a hole trap)k3
e + h → heat (on recombination sites) k4
h + O2-(a) → O2(g) ↑
Yield of photodesorbing oxygen scales proportionally with the square root of the incident light intensity:
][][
2
2 2/12/1
32
14 Oh
O Fkk
kkdt
dθ
θυ⎟⎟
⎠
⎞⎜⎜⎝
⎛+
−=
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Photoexcitation Behavior Above Fhv (crit.) in Kinetic Regime B
h + T T+k2
• Upon saturation of hole traps above Fhv (crit.)
k2 0
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][][
2
2 2/12/1
32
14 Oh
O Fkk
kk
dtd
θθ
υ⎟⎟⎠
⎞⎜⎜⎝
⎛+
−=
[ ]∫ ∫ ⎟⎟
⎠
⎞⎜⎜⎝
⎛−= 2
121
3
14
2ln
υ
θh
O Fkk
kdt
d
[ ] tkk
kF O
h
⋅⎟⎟⎠
⎞⎜⎜⎝
⎛−=⋅
2/1
3
142/1 2
ln1 θυ
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1.0 0.8 0.6 0.4 0.2
0
2
4
6
8
10
12
k 4 (k1/k
3)1/2 (x
10-9) (
arb.
uni
ts)
Relative O2 Coverage [ θ O ]2
Time (s)500 250
Range of Experimentally Measured Range of Experimentally Measured PhotodesorptionPhotodesorptionRate ConstantsRate Constants
k4 varies by a factor of ~100 over 250 seconds and is
shown to be time dependent, thus exhibiting fractal
character.
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Time-dependent rate constants are described by a Fractal Kinetic Process
Rate Coefficient
“Percolation Cluster” R. Kopelmann, Science 241 (1988) 1620.
R. Kopelman in “Fractal Approach to Heterogeneous Chemistry” (1989) 295.
R. Kopelman, J. Stat. Phys. 42 (1986) 185.
101 ≤≤= − hwheretkk h
instantaneous rate coefficient
fractal term
h = 0 – infinite mixing
h = ½ - one dimensional transport
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htkk −⋅= 1
.lnln constthk +−=
ln k versus ln t should give a straight line with slope = -h
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0 5
-27
-24
-21
-18
h = 0.33
h = 0.48
ln [k
4 (k1/k
3)1/2 ] (
arb.
uni
ts)
ln(time, (s))
Fractal Kinetic Behavior of the Photodesorption of O2 from TiO2(110)
k4 = k04 t
-0.51 +/- 0.03
h = 0.54
(2-D Fractal)
(1-D Fractal)
-
--
“k”4 = k4t -0.51±0.03
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A tentative new model explaining the A tentative new model explaining the 11--D fractal OD fractal O22 desorptiondesorption kinetics is kinetics is proposed.proposed.
T. L. Thompson and J. T. Yates, Jr., T. L. Thompson and J. T. Yates, Jr., ““Control of Control of a Surface Photochemical Process by Fractal a Surface Photochemical Process by Fractal Electron Transport Across the Surface: OElectron Transport Across the Surface: O22PhotodesorptionPhotodesorption from TiOfrom TiO22(110),(110),”” J. Phys. J. Phys. Chem. B Chem. B 110110, 7431 (2006)., 7431 (2006).
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defect state
h+
e-
Ef
Filled electronic state at defect site
e- e- e-
h+
O2-
TiO2
e- e-
Fractal Electron Transport via O-Vacancy Sites
• “Charge Cloud” associated with vacancy - Vijay and Metiu, J. Chem. Phys. 118 (2003) 6536.
• “Charge Cloud” causes enhanced electron transport across the crystal surface.
• Reduced efficiency of CT between photoproduced holes and adsorbed O2-.
Besen2.movBesen2.mov
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0
30
60
90
120
150
180
210
240
270
300
330
<110>
40
30
20
10
<001>
_
O. Byl and J. T. Yates, Jr. 2006
Anisotropy in the Bulk Resistivity of TiO2(110) Crystal –Four-point Probe Measurements at 300 K
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Summary and Conclusions:
•Charge clouds surrounding surface oxygen vacancies combine to form a percolation network for electron transport across the surface causing a decrease in the efficiency of photodesorption of O2.
•A kinetic analysis of the O2 photodesorption process reveals that the rate coefficient for photodesorptionchanges by a factor of > 100 over ~ 250 seconds.
•Electron transport across the surface is likely to be one-dimensional, based on the fractal analysis. Significant anisotropy in conduction through TiO2 has been observed in bulk measurements.
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Studies on TiO2 Powder
• ESR
• IR
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CB
- - -- - - - - -
hνshallow trapsT = 90K
T. Berger, M. T. Berger, M. SterrerSterrer, O. , O. DiwaldDiwald, E. , E. KnKnöözingerzinger, , D. D. PanayotovPanayotov, T. L. Thompson and J. T. Yates, Jr., T. L. Thompson and J. T. Yates, Jr.
J. Phys. Chem. B J. Phys. Chem. B 109109, 6061 (2005), 6061 (2005)
VB+ + + +
Ti3+
O-
Correlation Between Correlation Between PhotogeneratedPhotogeneratedElectron and Hole Production at 77K in TiOElectron and Hole Production at 77K in TiO22
PowderPowder
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EPR Signature for Ti3+ and O-
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OO2 2 (a)(a) as an Electron Scavenger:as an Electron Scavenger:Production of OProduction of O22
-- (ads.)(ads.)
TiO2 + O2(a)
lattice O- (hole)+
scavenged e (O2-(a))
hν
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IR Spectroscopy Can Be Used IR Spectroscopy Can Be Used To See Electrons Excited Into To See Electrons Excited Into
Conduction BandConduction Band
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UV-Induced Increase of IR Background – CB Trapping
CB
VB
2500 2000 1500 1000
IR A
bsor
banc
e
Wavenumber, cm-1
+
a. b.
++
hν hν hν
T. Berger, M. T. Berger, M. SterrerSterrer, O. , O. DiwaldDiwald, E. , E. KnKnöözingerzinger, , D. D. PanayotovPanayotov, T. L. Thompson and J. T. Yates, Jr., T. L. Thompson and J. T. Yates, Jr.
J. Phys. Chem. B J. Phys. Chem. B 109109, 6061 (2005, 6061 (2005))
IR
UV
UV on
UV off
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Conduction Band Electrons Transfer to Adsorbed Organic Molecules
CB
VB
ClC2H4SC2H5
e-
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-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.20.6
0.7
0.8
0.9
1.0
1.1
Bac
kgro
und
Abs
orba
nce
at ν
= 2
146
cm-1
Admitted Gas Pressure (Torr)
TiO2-SiO2
T = 200 K
Adsorbed 2-CEES
Adsorbed DES
SC2H5
C2H5
Preferential Electron Transfer: TiO2 CB Electrons to Electrophilic Molecules
Preferential Electron Transfer: TiO2 CB Electrons to Electrophilic Molecules
Xe-
CB
VB
ClC2H4SC2H5
e-
CB
VB
• CB electron transfer to electrophilic atom in adsorbed molecules
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SUMMARY
• Oxygen Vacancy Defects – Made by Heating Above 700 K
• O2 Adsorbs on Vacancies
• Hole Traps Retard Photochemistry of O2
• ESR and IR Effective Tools for Detection of Holes [O-1] and CB Electrons in TiO2 Powders
• CB Electrons Populated by UV at 130 K as Seen by IR Background Shift
• CB Electrons Depopulated by Electrophilic Organic Adsorbates – need electronegative atom
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A Recent Development- Explanation of UV-Induced TiO2 Hydrophilicity
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Rong Wang, Kazuhito Hashimoto, Akira Fujishima, Makota Chikuni, Eiichi Kojima,
Atsushi Kitamura, Mitsuhide Shimohigoshi, Toshiya Watanabe
Nature, 388 (1997) 870-873.
“Light-Induced Amphiphilic Surfaces”
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Taken from: Fujishima, Hashimoto, Watanabe, “TiO2-Photocatalysis, Fundamentals and Applications”, BKC Inc. Tokyo, 1999
UV
Taken from: Hata, Kai, Yamanaka, Oosaki, Hirota, Yamazaki, JSAE Review 21, 97-102, 2000
60% of Toyota automobiles already use this technology today.
UV
TiO2 - UV-induced Hydrophilicity - ApplicationsAnti-fogging
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What causes TiO2 to become hydrophilic in UV?3 Models:
•Photocatalytic removal of organic compounds ?UV
hydrophilic TiO2hydrophobic TiO2
•UV induced oxygen vacancies ?Nakajima, Koizumi, Watanabe, Hashimoto, J. Photochem. Photobiol. A 146, 129-132, 2001hydrophilic
OH
Ti Ti
OH+
hν
•UV-Induced bond breaking?
UV
Fujishima, Rao, Tryk, J. Photochem. Photobio C 1, 1 2000.
Sakai, Fujishima, Watanabe, Hashimoto, J. Phys. Chem. B 103, 2188, 1999.
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Problems with Existing Understanding of the UV-Induced Hydrophilicity Phenomena on TiO2
All current contact angle measurements have been
made in the ambient atmosphere on surfaces which
are not atomically clean. Hydrocarbon (and other
organic contamination) effects are uncontrolled.
Problem needs a clean surface-ultrahigh vacuum
approach and atomic resolution of surface atoms.
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STM Investigation of Vacancy Formation by STM Investigation of Vacancy Formation by Intense UV Irradiation Intense UV Irradiation –– TiOTiO22(110)(110)--(1x1)(1x1)
2.0nm
UV Exposure:
Sergey Mezhenny, Peter Maksymovych, Tracy L. Thompson, Oliver Diwald, Dirk Stahl and Scott D. Walck, Chemical Physics Letters, 369 (2003) 152-158.
2.0nm O-vacancy
Ti row
No Irradiation 5.4x1021 photons/cm2 (hν > 3.0eV)
Q ≤ 10-23.5±0.2 cm2
Implies that ~108 photons/site do not produce O vacancy defects!
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LiquidLiquid--Solid Contact Angle MeasurementsSolid Contact Angle Measurements
γSV
γLV
γSLθ
surface
droplet
γSV
γLV
γSLθ
surface
droplet
A γSV increases, θ decreases
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This technology allows study of contact angle This technology allows study of contact angle for pure Hfor pure H22O under conditions of:O under conditions of:
-- well controlled initial surface cleanlinesswell controlled initial surface cleanliness
-- well controlled atmospherewell controlled atmosphere
-- well controlled photon fluxwell controlled photon flux
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Typical H2O Contact Angle ShowingSudden Onset of Wetting of TiO2(110)
P = 1 atm; hexane = 120 ppmO2
a
b
c
0 s
154 s
155 s
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0 50 100 150 200 2500
10
20
30
40
T = 297-302 KP = 1 atm
Fhν= 1.1x1017 photons cm-2s-1 (2.1-4.4 eV)
Phν= 0.049 W cm-2
Region of uncertainty
O2
360 ppmhexane
120 ppmhexane
0 ppmhexane
Hexane Vapor Effect on the UV-Induced Wetting of TiO2(110)
Time (s)
θ C ,
Con
tact
Ang
le (d
egre
es)
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hν
t100 t200 t3000
300 ppm C6H14
200 ppm C6H14
TiO2
O2 and C6H14
100 ppm C6H14
Schematic Origin of Wetting Delay Period
Hex
ane
Cov
erag
e
Induction Periods0
hν
t100 t200 t3000
300 ppm C6H14
200 ppm C6H14
TiO2
O2 and C6H14
100 ppm C6H14
Schematic Origin of Wetting Delay Period
Hex
ane
Cov
erag
e
Induction Periods0
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0 100 200 300 400 5000
100
200
300
400
+2σ error
-2σ error
R = 0.906
Hexane Effect on the Wetting Delayof UV-Induced Wetting of TiO2(110)
Wet
ting
Del
ay (s
)
Hexane Concentration (ppm)4 data points
P = 1 atmT = 297-302 KFhν
= 1.1x1017 photons cm-2s-1 (2.1-4.4 eV)
Phν= 0.049 W cm-2
O2
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ConclusionsConclusions
Hydrophilicity model involving UV model involving UV
production of Oproduction of O--vacancy defect sites is vacancy defect sites is
unlikely based on STM results.unlikely based on STM results.
HydrophilicityHydrophilicity model involving hydrocarbon model involving hydrocarbon
photooxidationphotooxidation to produce clean to produce clean wettablewettable
TiOTiO22 is likely to be true. is likely to be true.
-- Induction period scales with Induction period scales with ppmppm
concentration of hexane in Oconcentration of hexane in O22..
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What Have We Learned?What Have We Learned?
Photochemistry on semiconductors occurs by e-hpair production in the substrate, accompanied by charge transfer to adsorbed species
Defect sites are important- On surface for adsorption of molecules- In bulk-promoting charge-carrier recombination
Second-order e-h recombination can give F dependence of photochemical kinetics
Hole traps reduce photochemical efficiency
Fractal photodesorption kinetics – controlled by anisotropy in crystal electrical conductivity
1/2hυ
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EPR – detects e & h states sensitivelye = Ti3+
h = O-
IR detects CB electrons – delocalization gives continuous optical absorption in IR region
Hydrophilicity induced by UV on TiO2 – caused by photooxidation of organic layers in equilibrium with hydrocarbons in atmosphere, causing cleanup of TiO2 surfaces
Don’t believe STM images unless you know you have chemical control
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AcknowledgementsAcknowledgements
Tracy L. ThompsonTracy L. Thompson University of PittsburghUniversity of PittsburghPeter Peter MaksymovychMaksymovych University of PittsburghUniversity of PittsburghDimitarDimitar PanayotovPanayotov University of Pittsburgh; Bulgarian University of Pittsburgh; Bulgarian
Academy of SciencesAcademy of SciencesSergey Sergey MezhennyMezhenny University of Pittsburgh University of Pittsburgh →→ University of University of
MarylandMarylandTykhonTykhon ZubkovZubkov University of PittsburghUniversity of Pittsburgh→→ PNNLPNNLDirk StahlDirk Stahl University of PittsburghUniversity of Pittsburgh→→ LeitzLeitzOliver Oliver DiwaldDiwald University of PittsburghUniversity of Pittsburgh→→ T. U. Wien, T. U. Wien,
AustriaAustriaProfessor Erich Professor Erich KnKnöözingerzinger T. U. Wien, AustriaT. U. Wien, AustriaThomas BergerThomas Berger T. U. Wien, AustriaT. U. Wien, AustriaMartin Martin SterrerSterrer T. U. Wien, AustriaT. U. Wien, Austria
This work has been supported by the Army Research Office underThis work has been supported by the Army Research Office underDr. Stephen Lee and DARPA; Also by PPG IndustriesDr. Stephen Lee and DARPA; Also by PPG Industries