John T. Yates, Jr.John T. Yates, Jr.

Department of ChemistryDepartment of Chemistry

University of University of VirginiaVirginia

Charlottesville, VA 22904Charlottesville, VA 22904

johnt@virginia.edujohnt@virginia.edu

Photochemistry on TiOPhotochemistry on TiO22 Semiconductor SurfacesSemiconductor Surfaces

1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

0

200

400

600

800

1000

1200

1400

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Num

be

r o

f P

ublic

atio

ns

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*)

CB

VB+++

hν

3.0 eV

Upon UV excitation, both electrons and holes are photochemically active

towards adsorbates on the TiO2 surface.

Donor

Molecule

Product

Product

Acceptor

Molecule

Semiconductor Semiconductor PhotoexcitationPhotoexcitation : TiO: TiO22

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

TiOTiO22 as a Photochemical Substrateas a Photochemical Substrate

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 oxygen

vacancies 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 annealing conditions

(time, presence of O(time, presence of O22))

Heating TiOHeating TiO22 above above ~ 700 ~ 700 ––

800 K 800 K →→→→→→→→ OO--vacancy defectsvacancy defects

(reduced TiO(reduced TiO22))

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

•••• TiO2 Photochemistry – Useful for Removal of Organics by Sequence of Reaction Events

i.e.hν

CH3OH H2C=O H-COOH CO2 + H2O

O2 + h O2 + h O2 + h

• Sequence of complex organic oxidation reactions makes the study of photophysics difficult

• ∴∴∴∴Investigate a simple photo reaction to obtaindetails;

hν + TiO2 h + eO2

-(a) + h O2 (g)

TiO2

Detailed Studies of ODetailed Studies of O22

PhotodesorptionPhotodesorption 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.

a)

b)

Oxygen Vacancies

h+e-c)

hν Oxygen

Photodesorption

--O

O

O

O

O

O

O

O

a)

b)

Oxygen Vacancies

h+e-

h+e-c)

hν Oxygen

Photodesorption

--O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

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.

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).

QMSHg Lamp

and

Filters

Photodiode

Detector

Quartz

Reflector

Fhν

0.096 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 1

0-9)

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.

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 2/1

2/1

32

1

4 Oh

O

Fkk

kk

dt

dθ

θυ

+−=

0.0 5.0x106

1.0x107

1.5x107

2.0x107

0

2

4

6

8

10

12

14

hν = 3.4 +/- 0.05 eV

T = 110 K

Flux 1/2

hν (photons

1/2cm

-1sec

-1/2)

Y0 (O

)

(Am

ps x

10-9

)

A

B

F1/2

hν (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 C

urr

ent (3

6 a

mu)

Time (s)

Fhν

= 4.08 x 1014

cm-2s

-1

Y (O2)

dhν

Fhν

O2- O2

-O2- O2

-O2-O2

-O2-O2

-

TiO2 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.

Careful studies of hole trapping enhancement Careful studies of hole trapping enhancement by added CHby added CH33OH have been made.OH have been made.

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 υ

½

QMSHg Lamp

and

Filters

Photodiode

Detector

Quartz

Reflector

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 1

0-9)

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

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 ∆∆t (sampling)].t (sampling)].

•• ∆∆t (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 ∆∆∆∆∆∆∆∆tt / 10/ 10--66 cm = ~ 3 x cm = ~ 3 x 10101818 cmcm--33

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.

A Recent Development- Explanation of UV-Induced TiO2 Hydrophilicity

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”

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

LiquidLiquid--Solid Contact Angle MeasurementsSolid Contact Angle Measurements

γSV

γLV

γSL

θ

surface

droplet

γSV

γLV

γSL

θ

surface

droplet

As γγγγSL increases, θθθθ decreases

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

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

0 50 100 150 200 2500

10

20

30

40

T = 297-302 K

P

= 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)

θC

, C

on

tact A

ng

le (

de

gre

es)

hν

t100 t200 t300

0

300 ppm C6H

14

200 ppm C6H

14

TiO2

O2 and C

6H

14

100 ppm C6H

14

Schematic Origin of Wetting Delay Period

He

xa

ne C

overa

ge

Induction Periods0

hν

t100 t200 t300

0

300 ppm C6H

14

200 ppm C6H

14

TiO2

O2 and C

6H

14

100 ppm C6H

14

Schematic Origin of Wetting Delay Period

He

xa

ne C

overa

ge

Induction Periods0

0 100 200 300 400 5000

100

200

300

400

+2σ erro

r

-2σ error

R = 0.906

Hexane Effect on the Wetting Delay

of UV-Induced Wetting of TiO2(110)

We

ttin

g D

ela

y (

s)

Hexane Concentration (ppm)4 data points

P = 1 atm

T = 297-302 K

Fhν

= 1.1x1017

photons cm-2s

-1 (2.1-4.4 eV)

Phν

= 0.049 W cm-2

O2

ConclusionsConclusions

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..

T. Zubkov, D. Stahl, T. L. Thompson, D. Panayotov, O. Diwald, and J. T. Yates, Jr.

J. Phys. Chem. B 109, 15454 (2005)

Photochemistry on semiconductors occurs by e-h pairproduction in the substrate, accompanied by chargetransfer 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 F1/2hυυυυ

dependence of photochemical kinetics.

Hole traps reduce photochemical efficiency.

Hydrophilicity induced by UV on TiO2 - caused byphotooxidation of organic layers in equilibrium withhydrocarbons in atmosphere, causing cleanup of TiO2

surfaces.

What Have We Learned?What Have We Learned?

AcknowledgementsAcknowledgements

Tracy L. ThompsonTracy L. Thompson University of PittsburghUniversity of PittsburghPeter MaksymovychPeter Maksymovych 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

MarylandMarylandTykhon ZubkovTykhon Zubkov University of PittsburghUniversity of Pittsburgh→→ PNNLPNNLDirk StahlDirk Stahl University of PittsburghUniversity of Pittsburgh→→ LeitzLeitzOliver DiwaldOliver Diwald 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