Photocatalysis on Single Crystal TiO22 SurfacesSurfaces · Photochemistry of TMAA on TiO 2 (110): M.A. Henderson, et al. JACS 125 (2003) 14974. Photochemical rate changes due to ‘hydrophobic-to-hydrophilic’

1

Michael A. HendersonInstitute for Interfacial Catalysis

Pacific Northwest National LaboratoryRichland, WA

Photocatalysis on Single Crystal TiO2 SurfacesPhotocatalysis on Single Crystal TiO2 Surfaces

2

OutlineOutline

Motivation for modeling heterogeneousphotocatalysis using single crystals

Examples studies on rutile TiO2(110)- oxygen and water- trimethyl acetic acid- acetone

Conclusions

3

Large and growing numbers of patents and publications involving TiO2-based photocatalysis(Source: SciFinder (Chem Abstracts) search on “TiO2+photo-”)

Growing Interest in TiO2-Based PhotocatalysisGrowing Interest in TiO2-Based Photocatalysis

titaniumart.com140012001000

800600400200

0200420022000

Year

publications patents

Numerous companies market products or services involving use of TiO2 as a photocatalyst(e.g., for self-cleaning glass, water treatment, air purification, disinfection, deodorization, etc.)

4

Important issuesexcitation (band structure and itsmodification)charge diffusion and trappingmolecular adsorptioncharge transferreaction mechanism (coupled redoxand thermal chemistries)poisons, promoters and spectatorssurface and material structure

EVB

ECB

e-

h+

hν

O2

R

OH HR´O

HNa+

Photocatalysis on TiO2Photocatalysis on TiO2

TiO2 photocatalysis cartoon typical found in the literature CO2

O2- ?

R• + H+

e-

CO2the analytical focus of most

photooxidation studies

5

• Annealing TiO2(110) in UHV results in creation of surface oxygen vacancy sites.• Coverage of vacancies is linked to the concentration of bulk defects.• Each vacancy is occupied by what is traditional referred to as 2 Ti3+ cations.

unit cell 2.96Å x 6.49Å

Ti cations (5.2 x 1014 cm-2)

O anions (shaded atoms are two coordinate)

[110]

[110]

[001]-

-

oxygenvacancy

Rutile TiO2(110) single crystal surfaceRutile TiO2(110) single crystal surface

6

OutlineOutline



Conclusions

7

e-?

O2 as an electron scavengerO2 as an electron scavenger

• Excited electrons trap at shallow band gap states on a sub-picosecond time scale.(Colombo and Bowman, JPC 99 (1995) 11752 and 100 (1996) 18445)

• Electrons trapped in surface states can have very long lifetimes (~minutes) depending on the concentration of electron scavenger present.

• Many surface trap sites are OH-related. (Szczepankiewicz, et al. JPCB 106 (2002) 2922)

EVB

ECB O2

O2-

e-

h+

hν

>OHO2

HO2e-

h+

hν

charge removedas an electron

charge removedas an H atom

8

Temperature programmed measurementsTemperature programmed measurements

crystal

QMS

100 WHg arclamp H2O

filter

fiberoptic

filterholder

lens

counts to computer

analog controlof QMS analog control

of power supply

heating leads

thermocouplefrom crystal

UHVchamber

doser

QMScontroller

Computer

DCpowersupply

9

Thermal desorption states of water on TiO2(110)Thermal desorption states of water on TiO2(110)

Desorption states fill sequentially.

Coverage in 500 K TPD peak is equal to the oxygen vacancy population.

2nd layer is H-bonded to bridging O/OH sites

Multilayer fills with ‘non-classical’coverage-dependent behavior.

Hugenschmidt et al., Surf. Sci. 302 (1994) 329. Henderson, Surf. Sci. 355 (1996) 151; Lang. 12 (1996) 5093.

recombinationof bridgingOH groups

ice

1

.5

0

m/e

= 1

8 Q

MS

sign

al (x

106cp

s)

600500400300200100Temperature (K)

Water coverage(ML)

0.61.21.62.33.54.9

x 10

‘2nd layer’water

chemisorbedmolecular water

10

Simulating the role of O2 via reaction with OH groups at Ti3+ sitesSimulating the role of O2 via reaction with OH groups at Ti3+ sites

(Henderson, et al. JPC B 107 (2003) 534)

1.5

1

.5

0

m/e

= 1

8 Q

MS

sign

al (x

105 c

ps)

600500400300200100Temperature (K)

0

0.024

0.064

0.16

0.40

0.80

2.0

postdosedO2 (L)

terminal OH

Ti4+

OH

TiO2(110) w/ 14% vac.bridging OH

Ti3+OH

Ti3+

trappedelectron

OH

Ti4+ Ti3+

O2

O2-

Ti4+ Ti4+ + {OOH}

O + OH

11

TMAA alone (no UV or O2)300 L O2 on TMAAUV irradiation in UHV (no O2)130 L O2 after UV irradiationin UHVUV irradiation in 5x10-7 O2

O2 reaction with Ti(3+)-OH during TMAA photodecompositionO2 reaction with Ti(3+)-OH during TMAA photodecomposition

• Light generated Ti3+-OH groups behave toward O2 like Ti3+-OH groups formed from water dissociation at oxygen vacancies

Water from TMAA on TiO2(110) w/ 7% vac.OH groupsOH*: modified by

trapped electrons?

m/e

=18

QM

S si

gnal

(arb

. uni

ts)

700600500400300200100Temperature (K)

no light or O2

O2, no light

light, no O2

light, then O2

light + O2

12

OH groupson O2- rows

Titration of Ti(3+)-OH groups with O2Titration of Ti(3+)-OH groups with O2

(11x11 nm2)

UV irradiation of TMAin UHV (no O2)

TMA

• Tunneling features located between TMA/Ti4+ rows before O2 exposure are mostly absent after O2 exposure at RT

• Assignment of spots on bridging O2- rows to Ti3+-OH groups is consistent withSTM literature: - water dissociation at vacancies (Brookes et al., PRL 87 (2001) 266103/1; Schaub, et al.

PRL 87 (2001) 266104/1)- H atom exposure to the clean surface (Suzuki, et al. PRL 84 (2000) 2156)

Exposure of UV-irradiated TMAto 100 L O2 in the dark

dashed lines on

TMA/Ti4+

rows

13

Detecting surface electronic/vibrational modes usingelectron energy loss spectroscopy (EELS/HREELS)

Detecting surface electronic/vibrational modes usingelectron energy loss spectroscopy (EELS/HREELS)

AB

e- (energy=E) e- (energy=E-ΔE)

ΔE is associated with electronexcitation of vibrational/electronictransitions at the surface.

14

HREELS of H2O and O2 coadsorbed on vacuum annealed TiO2(110)HREELS of H2O and O2 coadsorbed on vacuum annealed TiO2(110)

ν(OH) mode at 3665 cm-1 is due to OHbr groups formed from water dissociation at vacancies.

O-H product from the reaction of OHbr and O2 is transparent in HREELS. (product is H-bonded and/or tilted?)

OHbr groups are not formed from water adsorption if vacancies are not available.

2

1

0

Inte

nsity

(x 1

05cp

s)

40003000200010000

Electron energy loss (cm -1)

H2O3500

OHbr3665

1 ML H2O

O2 onOHbr

O2 onOHbrheated

to 260 K

1 MLH2O on

oxidizedsurface

54 cm-1

(x 1000)

H-bondedH2O3250

BE = 8.8 eVbkgd CO?

2200

1 ML H2O

heatedto 375 K

(OHbr)

15

Vacancy oxidation from reaction between O2 and OHbrVacancy oxidation from reaction between O2 and OHbr

• Loss at 0.8 eV is due to a excitation of Ti3+ (not due to excitation into TiO2conduction band)

• Ti3+ cations, associated with the 0.8 eV state, are oxidized during the reaction of O2 with OHbr

excitation ofTi3+ sites

band-to-bandexcitation

8

6

4

2

0

Inte

nsity

(x 1

04cp

s)

543210Electron energy loss (eV)

0.8

x 200

heated at 850 K

0.20 ML OHbr

O2 on 0.20 ML OHbr

16

OutlineOutline



Conclusions

17

(2x1) overlayer of TMA on TiO2(110)(2x1) overlayer of TMA on TiO2(110)

Review of carboxylic acids on TiO2(110): H. Onishi, Springer Ser. Chem. Phys., 70 (2003) 75.Photo-induced hydrophilicity on TiO2: J.M. White, et al. JPCB 107 (2003) 9029.Photochemistry of TMAA on TiO2(110): M.A. Henderson, et al. JACS 125 (2003) 14974.Photochemical rate changes due to ‘hydrophobic-to-hydrophilic’ transition: H. Uetsuka, et al.

JPCB 108 (2004) 10621.

Ti4+

Obridging

O2-

trimethylacetate(TMA) C

O

acidproton

VDW diameterof TMA• TMAA dosed at RT yields a

dense-packed TMA adlayer that is stable in UHV at RT and hydrophobic in nature.

18

TMA photodecomposition pathway on TiO2(110)TMA photodecomposition pathway on TiO2(110)

C4 alcohol(a) ?(CH3)3CH (a)

(CH3)2C=CH2 (a)

(CH3)2CO (a)CO2 (a)H2O (a)

hν O2

CH3

CO O-

O2

hν

CO2

(CH3)3C•C

O

H3C

O

C

CH3CH3

-hν(h+)C5

C4

C3

C2

hν, Ο2CO2 + ?

photo-desorption

+ CH3 (g)

Henderson, et al. JPC B 107 (2003) 9029; 108 (2004) 3592; 108 (2004) 10621; 108 (2004) 18932; 109 (2005) 12062; 109 (2005) 12417; Langmuir 21 (2005) 3443; JACS 125 (2003) 14974; J. Catal. 238 (2006) 111, 153.

19

STM of TMAA dosed on TiO2(110) at RTSTM of TMAA dosed on TiO2(110) at RT(b) satn. TMA on ‘a’(a) vacuum annealed surface

0.12 ML vacancies

(d) satn. TMA on ‘c’(c) 100 L O2 on ‘a’

0.09 ML O adatoms0.04 ML vacancies

0.50 ML TMA

0.45 ML TMA

oxygenvacancy

oxygenadatom

On the clean surface:- bright rows = Ti4+

- dark rows = O2-

(U. Diebold, SSR 48 (2003) 53.)

Pre-oxidation affects both TMA order and rate of TMA photodecomposition.

M.A. Henderson, et al. JACS 125 (2003) 14974.H. Uetsuka, et al. JPCB 108 (2004) 10621.

CO2 photodesorption

Sign

al (a

rb. u

nits

)

3002001000Time (seconds)

UV onin UHV

(16 x 16 nm2)

20

Inte

nsity

(arb

. uni

ts)

3210Electron energy loss (eV)

x 625

Ti3+

elasticpeak

phonon

(a) TMAA adsorbedon oxidized TiO2(110)

(b) 10 sec UV(c) 30 sec UV(d) 75 sec UV(e) 300 sec UV(f) 240 L O2 on 'e'

.2

.1

0.08.06.04.020

Ti3+ yield (ML)

isobutene CO2

Photo-desorption

yield(ML)

Photo-excited electron trapping on TMA-covered TiO2(110)Photo-excited electron trapping on TMA-covered TiO2(110)

• Ti3+ feature same as that observed from vacancies

• Electron trapping not observed on the clean surface

• Electron trapping yield correlates with the photo-desorption yields from hole transfer; both are needed!

• O2 titrates trapped electrons

no O2; UV irradiation at RT

21

STM during photodecomposition of TMASTM during photodecomposition of TMA

(a) TMA monolayer on a vacuum-annealed TiO2(110) surface, and after UV irradiation at 280 K in 1x10-7 torr of O2 for (b) 10, (c) 15, (d) 20, and (e) 30 min. (Image size: 88x88 nm2; Xe lamp)

(a) (b)

(e)(d)

(c)

TMA(on Ti-rows)

OH?

• Voids develop in TMA layer at an accelerated rate during photolysis

• Voids possess weak spots attributable to OH groups

• TMA groups on steps show lower reactivity than those on terraces.

no light

30 min

20 min

15 min10 min

22

1

.5

0

QM

S si

gnal

(x10

-3 M

L/se

c)


Influence of O2 on TMA photodecomposition selectivityInfluence of O2 on TMA photodecomposition selectivity

UV on UV off

isobutene(x1)

isobutane(x5)

0.5 ML TMAA at RTUV irradiation at RT

UHV

2x10-6 torr O2

UHV

2x10-6 torr O2

• Selectivity in the first step of TMA photooxidationshows strong O2 pressure dependence

• Temporal changes in the selectivity are linked to coverage dependence in TMA photodecomposition

(Henderson et al.J. Catal. 238 (2006) 153)

23

Photocatalytic selectivity and adlayer phase dynamicsPhotocatalytic selectivity and adlayer phase dynamics

1

.8

.6

.4

Isob

uten

e fr

actio

nal y

ield

3002001000Irradiation time (seconds)

2x10-6 5x10-7

2x10-75x10-8

1x10-8

5x10-9

UHV

0.5 ML TMA at RTUV irradiated at RT

in O2 (torr)

100%isobutene

1:1 ratio ofisobutene

and isobutane

TMA+hν+1/2O2

CO2+ene+OH (H2O)

2TMA+hν

2CO2+ene+ane

UV in1x10-6 torr

O2

UV inUHV

hydrophobic

hydrophillicdomains

hydrophobic

hydrophobic

(2x1) TMA/TiO2(110)

24

OutlineOutline



Conclusions

25

TMA photodecomposition mechanism for photolysis at 100 KTMA photodecomposition mechanism for photolysis at 100 K

C4 alcohol(a) ?(CH3)3CH (a)

(CH3)2C=CH2 (a)

(CH3)2CO (a)CO2 (a)H2O (a)

CO2

(CH3)3C•

hν O2

CH3

CO O-

O2

hν

CO

H3C

O

C

CH3CH3

-hν(h+)C5

C4

C3

C2

hν, Ο2CO2 + ?+ CH3 (g)

26

Mas

s 46

QM

S si

gnal

(arb

. uni

ts)

700600500400300200100Temperature (K)

d6-acetone on pre-reduced TiO2(110)

d6-acetone on pre-oxidized TiO2(110)

375

320

165

125

345

125

175

230

285 acetone coverage

(ML)

0.120.270.380.500.650.770.981.2

Acetone thermal chemistry on TiO2(110)Acetone thermal chemistry on TiO2(110)

• Little or no thermal decomposition

• Acetone desorption is influenced by coverage and the surface redoxcondition

• Pre-oxidation stabilizes acetone and minimizes acetone-acetone repulsions

acetone-oxygencomplex

η1-acetone

η1-acetoneice

μ(g) = 2.9 D

MeMe

Ti

OC

MeMe

Ti

OC

δ- δ-

δ+δ+

d6-acetone onreduced TiO2(110)

7% oxygen vacancies

filled vacanciesand reactive O species

d6-acetone onoxidized TiO2(110)

27

Inte

nsity

(arb

. uni

ts)

2000150010005000Electron energy loss (cm-1)

x 10

x 200

Organic and O2 thermal chemistry: Conversion of a photo-inactive species to a photo-active species

Organic and O2 thermal chemistry: Conversion of a photo-inactive species to a photo-active species

400 K

285 K

235 K

135 K

105 K

Multilayer d6-acetoneheated to:

20 L O2; ΔT to RT in the dark

ν(OCO) ofcomplex

Me

C

O O-hν (h+?)

Me•(g)

acetate

O

C

O

MeMe


MeMe

OC hν no

photo-reaction

28

Photodecomposition of 1 ML acetone on TiO2(110)Photodecomposition of 1 ML acetone on TiO2(110)M

ass 4

2 Q

MS

sign

al (a

rb. u

nits

)

700600500400300200100Temperature (K)

photolysis of 1 ML acetone at 95 K in 5x10-7 torr O2

1

.5

0Peak

are

a (a

.u.)

200150100500UV exposure (min)

acetone

ketene

acetone

180

80

0

10

50

ketene(from acetate

decomposition)UV exposure

(minutes)

• Acetone is photo-decomposed to acetate, which thermally decomposes to ketene at 620 K.

photolysis of 1 ML acetoneat 95 K in 5x10-7 torr O2

ketene

acetic acid

QM

S si

gnal

(arb

. uni

ts)

600400200Temperature (K)

mass 14mass 42

(both ÷ 2)

mass 60

RT saturation of acetic acid on TiO2(110)

RT satn. ofacetic acid

on TiO2(110)

Immediate reaction product does not leave the surface!

29

Cross sections for acetone photodecomposition from TPDCross sections for acetone photodecomposition from TPD

-1.5

-1

-.5

0

ln (

θ t / θ

i )

1.51.50Photon exposure (x 1021 photons/cm2)

acetoneTPD peak area

data taken from: mass 42 mass 43 mass 58

1 ML acetoneσ = 3 x 10-22 cm2

0.25 ML acetoneσ = 3 x 10-21 cm2

fast rate for both coverages• Cross section of acetone photo-decomposition is coverage dependent; greater for lower coverages

• Fast initial and slow subsequent photo-decomposition rates

• Cross sections resemble gas phase values

Photon exposure (x1021 cm-2)

30

• Methyl radicals ejected from the surface during photolysis

• Fast and slow evolution of methyl radicals

• H2CO is formed from reaction of methyl radical on the walls of the mass spectrometer. (H2COin powder studies from ejected CH3radicals?)

Phot

odes

orpt

ion

sign

al (a

rb. u

nits

)


mass 15mass 14mass 13

mass 29mass 30

mass 15(x 20)

mass 18

mass 30

photolysis of 0.25 ML acetone at 220 K in 5x10-8 torr O2

acetone

d6-acetone

mass 43

mass 20

UVon UV

offCD3no CD4signal!

CH3(fast rate)

H2CO

CH3(slow rate)

Photodesorption during UV irradiation of acetone on TiO2(110)

Photodesorption during UV irradiation of acetone on TiO2(110)

d6-acetone

acetone

photolysis of 0.75 ML acetoneat 200 K in 5x10-8 torr O2

no acetone

31

Cross sections for methyl radical ejectionCross sections for methyl radical ejection

fast rateσ ~ 10-18 cm2

slow rateσ = 6 x 10-21 cm2

• Cross section for fast rate is two orders of magnitude greater than the slow rate

• Cross section for slow rate (~6x10-21

cm2) matches that obtained by TPD (~3x10-21 cm2); this suggests conversion of acetone to acetate and methyl radical ejection are mechanistically linked.

-0.6

-0.4

-0.2

0

ln (

θ t / θ

i )

43210Photon exposure (x 1019 cm-2)

32

Proposed acetone photodecomposition mechanismProposed acetone photodecomposition mechanism

OTi3+

Ο2CO

CH3CH3 C

O

CH3CH3

in thedark


O

CO

CH3CH3

CH3

CO O-hν

•CH3 (g)UVfast

processacetate

UVslow

process(O2 flux

dependent)

Ti4+ hν Ti3+ + h+

Ti3+ + O2 Ti4+ + O2-

acetone + O/O2- acetone-oxygen complex

R

33

Hydrogen production on TiO2Hydrogen production on TiO2

trappedelectron

OH

Ti4+ Ti3+

ΔT + H2OTi4+ Ti3+

O2

O2-

Ti4+ Ti4+ + {OOH}

ΔTO2-

Ti4+ Ti4+ + H2

Pt HO2-

Ti4+ Ti4+TiO2

Pt

AA-H

What about the holes?

34

ConclusionsConclusions

AcknowledgementsPNNL: Dr. Janos Szanyi

External: Prof. Hiroshi Onishi and Dr. Hiroshi Uetsuka (Kobe University, Japan) Prof. Mike White (University of Texas, Austin, TX)Mr. Matt Robbins (Stanford University, Stanford, CA)

Funding: DOE Office of Basic Energy Sciences, Divisions of Chemical Sciences and Materials Sciences

Meaningful insights into the molecular-level details of heterogeneous photocatalysis can be obtained from model studies.- Identification of charge transfer and trapping sites - Detection of adsorbed and photodesorbed intermediates- Determination of reaction pathways and selectivities- Measurement of cross sections (rates)- Observation of spatial effects such as evolution of hydrophobic and

hydrophilic domains

Documents

Photocatalysis on Single Crystal TiO22 SurfacesSurfaces · Photochemistry of TMAA on TiO 2 (110): M.A. Henderson, et al. JACS 125 (2003) 14974. Photochemical rate changes due to ‘hydrophobic-to-hydrophilic’