1 Michael A. Henderson Institute for Interfacial Catalysis Pacific Northwest National Laboratory Richland, WA Photocatalysis on Single Crystal TiO 2 Surfaces Photocatalysis on Single Crystal TiO 2 Surfaces
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
Motivation for modeling heterogeneousphotocatalysis using single crystals
Examples studies on rutile TiO2(110)- oxygen and water- trimethyl acetic acid- acetone
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
Motivation for modeling heterogeneousphotocatalysis using single crystals
Examples studies on rutile TiO2(110)- oxygen and water- trimethyl acetic acid- acetone
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)
3002001000Time (seconds)
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
Motivation for modeling heterogeneousphotocatalysis using single crystals
Examples studies on rutile TiO2(110)- oxygen and water- trimethyl acetic acid- acetone
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
acetone-oxygencomplex
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
)
3002001000Time (seconds)
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
acetone-oxygencomplex
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