Role of Ultrafast Charge Dynamics in

Photocatalytic Water Oxidation

NanotekNanotekNanotekNanotek, , , , December 2014December 2014December 2014December 2014

Tanja Cuk

Assistant Professor, Chemistry, UCB

Faculty Scientist, CSD, LBNL

Transient Optical, Infrared

SpectroscopyTransient X-Ray Spectroscopy

Heterogeneous CatalystsControlling Charge Flow

CO2 + H2O + hνννν ����FUEL + O2

Fuel

O2

H2O , CO2

Water Oxidation Catalysts: Water Oxidation Catalysts: Water Oxidation Catalysts: Water Oxidation Catalysts:

Transition Metal OxidesTransition Metal OxidesTransition Metal OxidesTransition Metal Oxides

Hydroxilated Surfaces

Bluhm, Salmeron, Nilsson et. al.,

J. Phys. Chem. C (2010)

• Efficient and Sustainable Catalysis on TM Oxide Surfaces

• Hydroxilated surfaces common to TM Oxide Surfaces

Robust Catalysis

Fe2O3

4h+ + 2H2O O2 + 4H+ E0 = 0.6 V vs. Ag/AgCl

Heterogeneous CatalysisHeterogeneous CatalysisHeterogeneous CatalysisHeterogeneous Catalysis

Activation Barrier thru Transient Spectroscopy

Dynamic Catalytic Surface

Suggested First Hole Transfer (t=0):

Helmholtz Double Layer

[h+] + [OH-] [OH*]

Challenges Challenges Challenges Challenges in Applying Ultrafast in Applying Ultrafast in Applying Ultrafast in Applying Ultrafast

Spectroscopy to CatalysisSpectroscopy to CatalysisSpectroscopy to CatalysisSpectroscopy to Catalysis

• Interplay of recombination and interfacial charge transfer

• Sensitivity to the surface

• Studying hetero-junctions (solid-solid, solid-liquid)

• Multi-electron transfer processes and “clocking” the cycle

Strategy and TechniquesStrategy and TechniquesStrategy and TechniquesStrategy and Techniques

Working

electrodeCounter

electrode

Reference

electrode

h+

h+

h+

h+

h+

Catalysts Catalysts Catalysts Catalysts &&&& Devices Under InvestigationDevices Under InvestigationDevices Under InvestigationDevices Under Investigation

n-SrTiO3, High Over-Potential Co3O4 , Low Over-Potential

10 O2/site-s 0.01 O2/site-s

nnnn----type SC/Liquid type SC/Liquid type SC/Liquid type SC/Liquid

SchottkySchottkySchottkySchottky BarrierBarrierBarrierBarrier

nnnn----p GaAs photodiode/Cop GaAs photodiode/Cop GaAs photodiode/Cop GaAs photodiode/Co3333OOOO4444

HeteroHeteroHeteroHetero----junctionjunctionjunctionjunction

PhotoPhotoPhotoPhoto----electrochemistry of nelectrochemistry of nelectrochemistry of nelectrochemistry of n----SrTiOSrTiOSrTiOSrTiO3333

High Quantum Efficiency with 1 kHz Laser (W ∼∼∼∼ αααα)

• Single 150 fs Pulse Triggers Multi-electron Transfer Water Oxidation

• Achieve high quantum efficiency (75%) under laser excitation

-10

0

I (

µΑ

)µ

Α)

µΑ

)µ

Α)

1.0 0.5 0.0 -0.5 -1.0 V (vs. Ag/AgCl)

Dark

0.04 mJ/cm2

, Q.E. = 75%

VRB

VRB

Transient Reflectance of nTransient Reflectance of nTransient Reflectance of nTransient Reflectance of n----SrTiOSrTiOSrTiOSrTiO3333

Width of Electric Field at High Q.E.:

W ∼∼∼∼ 25 nm

Pump Band Gap (300 nm, 4 eV):

αααα = λλλλ/4ππππk ∼∼∼∼ 24 nm

Probe Hole Absorption (800 nm, 1.5 eV):

ααααREFL = λλλλ/4ππππn ∼∼∼∼ 27 nm

Surface Sensitivity Thru Reflectivity

UV-VIS Ellipsometry

Experimental conditions needed

to probe kCT , [h] :

W = ααααPUMP = ααααPROBE

Pump Band Gap, Probe Holes

Interfacial Charge TransferInterfacial Charge TransferInterfacial Charge TransferInterfacial Charge Transfer

Change in kinetics reflects kCT, [h]

SrNb0.001TiO3 (0.1%) SrNb0.007TiO3 (0.7%)

W = 25 nm &&&& ααααpump , ααααprobe ∼∼∼∼24 nm W = 9 nm &&&& ααααpump , ααααprobe ∼∼∼∼24 nm

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

No

rm ∆∆ ∆∆

R/R

1 10 100 1000

Time Delay(ps)

0.1M NaOH

0.2M NaSCN

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

No

rm ∆∆ ∆∆

R/R

1 10 100 1000

Time Delay (ps)

0.1M NaOH, τ = 3 τ = 3 τ = 3 τ = 3 ns

0.2M NaSCN, τ = 1 τ = 1 τ = 1 τ = 1 ns

Manipulating Interfacial Charge TransferManipulating Interfacial Charge TransferManipulating Interfacial Charge TransferManipulating Interfacial Charge Transfer

• Interfacial charge transfer rate increases with increasing oxidative voltages

• Rate changes while current and quantum efficiency constant

SrNb0.001TiO3 (0.1%)

-10

-5

0

5

I (

µΑ

)µ

Α)

µΑ

)µ

Α)

-1.0-0.50.00.51.0

V (vs. Ag/AgCl)

Dark

0.04 mJ/cm2, Q.E.= 75%

-1.0

-0.5

0.0

0.5

No

rm ∆∆ ∆∆

R/R

1 10 100 1000

Time Delay (ps)

-0.92V -0.4V 0V 0.2V 0.7V

-5.0

-4.0

-3.0

-2.0

-1.0

0.0

Lo

g k

-1.0 -0.5 0.0 0.5 1.0 V

log k

fit log k

-1.0

-0.5

0.0

0.5∆∆ ∆∆

R/R

1 10 100 1000

Time (ps)

0.7V fit 0.7V

-1.0

-0.5

0.0

∆∆ ∆∆R

/R

1 10 100 1000 Time (ps)

-0.4V fit -0.4V

-1.0

-0.5

0.0

∆∆ ∆∆R

/R

1 10 100 1000 Time (ps)

0V fit 0V

-1.0

-0.5

0.0

∆∆ ∆∆R

/R

1 10 100 1000 Time (ps)

-0.92V fit -0.92V

-1.0

-0.5

0.0

∆∆ ∆∆R

/R

1 10 100 1000 Time (ps)

0.2V fit 0.2V

Kinetics (Kinetics (Kinetics (Kinetics (VVVV))))

• Kinetics are fit to a single exponential at early time scales

• Rate constant depends exponentially on applied V (Arrhenius Law)

M. Waegele, X. Chen, D. Herlihy, T. Cuk, JACS 2014

Voltage Distribution at the SurfaceVoltage Distribution at the SurfaceVoltage Distribution at the SurfaceVoltage Distribution at the Surface

CH

CSC UFB

UH

USC

U

CSC CH

UH

USC

VB

CB

�

UFB + U = UH + USC

φφφφ��

φφφφ��

=

surface hole potential

• Since the solution potential is invariant, changes in UH are tied to

changes in the valence band edge potential, φφφφ��

• Changes in UH (Helmholtz Voltage) by applied U determined by

capacitances at n-type SC/liquid interface

U = applied voltage

φφφφOH-/OH

UUUUHHHH(V): Changing Surface Hole Potential(V): Changing Surface Hole Potential(V): Changing Surface Hole Potential(V): Changing Surface Hole Potential

(1) U + UFB = UH + USC

(2) UH = (qsc + qphoto) / CH

Brockeris, Bard, Bocarsly

n-SrTiO3 Electrolyte

η(V)

VB

φφφφ��/��

−

UH(0)

UH(V)

φφφφ��(�)

1.4

1.2

1.0

0.8

0.6

0.4

0.2

UH

(V

)

-1.0 -0.5 0.0 0.5 1.0

Uappl (V)

2.2

2.0

1.8

1.6

1.4

1.2

1.0

ηη ηη (V

)

0.045 mJ/cm2

0.065 mJ/cm2

Dark

CH = 21 µµµµF/cm2

UH(0) = 0.65 V give CH = 21 uF/cm2

αe0∆UH(V)

e0UH

OH*h+

e0∆UH(V)

k = k0 exp (∆∆∆∆G*/4kBT)

k = k0exp (αααα FUH / RT)

Activation Barrier of First Hole TransferActivation Barrier of First Hole TransferActivation Barrier of First Hole TransferActivation Barrier of First Hole Transfer

-4.0

-3.0

-2.0

-1.0

log

(k

)

1.21.00.80.6

UH (V)

αααα = 0.24

[h+] + [OH-] [OH*]

α = 0.2 ± 0.05

Quantifies barrier to localizing VB hole onto a molecular O2p bond

k

k0 = 7 x 10-7 ps-1 (µµµµs)

e0φφφφ�� /��∗

φφφφ�� /��∗

= 1 V vs. SCE

[From Photoemission, 1.2 V]

ConclusionsConclusionsConclusionsConclusions

• Quantified interfacial charge transfer at n-type

semiconductor/liquid interface

• Activation barrier (α, k0) for first hole transfer of water

oxidation reaction in n-SrTiO3

• Next step on n-SrTiO3: concomitant intermediates using

ultrafast infrared spectroscopy

• Investigate lower over-potential catalysts, and other n-

type semiconductors stable in aqueous solutions (e.g.

GaN)

Next StepsNext StepsNext StepsNext Steps

Graduate StudentsGraduate StudentsGraduate StudentsGraduate Students

Hoang Doan

Stephanie Choing

Kevin Pollack

David Herlihy

Xihan Chen

Jonathon Radberg

PostdocPostdocPostdocPostdoc

FundingFundingFundingFunding

Aayush Singh (Undergraduate)

Joseph Mosley (Volunteer)

CPIMS Program

AcknowledgementsAcknowledgementsAcknowledgementsAcknowledgements

Matthias Waegele

Facilities/DiscussionsFacilities/DiscussionsFacilities/DiscussionsFacilities/Discussions

Heinz Frei

Steve Leone

Gabor Somorjai

Eli Yablonovitch

Ian Sharp (JCAP)

Joel Ager (JCAP)

Michelle Chang

AFOSR Young Investigator (Co3O4)

Possible Intermediate Species FormedPossible Intermediate Species FormedPossible Intermediate Species FormedPossible Intermediate Species Formed

[h+] + [OH-] [OH*] [-OOH or -OOTi]

• -OOH is a likely intermediate: ms-FTIR (Frei)

• Single rate constant (k1) implies a highly populated –OOH surface?

• Probe nature of OH* (or O*) as surface trap (transient XAS)

k1 k1”

-1.0

-0.5

0.0

0.5

∆∆ ∆∆R

1 10 100 1000

Time Delay (ps)

0.7 V

k1

k1”

-10

-5

0

∆∆ ∆∆ m

OD

8006004002000

Time (ps)

-15

-10

-5

0

2079 cm-1

(SCN)

2202 cm-1

(Free Carriers)

-10

-5

0

∆∆ ∆∆ m

OD

-1.0 1.0

Time (ps)

-15

-10

-5

0

Ultrafast Transient IRRASUltrafast Transient IRRASUltrafast Transient IRRASUltrafast Transient IRRAS

• Small molecule rotations/bindings (-SCN, -NCS)

• Reaction Intermediate Rise Times

• Effects of surface potential/electric field vs. charge transfer

Transient -SCN / -NSC at n-SrTiO3 Decays with n-SrTiO3 Free Carriers

-15

-14

-13

-12

-11

∆

∆

∆

∆ m

OD

2200 2150 2100 2050 2000

Wavenumber (cm-1

)

2 ps

Transient Grating DiffractionTransient Grating DiffractionTransient Grating DiffractionTransient Grating Diffraction

Heterodyne Detection: TG(t), R(t), ϕϕϕϕ (t) determined by varying Ψ, probe-ref phase

∆I/I = R(t)cos(ϕϕϕϕ(t)) + TG(t)cos [Ψ - ϕϕϕϕ (t)]

Recombination (t) = R(t)cos(ϕϕϕϕ(t))

Diffusion (t) = εεεε (t) = TG(t)/R(t) = exp (-Dq2/t)

Catalysts Catalysts Catalysts Catalysts &&&& Devices Under InvestigationDevices Under InvestigationDevices Under InvestigationDevices Under Investigation

n-SrTiO3, High Over-Potential Co3O4 , Low Over-Potential

10 O2/site-s 0.01 O2/site-s

nnnn----type SC/Liquid type SC/Liquid type SC/Liquid type SC/Liquid

SchottkySchottkySchottkySchottky BarrierBarrierBarrierBarrier

nnnn----p GaAs photodiode/Cop GaAs photodiode/Cop GaAs photodiode/Cop GaAs photodiode/Co3333OOOO4444

HeteroHeteroHeteroHetero----junctionjunctionjunctionjunction

Pump From Back, Probe From Front at Open Circuit

e

h

τR

τINJECTh

-

-

-

-

-

-

+

+

+

+

+

+

+

p-G

aA

s

n+

-Ga

As

Co

3O

4

Transient Reflectivity on HeteroTransient Reflectivity on HeteroTransient Reflectivity on HeteroTransient Reflectivity on Hetero----junctions junctions junctions junctions

Probe depth

• Charge separation spatially separated from hole injection into catalyst

• Ultrafast charge injection (∼∼∼∼ 10 ps) unimpeded by slow diffusion kinetics

• Can recombination kinetics in GaAs & hole kinetics in Co3O4 be separated in a transient

reflectivity experiment?

• Surface sensitivity of ∆R w/ 400 nm probe

• Strong signal of surface holes with catalyst

• Time delay for hole injection (~10 ps)

Kinetics of Holes in Catalyst OverKinetics of Holes in Catalyst OverKinetics of Holes in Catalyst OverKinetics of Holes in Catalyst Over----layerlayerlayerlayer

• Kinetics vary w/ catalyst type

• Also with deposition technique

• Reproducibility for same batch

0.3

0.2

0.1

0.0

-0.1

-0.2

-0.3

∆∆ ∆∆R

/R (

%)

1 10 100 1000

Time Delay (ps)

GaAs 800 nm, Bulk Recombination GaAs 400 nm, Surface Holes Co3O4 ALD 400 nm, Surface Holes

x 0.03

Hole

injection

GaAs

Co3O4 ∆∆ ∆∆

R/R

(%

)

1 10 100 1000

Time Delay (ps)

Co3O4 ALD

Co3O4 Sputt

IrO2 PVD

~0.5 mJ/cm2

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0∆∆ ∆∆

R/R

(%

)

100806040200

Time Delay (ps)

Co3O4 ALD

Co3O4 Sputt

• 0.8 THz (& w/ 0.4 THz) signal generated from catalyst over-layer, independent of catalyst type

• Strength and decay of THz signal depends on catalyst type and deposition technique

Hole Carriers as an antenna to THz?Hole Carriers as an antenna to THz?Hole Carriers as an antenna to THz?Hole Carriers as an antenna to THz?2.0

1.5

1.0

0.5

0.0

∆∆ ∆∆R

/R (

%)

100806040200

Time Delay (ps)

IrO2 PVD

τmobile holes

0.87, 0.44 THz0.87 THz

p-GaAs

+

+

+

+

+

-

-

-

-

THz Generation THz Antenna

Free Carriers Injected in Catalyst

Responding to THz field

Plasma frequency of p-dopants

responding to ∆∆∆∆E field

Effects of Electrolyte?

• Potentially, mobile holes responding to THz in GaAs that decay to surface trapped states

~5 mJ/cm2

ConclusionsConclusionsConclusionsConclusions

• Activation barrier (α, k0) for first hole transfer of water

oxidation reaction in n-SrTiO3

• Next steps n-SrTiO3: (1) concomitant intermediates using

ultrafast infrared spectroscopy, (2) transient diffraction

gratings for interfacial hole diffusion

• Ultrafast photodiode for low over-potential catalysts:

dynamics of charge-separated carriers at electrolyte

interfaces

Graduate StudentsGraduate StudentsGraduate StudentsGraduate Students

Hoang Doan

Stephanie Choing

Kevin Pollack

David Herlihy

Xihan Chen

Jonathon Radberg

PostdocPostdocPostdocPostdoc

FundingFundingFundingFunding

Aayush Singh (Undergraduate)

Joseph Mosley (Volunteer)

CPIMS Program

AcknowledgementsAcknowledgementsAcknowledgementsAcknowledgements

Matthias Waegele

Facilities/DiscussionsFacilities/DiscussionsFacilities/DiscussionsFacilities/Discussions

Heinz Frei

Steve Leone

Gabor Somorjai

Eli Yablonovitch

Ian Sharp (JCAP)

Joel Ager (JCAP)

Michelle Chang

AFOSR Young Investigator (Co3O4)

Reaction via surface hole

(valance band oxygen)

Ti

O

O

O

Ti

Ti

h+ Ti

O

O

O

Ti

Ti

OH-

TiO

O

O

Ti

Ti

O

OH- h+

H

Ti

O

O

O

Ti

Ti

O

(1)

Ti

O

O

O

Ti

Ti

O

h+

OH-

Ti

O

O

O

Ti

Ti

O

OHOH-

h+

Ti

O

O

O

Ti

Ti

+ O2(2)

Ti

O

O

O

Ti

Ti

O

h+

OH-

Ti

O

O

O

Ti

Ti

O

OH

Ti

O

O

O

Ti

Ti

O

+ OH

(3)

Reaction via hot hole

(surface axial oxygen)

Number of surface site calculation

1. SrTiO3 has a lattice constant of a=3.905A

2. (1,0,0) surface for SrTiO3 is cubic, which looks like the graph on the right

3. Each (1,0,0) lattice has 3 surface atoms(1 Ti, 2 O)

4. Therefore, each lattice has area of A=a2=1.525*10^-15 cm2

5. Surface atom density=# of atoms on one lattice surface/lattice surface area

So, surface atom density= 3/1.525*10^-15cm2 = 1.967*10^15 atoms/cm2

Voltage Distribution (Dark, Equilibrium)Voltage Distribution (Dark, Equilibrium)Voltage Distribution (Dark, Equilibrium)Voltage Distribution (Dark, Equilibrium)

Mott-Schottky: UFB = 1.6 V Photo-Voltage: UOC = USC = 0.85 V

• Photo-voltage at open circuit roughly half of UFB

• UH= 0.65 V in the dark, at equilibrium (no applied U)

1.0

0.8

0.6

0.4

0.2

0.0

UO

C (

V)

1086420

Laser (µµµµJ/cm2)

0.1% Nb

0.7% Nb

5x1014

4

3

2

1

0

C-2

(cm

4/F

2)

-1.5 -1.0 -0.5 0.0 0.5 1.0

Uappl (V)

0.1% Nb

0.7% Nb

Ufb = USC + UH

Voltage Distribution (Illumination)Voltage Distribution (Illumination)Voltage Distribution (Illumination)Voltage Distribution (Illumination)

Capacitance Illuminated Junction

• Photo-induced holes at interface create an interfacial p-type layer

• This interfacial “carrier inversion” changes UH, even with no applied U

3

2

1

0

C (

F/c

m2)

x 1

0-3

-1.5 -1.0 -0.5 V (vs. Ag/AgCl)

Dark

0.01 mJ/cm2

0.04 mJ/cm2

VFB

VOC

Turner, Nozik APL 37 (1980)

CB

VB

n-type p-type

Reverse

BiasForward

Bias

qsc qphoto

Double

Layer

10μm 10μm

1μm 10μm

a) b)

c) d)

SEM images a)-d) show damage on 0.1% Nb doped

SrTiO3 under laser and applied bias condition

a) Damaged sample at 1500×magnification

b) Damaged sample at 1500×magnification

c) Damaged sample at 3500×magnification

d) Undamaged sample at 1500×

magnification

1. SEM image shows laser burned holes on sample surface, most of the holes have 5um as diameter while

some big holes have 10um as diameter

2. No damage is observed if only laser is applied to our sample, damage happens when current is running

through the sample and laser is applied.

3. Laser spot size is 500um(FWHM) but the peak is narrow around 10um

4. One possible mechanism for the damage is coulomb explosion where high density of excitons

generated by laser accumulate at sample surface causes explosion

a)

c)

Water Oxidation Water Oxidation Water Oxidation Water Oxidation &&&& OverOverOverOver----PotentialPotentialPotentialPotential

How do the surface dynamics modify the kinetic barriers

and thermodynamics of each intermediate in the cycle?

High over-potential,

Lower kinetic barriers

~10’s O2 /site-sec

Thermodynamic potential,

High kinetic barriers

~ 0.01 O2 /site-sec