Production of Hydrogen Using Tit i B d Ph t t l tTitania Based Photocatalysts

Wonyong Choi

School of Environmental Science and Engineering

D f Ch i l E i iDept. of Chemical Engineering

Pohang University of Science and Technology (POSTECH)

Pohang, KOREA

Solar Energy Based Hydrogen EconomySolar Energy Based Hydrogen Economy

H2 production

CO2-free CO2-neutralCO2-producing

Electrolysis(alkaline, PEM

PhotoelectrolysisPhotocatalysis

(PEC semiconductor

Thermo-chemical Natural Oil Coal

Thermo-chemical

Biological process

H2 from fossil fuelsWater splitting H2 from biomass

Wind

( ,electrolyzer)

(PEC, semiconductorpowder/colloid)

cycles(S-I cycle, etc)

PV+ Storage of l i i

High-T electrolysis

Solarth l

Nuclearth l

Natural gas

Oil Coal

Gasification

Steam

process(gasification,

pyrolysis, reforming)

process(anaerobic digestion,

fermentation)

+

Wind+EL

Proven technology but costly

PV+EL

electricity as H2

electrolysis(solar,

nuclear)

thermal thermal reforming/ water gas

shiftCO2 H2

but costly process

H2 CO2+Current

Ideal solar-to-hydrogen conversion process but far from

i l

Current industrial process

C & Scommercial realization

Capture & Storage(CCS)

Solar EnergySolar Energy

51.2 x 105 TW(10,000 x Current world demands)

• Abundant

• Environment-friendly energy source

• Safe and Clean Earth

~ 0.1% of the Earth’s surface

f

(5 times as big as South Korea)+

10% i ffi iGlobal need

13 TW ~ 10% conversion efficiency13 TW

Photocatalysis as a mean of solar energy conversion

A/A-

H O/H

e-

e-

H2O/H2

ΔG0(H O H + 1/2O )hν

ΔG0(H2O → H2 + 1/2O2)=56.7 kcal/mol

H2O/O2

D/D+

e-e-

Photocatalyst(usually semiconductors)

Water Splitting on a Photocatalyst Particle

1. Photon absorptionGeneration of e- and h+

with sufficient potentials for3. Reductionfor H2 evolution

Photonwith sufficient potentials forwater splitting (Band Engineering)

H O

H2

2

e- h++

H2O

e- h++2. Charge Separationand migration to surface reaction sites xreaction sites x

4. Oxidation

H2O

O2 for O2 evolution2

Band Gap Positions in Various Semiconductors

Vacuum level0 E vs. NHEeV

-3.0

-1.0

-4.0

-5.0

-4.5

2.3

1.1

3.0

2.5

H+/H20

-6.0 3.0

3.2

3.2 3.4

2.8

2.2

3.2

3.8

GaP

Si

2

+2 0

+1.0

-7.0

-8 0 iO TiO Z OFe2O3

GaPSiC

CdS

+2.0

+3.0

-8.0 TiO2Rutile

TiO2Anatase SrTiO3 Nb2O5

WO3

ZnO2

SnOSnO2

@ pH 0

Common Strategies for D l i Vi ibl Li ht Ph t t l tDeveloping Visible Light Photocatalysts

1. Impurity Doping in Wide Band-gap Oxide Semiconductors- transition metal ions (cations) - nitrogen, carbon (anions)

2. Sensitization of Wide Band-gap Oxide Semiconductors- organometallic complexes (e.g., ruthenium bipyridyl derivatives)- organic dyesorganic dyes- inorganic quantum dots (e.g., CdS)

3 Nanohybrid Systems3. Nanohybrid Systems (metal oxides & chalcogenides, metal nanoparticles, organic & inorganic sensitizers, polymers, etc.)

DyeDye--Sensitized TiOSensitized TiO22 Solar CellSolar Cell

R

Schematics of DSSC Performance of DSSC

Jsc : short circuit current

Dye*(-) ECB

ee-- Voc: open circuit voltageff : fill factor

ff =J V

PmaxDye+/0y

(eV

)

Pt

( )

A-A-

Jsc×Voc

Cur

rent

Jsc

Dye+/0

Ener

gy

(+)

AA

EVB

Eg(TiO2) ≈ 3.2 eV

ee--

I3- + 2e- ↔ 3I-II33-- + 2e+ 2e-- ↔↔ 3I3I--

Jsc = 18mA/cm2

Voc = 0.74V

0C

VocSemiconductor Solution

(+) VB 333 ff = 73%

0Voltage

HH22 Production on DyeProduction on Dye--Sensitized TiOSensitized TiO22

ee--ee--

PtPt CBCB SS++/S/S**HH2200

1/2H1/2H + OH+ OH--

SS++/S/S

D/DD/D++1/2H1/2H22 + OH+ OH

VBVB

//

2H+ + 2e- H2 (E0 = -0.41 V) (1)

2H2O O2 + 4H+ + 4e- (E0 = +0.82 V) (2)

H O H + 1/2O ( G 1 23 V) (3)ΔH2O H2 + 1/2O2 ( G = 1.23 V) (3)Δ

Hydrogen Production with Dye-Sensitized TiO2

Controlling/Modifying Interfacial Properties :

• Sensitizer anchoring mode• Ion-exchange resin coating• Barrier layer coating• Hybridization with carbon nanotubes

• Non-Ruthenium Dye sensitized systems

Anchoring GroupAnchoring Group

Different ways of anchoring molecules on surfacesDifferent ways of anchoring molecules on surfaces

HOOC

COOHc-RuIIL3

N

RuN

HOOC

N

N

COOH

COOH3

NN

HOOC

mol

g-1

)

15

20 c-RuL3p-RuL3

COOH

Tris(4,4’-dicarboxy-2,2’-bipyridyl) Ruthenium(II)

uL3]

ad (μ

m

5

10

H2 4 6 8 10 12

[Ru

0

5

N Np-RuIIL3

pH-dependent adsorption of the Ru-sensitizer on TiO2

pHN N

NN

Ru

([TiO2] = 0.5 g/L, [RuL3] = 10 μM) PP OO

OHHOOHHO

(B ) (4 4’ bi ( h h t ) 2 2’ bi id l) R th i (II)(Bpy)2(4,4’-bis(phosphonato)-2,2’-bipyridyl) Ruthenium(II)

Bae et al., J. Phys. Chem. B 2004, 108, 14903

Anchoring Groups in Ru-Sensitizers

O

OHO

OH

O

OHO

OH

O

OH

OH

Carboxyl

N NRu

NN N

N

N NRu

NN N

NO

HO

O

OHN N

RuN

N NN

O

HO

O

OH

O

OO O O

C2 C4 C6

PP

O OHO

OHOH

PP

O OH

OH

O

OHOH

PO

OHOHPhosphonic

N NRu

NN N

N

P P

P OH

O

HO

O

OH

OHN N

RuN

N NN

P P

P OH

O

HO

O

OH

OHN NRu

NN N

N

P

O

HO

p

P P

OH

O OHOH

O

OHOH

OHOHOH

P6P2 P4

A h i G Eff t H d d t H d P d tiAnchoring Group Effect: pH-dependent Hydrogen Production on RuII/Pt-TiO2 under Visible-light Illumination

30 160

ol) 20

25

C2 l) 100

120

140

P2P4P6

H2 (μm

o

10

15C2C4C6

H2 (μm

o

60

80

2 4 6 8 100

5

2 4 6 8 100

20

40

pH2 4 6 8 10

pH2 4 6 8 10

[Pt/TiO2] = 0.5 g/L; [RuLx]i = 10 μM; [EDTA] = 10 mM; λ > 420 nm[Pt/TiO2] 0.5 g/L; [RuLx]i 10 μM; [EDTA] 10 mM; λ 420 nm

(Bae and Choi, J. Phys. Chem. B 2006, 110, 14792)

TiOTiO22 Surface Modification with NafionSurface Modification with Nafion

(CF2CF2)x (CFCF2)( 2 2)x ( 2)

(CF2CFO)CF2CF2

O O

OSO- –

CF3O

Monomer unit of nafion–

––

–Cation-exchanger

Stable against photocatalytic oxidationTiO2

–

– –

–

5.0 nm

hydrophobiczone

– –

––

4.0 nm 1.0 nm

aqueouszone

Interface

Nafion-Coated TiO2 Particle

( H P k d W Ch i J Ph Ch Bsulfonicanion

fluorocarbonchain

zone ( H. Park and W. Choi, J. Phys. Chem. B2005, 109, 11667 )

Ru(dcbpy)Ru(dcbpy)33--TiOTiO22 vs.vs. Ru(bpy)Ru(bpy)332+2+//NafionNafion/TiO/TiO22

H+PtO

OH

O

OH

L2'RuII(bpy) CO Ti

e- H+

H2

hvPt

(a)N N

RuN

N N

N

O

HO

O

OHL2 Ru (bpy) CTiTiO2

OH2

OO O O

e--2+

H+ - H+

hv[H+]Nf > [H+]aq

(b) RuII(dcbpy)3

----

RuL32+

RuL 2+

H+

H+H+

-- - H+H2

RuN

N N

N

TiO2- -RuL3

2 H+

H+- H+

Nafion layer Nafion layerTiO2 Solution

N N

RuN N

RuII(bpy)32+

Photo-sensitized H2 Production in two anchoring systemstwo anchoring systems

25

30Ru(bpy)3

2+ + TiO2 Ru(dcbpy) + TiO2 )

20

Pt/P25

Nf-Pt/P25

mol

h-1

)

15

20

25Ru(bpy)3

2+ + Nf-TiO2

ex] a

ds ( μ

M)

10

15

[H2 ]

( μm

5

10

15

Ru-

com

ple

5

10

2 3 4 5 6 7 80

5

2 4 6 8 10

[R

0

pH pH

( H. Park and W. Choi, Langmuir 2006, 22, 2906 )

Photoelectrochemical Hydrogen Production

Carbon Nanotube Assisted Generation of Hydrogen in Dye-Sensitized Photoelectrochemical Cell under Visible Light

e-e- (-0.62 VNHE)

e-e- (-0.62 VNHE)

½ H2

H+

dye*/dye+

½ H2

H+

dye*/dye+

fi

hvCNT TiO2 CBTiO2 CB

dyee-e-

hv(~0 VNHE)

(-0.5 VNHE)

fi

hvCNT TiO2 CBTiO2 CB

dyee-e-

hv(~0 VNHE)

(-0.5 VNHE)

D

D+dye/dye+

Pt FTOD

D+dye/dye+

Pt FTO

nafionmatrix

dye

e-e-

e-e- FTO

nafionmatrix

dye

e-e-

e-e- FTO

TiO2/Nafion/CNT/Dyeelectrode

( J P k d W Ch i J Ph Ch C1 µm

( J. Park and W. Choi, J. Phys. Chem. C2009, 113, 20974)

Photoelectrochemical Hydrogen Production

1.2

10

12

14{CNT+TiO2}/Nf {CNT+TiO2}/Nf/Ru(bpy)3

2+

{TiO2}/Nf (mA)

0.30

l)0 8

1.0

1.2E1 (current)E2 (current)E1 (H2)E2 (H )

(1-R

)2 /2R

6

8

10

tocu

rren

t

0.20

ΔH

2 (μm

ol

0 4

0.6

0.8E2 (H2)

CNTCNT

0

2

4

Pho

t

0 00

0.10

Δ

0 0

0.2

0.4CNTCNT

Wavelength (nm)

300 400 500 600 7000

Irradiation time (min)0 10 20 30 40 50 60

0.00 0.0

E1: TiO2/Nf/RuL3 with CNTE2: without CNT

Dye-Sensitized TiO2 with Thin Overcoat of Al2O3Dye-Sensitized TiO2 with Thin Overcoat of Al2O3)

80

100

[H2]

( μm

ol

40

60

0

20Al2O3/TiO2/PtTiO2/Pt

0 50 100 150 200

Al 2pAl O /TiO /Pt

0.25 nm

Al 2pIntensity (a

Al2O3/TiO2/PtTiO2/Pt

Al2O3

arb. unit)

68707274767880

XPS Binding Energy (eV)(W. Kim et al., J. Phys. Chem. C 2009, 113, 10603)

Organic DyeOrganic Dye

Donor Acceptor

Strong Intra-molecular Charge Transfer

Organic DyeOO

(LUMO)

g y

N SS

O

OCOOH

NC

O

e−CB

hν

O

VB

TiO2hν

22

VB

(HOMO)(Y. Park et al., Chem. Commun. 2010, 46, 2477)

Organic Dye vs. Organic Dye vs. RuRu--complex Dyecomplex Dye

Ru-dye (RuL3)Organic-dye (OD)

λDye λmax(nm)

εmax(M-1 cm-1) ΔE (V) E0(dye/dye·+) (VNHE) E0(dye*/dye·+) (VNHE)

OD 445 24500 2.45 1.35 -1.0

MetalMetal--free organic dye sensitizersfree organic dye sensitizers

RuL3c 465 19500 2.20 1.39 -0.81

Low-cost production

High visible light absorption

MetalMetal free organic dye sensitizers free organic dye sensitizers

23

High visible light absorption

Facile molecular design

H2 Production using a Dye Sensitized TiO2 SystemHydrophilicity of Organic Dyes

NCLee at al., Org. Lett. 2010, 12, 460

reference

O

OO

O

O

O

OO

N SS

COOH

HOD 1 (5a) HOD 2 (5b) HOD 3 (5c) HOD 4 (5d)

N SS

O

O

OO

OCOOH

NC

N SS

O

O

OCOOH

NCN S

S

O

O

OCOOH

NC

N SS

O

O

COOH

NCO

HOD-1 (5a) HOD-2 (5b) HOD-3 (5c) HOD-4 (5d)

100

120 H2

60

80

100

mol

) @ 5

h

TiO2

Pt

2 H+

OO

O

20

40

[H2] (

μm

N

OO

0 HOD-4 HOD-3 HOD-2 HOD-1Reference

Hydrophilic group length Enhanced proximity of proton source to the reduction center (Pt)

O

[Dye/Pt/TiO2]= 10 μmol/g, [EDTA] = 10 mM, [Cat] =1g/L , pH0=3, λ>420nm

Fullerol/TiO2 Charge Transfer Mediated Visible Light Photocatalysis

Fullerol (C60(OH)x) C60(OH)x / TiO2

g t otocata ys s

-O

O-

( 60( )x) 60( )x / 2

O-

O

O

-O

-OOH

HH TiO

OO

-O

O

O

O

O-

H

H

H

H

TiO2

C60

O-

O--O

Surface-Complex Formation

Water Soluble !- Polyhydroxylate water-soluble form of the fullerene

C60

p

Ligand(C60) to Metal (Ti) charge transfer(LMCT)

60

-C60(OH)x(ONa)y (x+y=24) y generally around 10-15Visible light activity

Theoretical Calculation of Fullerol/TiO2 Complex<Fullerol/TiO ><Fullerol + TiO > <Fullerol/TiO2><Fullerol + TiO2>

Charge TransferTransition

LUMO

Transition

hν

HOMO

•These absorption spectra are calculated using intermediate neglect of differential overlap (INDO) model parameterized for spectroscopy at the configuration interaction (CI) level of theory (ZINDO/S-CIS)

Photocatalytic Activity of Fullerol/TiOFullerol/TiO2

100

Photocurrent20

H2 Production

m-2

60

80C60(OH)x/ TiO2

ol)

15

C60(OH)x /Pt /TiO2C60 /Pt /TiO2Pt /TiO2

I ph

/ μA

cm

40

60

[H2]

( μm

o

5

10

0 1000 2000 3000 40000

20 C60/ TiO2

TiO2

0 1 2 3 4 5 60

5

t / sCollector Electrode

Visible Light Illumination Time (h)

[EDTA] = 10 mM, [Cat] =1g/L, pH0=3, λ>420nm

e-Fe3+

Fe2+

photo-catalyst

e-

Light

[Fe3+] = 1 mM, [LiClO4] = 0.1 M, pH 1.8, [Cat]=1g/L, λ>420nm,E= 0.7 V, RE: Ag/AgCl, CE: Graphite rod, WE: Pt plate

Y. Park et al., Chem. Eur. J. 2009, 15, 10843

Conclusions

Dye-sensitized TiO2 nanoparticles can be modified in various ways for Dye sensitized TiO2 nanoparticles can be modified in various ways for H2 production.

The hydrogen production on dye-sensitized TiO2 is critically y g p y O2 yinfluenced by the kind of surface anchoring groups of the dye.

Nafion-coated TiO2 can anchor non-derivatized ruthenium bipyridyl 2 py ycomplexes via ion exchange for efficient hydrogen production.

The presence of alumina overcoat on TiO2 enhanced the efficiency of p 2 ydye-sensitization for hydrogen production.