Production of Hydrogen Using
Tit i Based
Titania
B d Photocatalysts
Ph t t l t
Wonyong Choi
School of Environmental Science and Engineering
D
Dept.
off Ch
Chemical
i l Engineering
E i
i
Pohang University of Science and Technology (POSTECH)
Pohang, KOREA
Solar Energy Based Hydrogen Economy
H2 production
CO2-producing
CO2-free
H2 from fossil fuels
Water splitting
Electrolysis
((alkaline,, PEM
electrolyzer)
Photoelectrolysis
Photocatalysis
(PEC, semiconductor
(PEC
powder/colloid)
CO2-neutral
Thermochemical
cycles
(S-I cycle, etc)
Oil
Natural
gas
H2 from biomass
Coal
Gasification
Wind
+EL
PV+
EL
High-T
electrolysis
(solar,
nuclear)
Storage of
electricity
l
i i
as H2
Solar
thermall
th
Proven
technology
but costly
process
Nuclear
thermall
th
Steam
reforming/
water gas
shift
H2
Ideal solar-tohydrogen conversion
process but far from
commercial
i l
realization
+
Thermochemical
process
(gasification,
pyrolysis,
reforming)
CO2
CO2
Current
industrial
process
Capture & Storage
C
S
(CCS)
Biological
process
(anaerobic
digestion,
fermentation)
+
H2
Solar Energy
Solar Energy
1.2 x 105 TW
(10,000 x Current world demands)
• Abundant
• Environment-friendly energy source
• Safe
f and Clean
Earth
Global need
13 TW
~ 0.1% of the Earth’s surface
(5 times as big as South Korea)
+
~ 10% conversion
i efficiency
ffi i
Photocatalysis as a mean of solar energy
conversion
eA/A-
eH2O/H2
hν
H2O/O2
e-
ΔG0(H2O → H2 + 1/2O2)
=56.7 kcal/mol
e-
D/D+
Photocatalyst
(usually semiconductors)
Water Splitting
on a Photocatalyst Particle
3. Reduction
for H2 evolution
H2
Photon
1. Photon absorption
Generation of e- and h+
with sufficient potentials for
water splitting (Band Engineering)
H2O
2. Charge Separation
and migration to surface
reaction sites
e- + h+
x
H2O
O2
4. Oxidation
for O2 evolution
Band Gap Positions in Various Semiconductors
eV
0
E vs. NHE
Vacuum level
-3.0
-1.0
1.1
-4.0
-5.0
0 H+/H2
2.5
2
2.3
3.0
-4.5
+1.0
3.8
3.2
2.2
2.8
3.4
3.2
3.2
-6.0
3.0
Si
GaP
SiC
-7.0
-8 0
-8.0
+2 0
+2.0
CdS
+3.0
Fe2O3
TiO
iO2
Rutile
TiO2
Anatase
SrTiO3
Z O2
ZnO
Nb2O5
WO3
SnO2
@ pH 0
Common Strategies for
D
Developing
l i Visible
Vi ibl Light
Li ht Photocatalysts
Ph t
t l t
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 dyes
- inorganic quantum dots (e.g., CdS)
3 Nanohybrid Systems
3.
(metal oxides & chalcogenides, metal nanoparticles, organic &
inorganic sensitizers, polymers, etc.)
Dye--Sensitized TiO2 Solar Cell
Dye
Performance of DSSC
Schematics of DSSC
R
Energy
y (eV)
((-))
(+)
ECB
Dye*
ff =
Dye+/0
Eg(TiO2)
≈ 3.2 eV
EVB
Semiconductor
A-
Pt
A
e-
II33-- +
+ 2e
2e-- ↔
↔ 3I
3I-Solution
Jsc×V
Voc
Pmax
Current
C
e-
Jsc : short circuit current
Voc: open circuit voltage
ff : fill factor
0
Jsc = 18mA/cm2
Voc = 0.74V
ff = 73%
Jsc
Voc
Voltage
0
H2 Production on DyeDye-Sensitized TiO2
H20
Pt
CB
e-
S+/S*
1/2H2 + OH-
D/D+
S+ /
/S
VB
2H+ + 2e2H2O
H 2O
H2 (E0 = -0.41 V)
(1)
O2 + 4H+ + 4e- (E0 = +0.82 V)
(2)
H2 + 1/2O2 ( Δ G = 1.23
1 23 V)
(3)
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 Group
Different ways of anchoring molecules on surfaces
HOOC
c-RuIIL3
COOH
HOOC
N
N
N
Ru
COOH
N
20
N
HOOC
COOH
Tris(4,4’-dicarboxy-2,2’-bipyridyl) Ruthenium(II)
p-RuIIL3
[Ru
uL3]ad (μm
mol g-1)
N
N
N
c-RuL3
p-RuL3
15
10
5
0
2
Ru
N
N
O
HO
6
8
10
12
pH
H
N
N
P
P
OH
4
HO
O
pH-dependent adsorption of the Ru-sensitizer on TiO2
([TiO2] = 0.5 g/L, [RuL3] = 10 μM)
OH
(B )2(4,4’-bis(phosphonato)-2,2’-bipyridyl)
(Bpy)
(4 4’ bi ( h
h
t ) 2 2’ bi
id l) Ruthenium(II)
R th i
(II)
Bae et al., J. Phys. Chem. B 2004, 108, 14903
Anchoring Groups in Ru-Sensitizers
Carboxyl
OH
OH
OH
OH
O
OH
O
O
O
O
OH
N N
N Ru N
N N
O
N N
N Ru N
N N
O
O
HO
OH
N N
N Ru N
N N
O
HO
OH
O
Phosphonic
p
O
HO
P
O
OH
O
P
N N
N Ru N
N N
O
HO
P
OH
P
OH
OH
OH
O
OH
P
N N
N Ru N
N N
O
OH
O
P
OH
OH
HO
OH
O
P
OH
P
OH
N N
N Ru N
N N
O
P
O
P
P
P
P4
OH
P6
O
OH
OH
OH
OH
OH
OH
O
P2
O
C6
OH
OH
O
O
C4
C2
O
Anchoring
A
h i G
Group Eff
Effect:
t pH-dependent
Hd
d tH
Hydrogen
d
P
Production
d ti
on RuII/Pt-TiO2 under Visible-light Illumination
30
160
25
140
120
P2
P4
P6
H2 (μmol)
H2 (μmo
ol)
20
C2
C4
C6
15
10
100
80
60
40
5
20
0
0
2
4
6
8
10
2
4
pH
6
8
pH
[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)
10
TiO2 Surface Modification with Nafion
((CF2CF2)x
((CFCF2)
O
O
–
-
(CF2CFO)CF2CF2 SO
–
O
CF3
–
Monomer unit of nafion
–
Cation-exchanger
Stable against photocatalytic oxidation
4.0 nm
–
TiO2
–
–
–
–
5.0 nm
hydrophobic
zone
–
–
–
Interface
1.0 nm
Nafion-Coated TiO2 Particle
aqueous
zone
sulfonic
anion
fluorocarbon
chain
(H
H. P
Park
k and
dW
W. Ch
Choi,
i J.
J Phys.
Ph Chem.
Ch
B
2005, 109, 11667 )
Ru(dcbpy)3-TiO2 vs. Ru(bpy)32+/Nafion
Nafion/TiO
/TiO2
OH
OH
O
(a) hv
e
Pt
-
O
H+
O
II
L2'Ru
Ru (bpy)
C
Ti
Ti
O
O
H2
TiO2
(b) hv
RuL3
2+
e-
-
O
N
N
N
OH
O
O
O
RuII(dcbpy)3
H+
H+
- RuL
-
TiO2
Nafion layer
TiO2
2+
2
3
H+
Ru
HO
O
[H+]Nf > [H+]aq
-
N
N
N
H+
-
H2
- H+
H+
H+
N
N
H+
Ru
N
N
N
N
Nafion layer Solution
RuII(bpy)32+
Photo-sensitized H2 Production in
two anchoring systems
30
Ru(bpy)32+ + Nf-TiO2
[R
Ru-comple
ex]ads (μM))
Ru(dcbpy) + TiO2
25
[H2 ] (μm
mol h-1)
20
Ru(bpy)32+ + TiO2
Pt/P25
20
Nf-Pt/P25
15
10
5
0
15
10
5
0
2
3
4
5
6
7
8
2
pH
( H. Park and W. Choi, Langmuir 2006, 22, 2906 )
4
6
pH
8
10
Photoelectrochemical Hydrogen Production
Carbon Nanotube Assisted Generation of Hydrogen
in Dye-Sensitized Photoelectrochemical Cell under Visible Light
e-
e½ H2
dye*/dye+
CNT
(~0 VNHE)
H+
D
Pt
D+
dye/dye+
FTO
nafion
fi
matrix
(-0.62 VNHE)
TiO2 CB
(-0.5 VNHE)
hv
hv
dye
edye
e-
FTO
TiO2/Nafion/CNT/Dye
electrode
( J.
J P
Park
k and
dW
W. Ch
Choi,
i J.
J Phys.
Ph Chem.
Ch
C
2009, 113, 20974)
1 µm
Photoelectrochemical Hydrogen Production
14
{CNT+TiO2}/Nf/Ru(bpy)3
2+
{TiO2}/Nf
10
2
Phottocurrent (mA)
12
(1-R) /2R
0.30
{CNT+TiO2}/Nf
8
6
4
E1 (current)
E2 (current)
E1 (H2)
1.0
E2 (H2)
08
0.8
0.20
0.6
CNT
0.10
04
0.4
0.2
2
0
300
0 00
0.00
400
500
Wavelength (nm)
600
700
00
0.0
0
10
20
30
40
50
60
Irradiation time (min)
E1: TiO2/Nf/RuL3 with CNT
E2:
without CNT
ΔH2 (μmoll)
Δ
1.2
Dye-Sensitized TiO2 with Thin Overcoat of Al2O3
100
[H2] (μmol)
80
60
40
Al2O3/TiO2/Pt
TiO2/Pt
20
0
0
50
100
150
Al2 O3
Al2 O3 /TiO2 /Pt
Intensity (a
arb. unit)
0.25 nm
200
TiO2 /Pt
80
(W. Kim et al., J. Phys. Chem. C 2009, 113, 10603)
Al 2p
78
76
74
72
70
XPS Binding Energy (eV)
68
Organic Dye
Donor
Acceptor
Strong Intra-molecular Charge Transfer
O
Organic
g
Dye
y
O
O
(LUMO)
NC
O
N
COOH
S
S
O
O
CB e−
TiO2
hν
VB
(HOMO)
(Y. Park et al., Chem. Commun. 2010, 46, 2477)
22
Organic Dye vs. RuRu-complex Dye
Organic-dye (OD)
Ru-dye (RuL3)
Dye
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
RuL3c
465
19500
2.20
1.39
-0.81
Metal--free organic dye sensitizers
Metal
Low-cost production
High visible light absorption
Facile molecular design
23
H2 Production using a Dye Sensitized TiO2 System
Hydrophilicity of Organic Dyes
Lee at al., Org. Lett. 2010, 12, 460
NC
N
COOH
S
S
O
reference
O
O
O
N
COOH
S
S
COOH
S
S
O
O
N
HOD 1 (5a)
HOD-1
N
O
S
O
COOH
S
S
O
O
O
O
O
COOH
S
NC
O
NC
NC
NC
N
O
O
O
O
O
O
HOD 3 (5c)
HOD-3
HOD 2 (5b)
HOD-2
HOD 4 (5d)
HOD-4
120
H2
[H2] (μm
mol) @ 5h
100
2 H+
O
80
Pt
O
60
TiO2
O
40
N
20
O
O
0
Reference
HOD-1
HOD-2
HOD-3
Hydrophilic group length
HOD-4
Enhanced proximity of proton source to the reduction center (Pt)
[Dye/Pt/TiO2]= 10 μmol/g, [EDTA] = 10 mM, [Cat] =1g/L , pH0=3, λ>420nm
Fullerol/TiO2 Charge Transfer Mediated Visible
Light
g t Photocatalysis
otocata ys s
Fullerol ((C60((OH))x)
-
C60((OH))x / TiO2
O
O-
-
O
O-
H
-
O
O
O
H
O
H
O
-
TiO2
H
O
O-
H
O
O
H
O
C60
O-
O
O-
p
Surface-Complex
Formation
Water Soluble !
- Polyhydroxylate water-soluble form of the fullerene
C60
-C60(OH)x(ONa)y (x+y=24) y generally around 10-15
Ligand(C60) to Metal (Ti) charge transfer
(LMCT)
Visible light activity
Theoretical Calculation of Fullerol/TiO2 Complex
<Fullerol + TiO2>
<Fullerol/TiO2>
Charge Transfer
Transition
LUMO
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/TiO2
Photocurrent
H2 Production
20
100
C60(OH)x /Pt /TiO2
C60(OH)x/ TiO2
60
40
TiO2
0
0
1000
2000
3000
10
4000
0
t/s
Light
Fe3+
Fe2+
0
1
2
3
4
5
6
Visible Light Illumination Time (h)
Collector Electrode
e-
Pt /TiO2
5
C60/ TiO2
20
[Fe3+]
C60 /Pt /TiO2
15
[H2] (μmo
ol)
m-2
Iph / μA cm
80
[EDTA] = 10 mM, [Cat] =1g/L, pH0=3,
λ>420nm
ephotocatalyst
= 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
z Dye-sensitized
Dye sensitized TiO2 nanoparticles can be modified in various ways for
H2 production.
z The hydrogen
y
g production
p
on dye-sensitized
y
TiO
O2 is critically
y
influenced by the kind of surface anchoring groups of the dye.
z Nafion-coated TiO2 can anchor non-derivatized ruthenium bipyridyl
py y
complexes via ion exchange for efficient hydrogen production.
z The p
presence of alumina overcoat on TiO2 enhanced the efficiency
y of
dye-sensitization for hydrogen production.