electrochemistry: from soft interfaces to bioanalytics · 2018. 8. 16. · ens lyon...

Electrochemistry: From soft interfaces to

bioanalyticsHubert H. Girault

ENS Lyon Septembre-Octobre 2010

Saturday, September 18, 2010

Liquid-liquid interfaces

• Interfacial structure

• Polarised liquid-liquid interfaces

• Electrocapillary phenomena

• Charge transfer reactions

• Photocurrent

• Nanoparticle adsorption - Plasmonics

• Artificial photosynthesis


Plan

• Hydrogen production by decamethylferrocene

• Oxygen reduction by decamethylferrocene

• CO2 reduction


Ferrocenes

ferrocene(Fc)

1,1’-dimethylferrocene(DFc)

decamethylferrocene(DMFc)

Fe FeFe


Ferrocenes

Ag AgCl 5 mM BATB5 mM Fc or DFc

0.01 M LiCl(pH = 1)

0.01 M LiCl1 mM BACl

AgCl AgCell:

Fc & DFc remains unreactive at the water/DCE interface under both acidic and basic conditions.

-10

0

10

j / µ

A c

m-2

0.40.20.0-0.2

Δowφ / V

pH=1 Blank 5mM Fc

a

-10

-5

0

5

10

j / µ

A c

m-2

0.40.20.0-0.2

Δowφ / V

pH=1 Blank 5mM DFc

a

Ferrocene 1,1’-dimethylferrocene


Acid - DMFc voltammetry

0 Volt

Fc+/FcDMFc+/DMFc

–0.6

0.640.04

Does the current at the positive limit originate from H+ assisted transfer or H+ reduction?

Fe3+ /Fe

2+

H+ /H

2

0 0.77Volt

Fe(C

N)63– /Fe

(CN)6

4–

0.36

Fe3+ /Fe

2+

0.77

O2/H2O

1.23

Ag AgCl 5 mM BATB5 mM DMFc

0.01 M LiClHCl (pH=1)

0.01 M LiCl0.001 M BACl

AgCl AgCell:

+

15

10

5

0

-5

j / µ

A c

m-2

0.20.0-0.2Δo

wφ / V

pH=1 Blank Air Glovebox

Strong offset current


Scan rate independence

15

10

5

0

-5

j / µ

A c

m-2

0.20.0-0.2Δo

wφ / V

pH=1 GB 20 30 50 mV/s


0.01 M LiClHCl (pH=1)


AgCl AgCell:


Michael Buhl* and Sonja GrigoleitOrganometallics, 24 (7), 1516 -1527, 2005

Ferrocene Protonation

Until the early 1990s the situation was that “while the proton affinity of ferrocene is well established, the site of protonation is not”.

In weakly protic solvents, such as chloroform, hydrogen bonding is found to occur preferentially with the Cp ring of ferrocene, while, in strongly acidic media, metal protonation occurs.

Michael L. McKeeJ. Am. Chem. SOC. 1993, 115, 2818-2824


http://pubs.acs.org/cgi-bin/article.cgi/orgnd7/2005/24/i07/html/om049243g.html/QueryZIP/C3-I/((((grigoleit)%3CIN%3E(au,aul)))%3CAND%3E(PUBYR@@%3E=@@1879))%3COR%3E((((grigoleit)%3CIN%3E(au,aul)))%3CAND%3E(ASAP@@%3CIN%3E@@VOL))#om049243gAF1

http://pubs.acs.org/cgi-bin/article.cgi/orgnd7/2005/24/i07/html/om049243g.html/QueryZIP/C3-I/((((grigoleit)%3CIN%3E(au,aul)))%3CAND%3E(PUBYR@@%3E=@@1879))%3COR%3E((((grigoleit)%3CIN%3E(au,aul)))%3CAND%3E(ASAP@@%3CIN%3E@@VOL))#om049243gAF1

pH dependence

15

10

5

0

-5

j /

µA

cm

-2

0.40.20.0-0.2

!o

w" / V

1

2

3

4

a

!

0.35

0.30

0.25

0.20

!o

w"

onset /

V4321

!o

w" / V

b

!

slope of 56.4 mV

pKaDCE = 6.6Anaerobic conditions


0.01 M LiClHCl (pH=x)


AgCl AgCell:


Electrolysis

PH=1

120

100

80

60

40

20

0

-20

-40j

/ !

A c

m-2

-0.6 -0.4 -0.2 0.0 0.2 0.4

"o

w#$/ V

pH=1 , 50 mV/s in glovebox Blank DMFc 5mm,before chronA After 1st chronA 2 st chronA 3 st chronA

140

120

100

80

60

40

20

j /

!A

cm

-2

3000200010000

Time/s

pH=1 ,850 mV,glovebox 1 st chronoA 2 st 3 st 4 st

DMFc at pH=1 and 11 in the glove box

Ag

Ag2SO4

Li2So4

(10mM)

H2SO4 pH=1,11

(aq)

BTPPATPFB

(5 mM)

DMFC 5mM

DCE

BTPPACl

(1mM)

LiCl

(10mM)

(aq)

AgCl

Ag

Bubble

15

10

5

0

-5

j / µ

A c

m-2

0.20.0-0.2Δo

wφ / V



Chemical Potential Control

ion

TB− very positive

TMA+ 0.160 V

TEA+ 0.018 V

TBA+ -0.230 V

BA+ very negative

FF

F F

F

B-

FF

F

F F F

FF

F

F F F

F

FF

TB−

P N P+

BA+

TB−Li+TMA+BA+

0.40.20.0-0.2Δo

wφ / V

TEA+Cl–

Fixing the polarisation with salts

Δowφ = Δo

wφ o ′ +RTziF

ln cio

ciw

⎛

⎝⎜⎞

⎠⎟

Nernst equation for the common ion

TBA+

LiTB | BATB


How to pump protons to the organic phase?

HCl

BATB

H+

TB–

Potentiostatic control

HCl + LiTB HTB

Chemical control

+

–F

F

F F

F

B-

FF

F

F F F

FF

F

F F F

F

FF

TB−


Distribution concentration10 mM HCl + 5 mM LiTB

5 mM BATB

Δowφtr

o / V

H+ Li+ BA+ TB– Cl–

0.55 0.59 –0.6 0.65 –0.53

At equilibrium Δo

wφeq = 0.541V

5.8 mM H+ 4.3 mM Li+ 0.14 mM TB– 10 mM Cl–

5 mM BA+ 4.2 mM H+ 0.7 mM Li+ 9.86 mM TB–


Hydrogen Production

!

1.5

1.0

0.5

0.0

A

800700600500400

! / nm

b

425

779

15

10

5

0

Int.

1.00.80.60.4

t / min

c

N2

H2 -10

0

10

I / nA

1.00.50.0-0.5

E / V vs Fc+/Fc

d

DMFc ! DMFc+ +e

–

1/2H2 ! H

+ +e

–

DMFc+ +e

– ! DMFc

LiTB + HCl

BATB· DMFc DMFc+

H2pH=1


Distribution concentration100 mM HCl + 5 mM LiTB

5 mM BATB + 5mM DMFC

Δowφtr

o / V

H+ Li+ BA+ TB– Cl–

0.55 0.59 –0.6 0.65 –0.53

95 mM H+ 5 mM Li+ 0 mM TB– 100 mM Cl–

5 mM BA+ 0 mM H+ 0 mM Li+ 10 mM TB– 5mM DMFc+

At equilibrium Δo

wφeq = 0.107V

Complete oxidation


Bulk reaction DMFc - Acid

N2 Atmosphere

GC

UV-vis


Mechanism

+1. DMFc protonation - Volmer reaction

FeII HFeIV

2. Bimolecular pathway - Tafel reaction

+ H2+

+ +

2

+

HFeIV H FeIV FeIII


Mechanism

H+

2+

+ H2

+

3. Proton attack - Heyrovsky reaction

HFeIV FeIV

4. Reduction+

e– H+

+ H2

+

HFeIV HFeIII FeIII


Electrochemical mechanism

DMFc DMFc+

H2H+

Ferrocene partitionProton transfer independent

H+

DMFc

DMFc+ + H2

DMFc : Positive potentials

Proton transfer dependent

DMFc–H+ DMFc

DMFc–H+

15

10

5

0

-5

j / µ

A c

m-2

0.20.0-0.2Δo

wφ / V



DMFc+ Reduction

!

DMFc+

BAClCdS NPs

After illumination

!!


ConclusionDMFc can react with organic acids in bulk 1,2-DCE to form H2

DMFc can act as a base to undergo interfacial protonation to form H2

TB– partition can drive proton transfer to the organic phase

Long term goal : Photosynthesis of hydrogen


Plan



• CO2 reduction


pH dependence

15

10

5

0

-5

j / µ

A c

m-2

0.40.20.0-0.2Δo

wφ / V

1 2 3 4


0.01 M LiClHCl (pH=x)

0.01 M LiCl1 mM BACl

AgCl AgCell:

No effect of O2


H2O2 production

2

1

0

Abs

/ a.

u.

800600400λ / nm

λmax, DMFc = 425 nmλmax, DMFc+ = 779 nm

A

B

10

0

-10

I / n

A0.40.20.0-0.2

E / V vs. Fc+/Fc

ISS

ISA

ISC

ISC/ISS = 74%ISC + ISA ≅ ISS

A

B

1: Separated Aqueous Phase2: 1+NaI, H2O2 + 2H+ + 3I- → 2H2O + I3-

3: 2 + Starch, Starch + I3- → Starch-I3-

4: Fresh aqueous solution + NaI

3

2

1

0

Abs

/ a.

u.

500400300λ / nm

λmax, I3- = 325 nm

Analysis of aqueous phase

5 mM BATB5 mM DMFc

5 mM LiTB5 mM H2SO4

A

DMFc DMFc+

Shake

B


0.6

0.4

0.2

0.0

Abs

/ a.

u.

700600500400λ / nm

620439c

O2 reduction by ferrocenes

Ferrocene

1.0

0.5

0.0

Abs

/ a.

u.

700600500400λ / nm

435652

b

Dimethylferrocene

After 24 hours

1.5

1.0

0.5

0.0

Abs

/ a.

u.

800600400λ / nm

425

779a

After 30 minutes

Decamethylferrocene

Two phase system with TB– as common ionLiTB | BATB


In Situ Detection of H2O2

Scanning ElectroChemical Microscopy (SECM)

2DMFc+O2 2DMFc++

O2+2H+ H2O2 2e–

Pt CE Pt UME

10 mM LiCl (pH = 1)

Glass Teflon

H2O2

Ag/AgCl RE

Ag/AgTPFB electrode

5 µL DCE droplet:

5 mM DMFc

5 mM BTPPATPBFB H+

BATB

Ag/AgTB electrode


LL Generation-Tip Detection

The separation between the tip and the interface was 15 μm. The tip potential was 0.6 V (vs Ag/AgCl) and the substrate potential was scanned from -0.25 to 0.45 V with a scan rate of 10 mV s-1.

ITIES current

Tip currentH2O2 detection


Acid catalysis

4-Dodecylaniline (DA)


DA: Shake Flask

!

TB−TMA+TBA+BA+

0.40.20.0-0.2Δo

wφ / V

TEA+

After 20 min - TMA+ common ion

5 mM DMFc +1 mM DA5 mM DMFc + 0.1 mM DA5 mM DMFc

UV-vis DCE!

5 mM DMFc +1 mM DA5 mM DMFc + 0.1 mM DA5 mM DMFc

UV-vis waterafter addition of I–


DA: Protons are necessary

4-Dodecylaniline (DA) in the bare organic phase does not catalyse DMFc oxidation


O2 reduction by DMFc

!Saturday, September 18, 2010

Electrochemical mechanism

DMFc DMFc+

O2 H2O2

H+

DMFc : Negative potentialsDFc & Fc all potentials

DMFc partitionProton transfer independent

B BH+

H+

H2O2

BH+

+DMFc

DMFc+ + H2O2

DMFc : Positive potentialsB = DMFc or DMFc-O2

Proton transfer dependentRequire an organic base


Conclusion

We can produce H2O2 in a biphasic system very efficiently using DMFc as donor.

The rate of the reaction is controlled by the interfacial polarisation, i.e. the proton pump

Long term goal : H2O2 producing fuel cell


H2O2 producing fuel cell

H2O

H2O2

H2

Pt/C

Nafion

H+

O2

DCE

O2

+

DMFc

H2O2

+

DMFc+

C

PVDF

DCE


Plan



• Oxygen reduction by ferrocene catalysed by an an amphiphilic cobalt porphyrin (CoAP)

• CO2 reduction


Water–scCO2 Interface

Aqueous solution of Bromophenol Blue in contact with scCo2:


24 °C and 30 bar 30 °C and 70 bar 40 °C and 80 bar

Solubility of DMFc in scCo2:

‣ DMFc is quite soluble in scCO2. Nonetheless its solubility is limited in sub–critical CO2


Common ion = TB– 2mM in each phaseDMFc 5 mM - P = 160 Bars and T = 72 °C

0 min 30 min 60 min 120 min

180 min260 min330 minSaturday, September 18, 2010






• Photocurrent




Photocurrent at dye sensitised molecular interfaces

AInterfacial

electron transfer


Heterogeneous photo-inducedelectron transfer reactions

S* ≈100 nm

Sensitisation in the evanescent wave

QInterfacial redox quenching


Photocurrent measurements

M

MF

M

MDCE

TIR : 75°IntensityFilter

PM

Lock-inAmplifier

Slit

Potentiostat

Reference

Input

Chopper

LaserHe-Cd442nm

(He-Ne 543nm)


Photocurrent transients

6

4

2

0

-2

43210

6

4

2

0

-2

43210

6

4

2

0

-2

6

4

2

0

-2

43210 43210

6

4

2

0

-2 -2

107 J

photo/A

cm

-210

7 Jphoto/A

cm

-210

7 Jphoto/A

cm

-2

t/s t/s

0.10 V

0.30 V 6

4

2

0

43210 43210

0.40 V

on off on off

0.20 V

0.00 V-0.10 V

[ZnTPPC4-] = 10-4 M[DFcE]=10-3 M

Photon flux = 6.41 1015 cm-2·s-1

1.19 V

-0.45 V

ZnTPPCTriplet state

Diferrocenylethane

0.55 V

On-Off

Eo


Action spectrafor ZnTPPC4–and DMeFc

2.0

1.5

1.0

0.5

0.0

J Pho

to /

a.u.

600550500450

λ / nm

Absorption 0.32 V 0.12 0.02

Q band S0 ---> S1


Interfacial ET time-scale

S* + Q

S + Q

[S–••• Q+]

S– + Q+

hν krel ~ 105 s–1

kel ~ 105-106 s–1

krec ~ 10-102 s–1

kps ~ 10 s–1

kisc ~ 1010 s–1

Cage effect at the interface?


Intensity Modulated Photocurrent Spectroscopy (IMPS)

M

MF

M

MDCE

TIR : 75°PM

Slit

Potentiostat

Reference

Input

LaserHe-Cd442nm

(He-Ne 543nm)

Photoacousticmodulator

Frequency response analyser


IMPS

j(ω)g =

kps+iωkps+kr+iω ( 1

1+iωRC)g : flux of electrons injected from the excited moleculeskps: product separation rate constantkr: recombination rate constant

-0.4

-0.2

0.0

0.2

0.4

j imag

/g

1.00.80.60.40.20.0jrel/g

1.8

15100

5k

-0.4

-0.2

0.0

0.2

0.4j im

ag/g

1.00.80.60.40.20.0jrel/g

1.8

15

100

5k

-0.4

-0.2

0.0

0.2

0.4

j imag

/g

1.00.80.60.40.20.0jrel/g

1.815

100

5k

Complete Recombination Partial Recombination No Recombination

kps / kr =0 kps / kr =1 kps / kr = ∞

Steady state current Initial current

RC


IMPS Data

108 Jphotore A cm−2

108J p

hoto

imA

cm−2

2.5

2.0

1.5

1.0

0.5

0.0

-0.5

-1.0

2.01.00.0-1.0

Jphotoim10

8J ph

oto

Acm

−2

log (ω / s-1)

b

Nyquist Plot Bode plot

-1.5

-1.0

-0.5

0.0

0.5

1.0

2.52.01.51.00.50.0

310 mVSaturday, September 18, 2010

IMPS Kinetic Analysis

0.400.300.200.100.00

θ ZnT

PPC ln (

ket )

1.0

0.8

0.6

0.4

0.2

0.0

14

12

10

8

6

θ

k et

4

3

2

1

00.400.350.300.250.200.15

108 g 1/A

cm

-2

DFcET

Fc

Δowφ V

Flux of electron injectionΔowφ V

Rate of electron transferButler-Volmer

Surface coverage


Polarised light studies

Study the orientation of the transition dipole moment


Photocurrent vs light polarisation20

15

10

5

0

109 I P

hoto /

A

150100500Ψ / deg.

0.06 0.16 0.26 0.36 V

ZnTPPC / Ferrocene

90°= p-polarised

Photocurrent maximumFor s polarised


Orientation vs coverage

80

75

70

65

60angl

e be

twee

n rin

g an

d no

rmal

0.40.30.20.10.0

Δφ / V

2.5x10-5

1.0x10-5

1.0x10-6 mol dm-3

1.0

0.8

0.6

0.4

0.2

0.0

cove

rage

0.40.30.20.10.0Δφ / V

2.5x10-5

1.0x10-5

1.0x10-6 mol dm-3

Positive Potentials - Low coverageMolecules flat on the interface

Negative Potentials - High coverageMolecules tilted vs the interface


Surface Second Harmonic Generation (SSHG)


SSHG of Na4ZnTPPC

SH spectrum without applying potentials. (p-input/p-output)

[ZnTPPC4-]aq = 1.0×10-5 M

Large red-shift (34 nm) ofthe Soret band.

↓J-aggregation ?

λmax/nm

Aqueous phase 422.5

Organic phase 423.5

SSHG 456


SSHG at polarised ITIES

-2

0

2

I / 1

0-6 A

cm

-2

-0.4 -0.2 0

Δwoφ / V

o → w

o ← w

[ZnTPPC4-]aq = 5×10-6 mol dm-3

Sweep rate : 5, 15, 25 mV s-1


Assembly of ZnTPPC4– at ITIES

0

-0.25 -0.15 -0.05

no adsorption

monomer(423 nm)

Surface SH spectra at () -0.15 V,() -0.20 V, () -0.25 V() non polarised

Strong adsorption followedby the J-aggregation.


Aggregation vs coverage

Positive Potentials - Low coverageMolecules flat on the interface

Negative Potentials - High coverageMolecules tilted vs the interface

J aggregatesSlow ET kinetics

SSHG studies

Single moleculesFast ET kinetics

Photocurrent studies


Porphyrin Dimers

N

N

N

N

N+N+

N+N+

Zn

N

N N

N-O3S

SO3-

SO3-

-O3S

Zn

N

N

N

N

N+ N+

N+N+

Zn

ZnTMPyP4+

N

N

N

N

-O3S SO3-

SO3--O3S

Zn

ZnTPPS4-

Ka ~ 5·108 M-1 δ ~ 5 Å

perpendicular to the axis z at relatively low concentrations. The

porphyrin rings lie flat on the interface, suggesting that at

incomplete coverage the orientation of the dimer in the absence

of applied potential is controlled by the solubility of the paired

dyes. When the surface coverage of the heterodimer approaches

its maximum, the angle ! decreases to values close to what isobserved in the case of the monomers. As shall be discussed in

the following section, the orientation at high surface concentra-

tions is determined by the formation of a film of aggregated

porphyrins at the interface.

3.2. Adsorption as a Function of the Galvani Potential

Difference between the Two Phases. As mentioned in the

Introduction, the ZnTPPS-ZnTMPyP heterodimer has beenextensively used as a sensitizer for photocurrent generation at

the liquid|liquid interfaces. These studies have allowed char-acterization of the dependence of the rate of electron transfer on

the Galvani potential difference ∆owφ between the two im-

miscible liquids. The analysis of the photocurrent responses was

based on the assumption that the surface concentration of dimer

remains independent of the potential.21,22 Such a behavior appears

reasonable considering that the dimer forms an overall neutral

entity. Furthermore, surface concentration isotherms inferred

from photocurrent measurements have shown that the energy of

adsorption does not vary with the applied potential.19 Here we

shall support these observations employing QELS, capacitance,

and LPMR measurements.

Figure 6 shows the evolution of the surface tension measured

with the QELS technique as a function of the applied potential.

Open squares correspond to the bare water|DCE interface, whilethe solid squares were obtained in the presence of 10-4 mol

dm-3 of ZnTPPS and 10-4mol dm-3 of ZnTMPyP in the aqueous

phase. The supporting electrolytesLi2SO4 andBTPPATPFBwere

present in the aqueous and organic phases, respectively. These

salts are responsible for the lower surface tension observed here

compared to the case of Figure 2. The potential corresponding

to the maximum of the electrocapillary curve (Emax) appears un-

affected by the presence of heterodimer, although the surface

tension is substantially decreased. Emax is commonly referred to

as the potential of zero charge, i.e., the potential in which the

charge of the diffuse layer on each side of the interface is zero.

The results in Figure 6 confirm that the potential difference

between the two phases has little effect on the surface coverage

of the heterodimer, as the surface tension decreases symmetrically

around E0.

The specific adsorption of ionic species at the liquid|liquidboundary alsomanifests itself by perturbations of the differential

capacitance, as illustrated by Figure 7. The symmetrical potential

dependence of the capacitance around the potential of zero charge

for the water|DCE junction is strongly affected in the presenceof the charged monomers. In the case of ZnTPPS, the minimum

of the capacitance shifts to positive potentials, showing a steep

increment of the capacitance toward negative potentials. This

behavior is consistent with the specific adsorption of hydrophilic

anionic species featuring a strong affinity for the liquid|liquidboundary.42,45 In the case of ZnTMPyP, the minimum of the

capacitance curve shifts toward more negative potentials due to

the positive charge on the dye. On the other hand, substantial

changes to the capacitance-potential curves are observed whenboth ZnTPPS and ZnTMPyP are present in the aqueous phase.

Theminimumof the capacitance is observed at negativepotentials.

The potential dependence appears somewhat weakened, sug-

gesting a change in the interfacial relative permittivity. These

phenomena have been observed during the formation of a dense

(45) Su, B.; Eugster, N.; Girault, H. H. J. Electroanal. Chem. 2005, 577, 187.

Figure 5. Modulation amplitude C of the reflectance signal (a) and

orientation angle ! (b) as functions of the bulk concentration cSwof

ZnTPPS, ZnTMPyP, or heterodimer, as extracted from the data inFigure 3.

Figure 6. Electrocapillary curvesmeasured byQELS in the absence(white squares) and in the presence of 10-4mol dm-3 of ZnTMPyP-ZnTPPS heterodimer (black squares). The aqueous phase contained10-2 mol dm-3 Li2SO4, whereas the DCE phase contained 5× 10-3

mol dm-3 BTPPATPFB.

1116 Langmuir, Vol. 22, No. 3, 2006 Eugster et al.

Very strongadsorption


Photo-oxidation of ferrocenes

ZnTMPyP4+* /ZnTMpyP3+

0.07

0.550.64

1.19

DCMFc+ / DCMFc

DMFc+ / DMFc

Fc+ / Fc

Decamethylferrocene

Dimethylferrocene

Ferrocene

E°SHE /V

-0.45ZnTMPyP5+ / ZnTMPyP4+*

recombination path

electron transfer

E°SHE /V

Redox levelof the triplet state

a

b

Fermin and Eugster Figure 23


Potential dependence of photo-ET

Jphoto = Jmax ket / (ket + kd)

ΔGact = (λ + ΔG°et)2 / 4 λ

ketII = k0 exp (- ΔGact / RT )

ferrocenedimethylferrocenebutylferrocene

diferrocenylethanedecamethylferrocene

λsw|DCE ≈ 1.05 eV

-ΔG°et / eV -ΔG°et / eV

0.01

0.1

110

1.20.80.40.0

0.01

0.1

110

106 J

phot

o /A

cm

-2

10-2

3

10-2

1

10-1

9

1.20.80.40.0k e

tII / c

m4 s-1

10-2

310

-21

10-1

9







• Photocurrent




Surface Second Harmonic Generation (SSHG)


SHG spectrum of Au-NPs

Au-citrate 19 ± 2 nm at the water|DCE interface


SHG of Ag0.4CladAu0.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Abs

orba

nce

700650600550500450400350Wavelength / nm

1.2

1.0

0.8

0.6

0.4

0.2

0.0

SSH

G in

tens

ity /

a.u

700650600550500450400350Wavelength / nmSize = 23 nm (21 + 2)

Au

Ag


Potential controlled adsorption at ITIES

Red : Ag0.1CladAu0.9

Blue : Ag0.4CladAu0.6

LiCl 2.5mMBTPPATPBCl 2.5mM

20 mV·s–1

1.0

0.8

0.6

0.4

0.2

0

-0.2

SH in

tens

ity /

a.u.

12008004000

Time / s

-0.4

-0.2

0.0

0.2

0.4

Cu

rren

t /

mA

0.60.40.20.0-0.2

�

Δowφ /V


Metallic interfaces

21 nm Au-NPs with citrate

Polarisation or addition of alcohol


Light reflection and refraction from the monolayer of nanoparticles adsorbed at the ITIES, with an incident beam coming from the oil side. In reality, nanoparticles are orders of magnitude larger than the cations and anions of both inorganic and organic electrolytes, shown as smaller spheres.

In the absence of adsorbed nanoparticles, light propagates through the system with a minor absorption in the aqueous phase, as both phases are transparent to visible light. Refractive index of the aqueous phase is only slightly affected by the minute concentration of nanoparticles in it, as their surface plasmon resonance lies outside the visible band. Reflection and refraction at the interface is standard for the given difference between the refractive indices of the oil and the aqueous phase (if they are balanced by additives or by adjusting concentrations of electrolytes, the light will go straight through the interface).


Nanoparticle adsorption

Faraday Discuss., 2009, 143, 109–115M. E. Flatté, A. A. Kornyshev and M. Urbakh

Nanoparticle energy profile at ITIES: Effect of potential drop across the interface.Parameters: nanoparticle radius = 1.5 nm; dielectric constants εw = 78.8 , εo = 10.7 , charge of the nanoparticles, z =–5; concentrations of electrolytes in water and 1,2 DCE, c = 5 10–3 M (i.e. Debye lengths 4.31 nm in water and 1.6 nm in oil); interfacial tension between water and oil ϒ= 30 mN m–1; line tension = 10–11 N; three phase contact angle between the particle surface and water–oil interface θ = 0.55π.


s-pol p-pol

Frequency dependence of the coefficient of reflection of a water/1,2 DCE interface covered by 20 nm radius silver nanoparticles. Drude function parameters for silver13: εm∞ = 5.266, ωp=.6eV, η =0.0544 eV. Incident wave comes from the organic electrolyte phase.Surface coverages: Γ = 1.00, 0.57, and 0.335 (red, green and blue curves), corresponding to three depths of the trapping well: u = –2·104 kBT, –0.6 ·104 kBT, and –0.2·104 kBT, in turn corresponding to voltages across the interface V/kBT = 30, 12.3, and 5.7. Angle of incidence, θ=45°.


Film formationMethanol injection at the NPs sol | DCE interface followed by stirring

Gold

Gold/silver


SPR -Kretschmann configuration

Reflectivity and phase for light wave exciting an SPW in the Kretschmann geometry (SF14 glass prism – 50 nm thick gold layer – dielectric) versus the angle of incidence for two different refractive indices of the dielectric (wavelength 682 nm),

Anal Bioanal Chem (2003) 377 : 528–539Saturday, September 18, 2010

Plasmon coupling

Plasmon coupling takes place in the nearfield when the particles are in close proximity to each other so that the electron oscillations in each particle are affected by the local field associated with the electron oscillations in neighboring particles.

Plasmonics (2007) 2:89–94

The contour plots of the electric fields, |E|2 of two 18 nm diameter particles with a 10.8-nm separation at a 532 nm and b 580 nm


SPR setup


Concentration effect on SPR of MeOH-induced Au films

1.0

0.8

0.6

0.4

0.2

0.0

Ref

lect

ance

80787674Angle / degrees

Concentration

Blank Au sol 16nm C0 Au sol 2x dilute Au sol 3x dilute


Concentration effect on SPR of MeOH-induced Au/Ag films

1.0

0.8

0.6

0.4

0.2

0.0

Ref

lect

ance

8280787674Angle / degrees

Blank Au 0.5 -Ag 0.5 film (2x dilute sol) Au 0.5 -Ag 0.5 film (3x dilute sol)


Fluorescence enhancement

Emission spectra of the Coumarin 343 at water│Au film (16 nm or 13 nm)│DCE

interface for TIR condition and at the SPR. The Excitation wavelength equals 450 nm.


Large films

Journal of Colloid and Interface Science 346 (2010) 1–7

The particle suspension is mixed with a mercaptosuccinic acidsolution with large excess of MCSA (1 mM). (2) A solution of TOABr in toluene is added. After dissociation, the TOA ion is attached to the carboxy groups of MCSA at the toluene side. The amphiphilic particles arrange at the interface

(a) Sketch of the horizonal deposition setup. The water phase drains slowly between teflon rim and glass substrate, whereby the layer attaches to the surface.Below are shown photographs of a glass slide in reflection (b) and transmission illumination.


Mirror window

On a sunny day, energy saving level reached to about 35%.







• Photocurrent




Chemical Mechanical Electrical

Energy conversion

Wind

Tide

Solar

Hydro

Grid

CoalGasOil

CO2

Oil

Photovoltaics

Solar plant

Fuel cells

Photosynthesis

H2, CH4

Water splitting

CO2 reduction

Storage

Batteries


Artificial Photosynthesis

H2H2O

CH3OH

CH4

CO2

Glucose

DREAM


Natural Photosynthesis

Key aspects:•Water/oil/water polarized interfaces•Most reactions are proton coupled electron transfer reactions•Short lived photosystems (seconds)


Artificial photosynthesisReverse Engineering

water

oil

waterH2O O2

H+ H2

D

A

P H+e–


Artificial photosynthesisReverse Engineering

Key aspects:•Water/oil/water polarized interfaces•Most reactions are proton coupled electron transfer reactions•Alternative reduction : Carbon dioxide


Basic approach

H+/H2

E o⎡⎣ ⎤⎦SHE

1.23 O2/H2O

Cat-Ox

Cat-Red

4e–

e–

2e–

532 nm2.33 eV

0

at pH=0


Z - Scheme

H+/H2

E o⎡⎣ ⎤⎦SHE

1.23 O2/H2O

Cat-Ox

Cat-Red

4e–

e–

2e–

Redoxshuttle

0

at pH=0

Dual Photosystem

Photo-oxidation

Photo-reduction


Water Photo-oxidation

E o⎡⎣ ⎤⎦SHE

O2/H2O

4e–

e–

Photocatalyst :Si doped Fe2O3 ,IrO2 doped Fe2O3

WO3 , TiO2

with a suitable width. When the energy of incident light islarger than that of a band gap, electrons and holes aregenerated in the conduction and valence bands, respectively.The photogenerated electrons and holes cause redox reactionssimilarly to electrolysis. Water molecules are reduced by theelectrons to form H2 and are oxidized by the holes to form O2

for overall water splitting. Important points in the semicon-ductor photocatalyst materials are the width of the band gapand levels of the conduction and valence bands. The bottomlevel of the conduction band has to be more negative than theredox potential of H+/H2 (0 V vs.NHE), while the top level ofthe valence band be more positive than the redox potential ofO2/H2O (1.23 V). Therefore, the theoretical minimum bandgap for water splitting is 1.23 eV that corresponds to light ofabout 1100 nm.

Band gap (eV) = 1240/l (nm) (3)

Band levels of various semiconductor materials are shown inFig. 6. The band levels usually shift with a change in pH(!0.059 V/pH) for oxide materials.4,29,30 ZrO2, KTaO3,SrTiO3 and TiO2 possess suitable band structures for watersplitting. These materials are active for water splitting whenthey are suitably modified with co-catalysts. Although CdSseems to have a suitable band position and a band gap withvisible light response it is not active for water splitting into H2

and O2. S2! in CdS rather than H2O is oxidized by photo-

generated holes accompanied with elution of Cd2+ accordingto the eqn (4).30

CdS + 2h+ - Cd2+ + S (4)

This reaction is called photocorrosion and is often a demeritof a metal sulfide photocatalyst. ZnO is also photo-

corroded under band gap excitation even if it is an oxidephotocatalyst.

ZnO + 2h+ - Zn2+ + 1/2O2 (5)

However, CdS is an excellent photocatalyst for H2 evolutionunder visible light irradiation if a hole scavenger exists asmentioned in section 2.2. On the other hand, WO3 is a goodphotocatalyst for O2 evolution under visible light irradiationin the presence of an electron acceptor such as Ag+ and Fe3+

but is not active for H2 evolution because of its low conductionband level. The band structure is just a thermodynamicrequirement but not a sufficient condition. The band gap ofa visible-light-driven photocatalyst should be narrower than3.0 eV (l 4 415 nm). Therefore, suitable band engineering isnecessary for the design of photocatalysts with visible lightresponse as mentioned in section 7.1.1.The second step (ii) in Fig. 4 consists of charge separation

and migration of photogenerated carriers. Crystal structure,crystallinity and particle size strongly affect the step as shownin Fig. 7. The higher the crystalline quality is, the smaller theamount of defects is. The defects operate as trapping andrecombination centers between photogenerated electrons andholes, resulting in a decrease in the photocatalytic activity. Ifthe particle size becomes small, the distance that photogener-ated electrons and holes have to migrate to reaction sites onthe surface becomes short and this results in a decrease in therecombination probability.The final step (iii) in Fig. 4 involves the surface chemical

reactions. The important points for this step aresurface character (active sites) and quantity (surface area).Even if the photogenerated electrons and holes possess

Fig. 4 Main processes in photocatalytic water splitting.

Fig. 5 Principle of water splitting using semiconductor photocatalysts.

Fig. 6 Relationship between band structure of semiconductor and

redox potentials of water splitting.5

Fig. 7 Effects of particle size and boundary on photocatalytic

activity.

This journal is "c The Royal Society of Chemistry 2009 Chem. Soc. Rev., 2009, 38, 253–278 | 255

Chem. Soc. Rev., 2009, 38, 253

Photocatalyst :Dye sensitised p-type NiO

SC

pH➚

Biomassoxidation


Photo-reduction of TCNQ

e–

TiO2

Biomass/Wateroxidation

TCNQ/TCNQ–

0.29 V SHE

pH=11

E=3.07–11x0.06=2.41V SHE

E=1.23–11x0.06=0.57 V SHE

E=0.11–11x0.06=–0.55V SHE

O2/H2O


Water oxidation on TiO2

OH–

OH•

h+

Competing to avoidO2•– ← O2 + e– H2 ← H+ + e– ×

TCNQ

TCNQ–

e–

Recombination10-200 ns,

μs at sun intensity

Basic pH

τ < 2μs

Influence of the local static electric field

200 fs

200 fs

Surface trapped electrons adsorb at

800nm

Surface trapped holes adsorb at 450nm

No donor

E=0.29 V SHE


Schematic view of ITIES with negatively charged nanoparticle adsorbed at the interface, with and without applied electric field. Nanoparticle color indicates peak optical absorption, which is changed by the electric field.Positive and negative ions are shown as small spheres: violet and blue (in water) and brown and white (in oil). The gray arrow indicates the electric field direction, and the gray filled region indicates the field strength. The electric field pushes the negatively charged nanoparticle toward the oil phase, and a dashed line sketches the total potential that confines the nanoparticles to the interface

Calculated optical absorbance spectrum for a CdSe/ZnS quantum dot with a peak absorption at 560 nm for zero field (dashed line). In the ITIES region an applied voltage of 1 V shifts the peak to 620 nm (solid line).

18212–18214 PNAS 2008 vol. 105


Water Oxidation

E o⎡⎣ ⎤⎦SHE

0.57 O2/H2O

4e–

e–

Cat-Ox:Water oxidation catalystAt least 4 metal atoms

IrO2 nanoparticles adsorbed Φ=4.23 eVBandgap=3.12 eV

IrO2

Organic Photosystem: bis(4,4ʹ′-tridecyl-2,2ʹ′-bipyridine)-(4,4ʹ′- dicarboxy-2,2ʹ′-bipyridine) ruthenium(II)-bis(chloride) (N965)

effectiveness of the use of an ionic molecular monolayer onimproving the charge injection.

To verify if a monolayer of an ionic molecule is able to modifythe charge injection, we prepared a special ionic transition-metalcomplex capable of binding to metal oxide surfaces and containingaliphatic chains to facilitate monolayer deposition using theLangmuir-Blodgett technique (LB). The formation of themonolayer can be achieved by self-assembly; however, LB wasused because it offers a level of control over the orientation andplacement of the molecules that is not available with othertechniques.13,14 There are many examples of the use of thistechnique to prepare emissive,15-18 hole-injecting,19 and insulat-ing layers20,21 for OLED applications. However, most of theseLB films are formed by neutral species.

The observation of electroluminescence in a hole-dominatedhybrid metal oxide-organic light-emitting diode is a good wayto prove the effect of a monolayer of a molecular ionic specieson charge injection because electroluminescence is limited byelectron injection.

In this letter, a hybrid organic-inorganic electroluminescentdevice based on nanofunctionalized titanium oxide, using thepolymeric semiconductor poly[2-methoxy,5-(2!-ethyl-hexyloxy)-phenylene-vinylene] (MEH-PPV) as the light-emitting material,was prepared with and without a monolayer of an ionic transition-metal complex to verify the effect on electron injection. MEH-PPV was chosen because it is a well-studied polymer that hasfound applications in both light-emitting and photovoltaicdevices.22,23 We show that is possible to significantly improvethe injection of electrons into the LUMO of the MEH-PPV bymodifying the metal oxide cathode with a monolayer of an ionictransition-metal complex.

Experimental SectionTiO2 layers were prepared by using spray pyrolysis following a

method described previously.24 Briefly, an ethanolic solution ofdiisopropoxy titanium bis(acetyl acetonate) was sprayed with nitrogengas on hot ITO substrates that were subsequently annealed at 520°C for 2 h. Then, a monolayer of the ruthenium charged complexwas deposited onto the ITO substrate covered with TiO2 by usingthe Langmuir-Blodgett technique.

A solution of the chloride salt of the N965 complex in CHCl3 wasused as a spreading solution. An appropriate amount of this solutionwas carefully spread onto a 10-3 M KPF6 aqueous subphase, andthe spreading solvent was allowed to evaporate for 10 min prior tocompression. The monolayer was compressed up to a surface pressureof 27 mN m-1 for transfer. The compression isotherm is shown ascomplementary information. The LB film was assembled on thesubstrate by the vertical lifting method (i.e., immersion andwithdrawal of the substrate through the interface covered with thecharged complex monolayer). The dipping speed of the substrates

was 1 cm min-1, and one dipping cycle was performed. Because ofthe hydrophility of TiO2, transfer took place only for the upstrokeof the substrate with a transfer ratio close to unity. Therefore, onemonolayer of the Ru complex was deposited. A KSV3000 troughwas used to prepare these samples. The PF6

- anions of the aqueoussubphase were adsorbed onto the Ru complex monolayer, replacingthe Cl- anions of the Ru salt used to make the spreading solution.The presence of PF6

- on the LB film has been confirmed by IRspectra of multilayer films deposited onto a CaF2 substrate (FigureS1).

Devices were prepared by spin coating a thin layer (50-100 nm)of MEH-PPV from a toluene solution on the TiO2/ITO glass-coveredsubstrates. Before spin coating, the solutions were filtered over a0.20 µm PTFE filter. Afterward, the thin films were dried andtransferred to a high-vacuum chamber integrated in an inertatmosphere (<0.1 ppm O2 and H2O) glovebox. Gold was thermallyevaporated under a base pressure of 10-6 mbar and served as theanode contact and as an optical mirror to enhance the unidirectionalillumination of the device. The layer thickness was determined usingan Ambios XP1 profilometer. J-V characteristics were collectedusing either a Keithley 2400 source measurement unit or an AutoLabPGSTAT30 potentiostat. Electroluminescence was detected usinga Si photodiode coupled to a Keithley 6485 picoamperometer. Thephotocurrent was calibrated using a Minolta LS100 luminance meter.Electroluminescent spectra were recorded using an Avantes fiberoptics photospectrometer.

Results and Discussion

Bis(4,4!-tridecyl-2,2!-bipyridine)-(4,4!-dicarboxy-2,2!-bipyri-dine) ruthenium(II)-bis(chloride) (N965) was chosen because ofits peculiar characteristics (Figure 2). First, it is a charged complex,and for this reason, it is suitable as a charge-injecting layer fromthe metal oxide to MEH-PPV. Second, it can form LB filmsthanks to its amphiphilic character and because the carboxylicgroups can covalently bond to the TiO2 surface.25 Cl- has beenreplaced with PF6

- because larger ions have higher mobility asa result of diminished electrostatic interaction with the coun-terion.26

Proof of the presence of the molecular layer is given by thedistinct contact angle of a water droplet on the bare TiO2 andthe monolayer-modified substrate (Supporting Information).Although TiO2 is hydrophilic, after the deposition of N965 thesurface becomes hydrophobic because of the long aliphatic chains.In contrast to what is usually observed for LB films, the monolayerof the N965 complex is strongly grafted onto the TiO2 surfacethrough its hydrophilic part (the bpy ligand functionalized withtwo carboxylate groups) whereas the hydrophobic part (the bpyligand functionalized with long alkyl chains) is directed awayfrom the substrate.

(13) Talham, D. R. Chem. ReV. 2004, 104, 5479.(14) Ulman,A.IntroductiontoUltrathinOrganicFilms:FromLangmuir-Blodgett

to Self-Assembly; Academic Press : Boston , 1991; p 442.(15) Pal, A. J.; Ouyang, J.; Li, L.; Tai, Z.; Lu, Z.; Wang, G. Chem. Commun.

1997, 9, 815.(16) Jung, G. Y.; Arias-Marin, E.; Arnault, J. C.; Guillon, D.; Maillou, T.; Le

Moigne, J.; Geffroy, B.; Nunzi, J. M. Langmuir 2000, 16, 4309.(17) Yam, V. W. W.; Yang, Y. B.; Chu, W. K.; Wong, K. M. C.; Cheung, K. K.

Eur. J. Inorg. Chem. 2003, 4035.(18) Olivati, C. A.; Ferreira, M.; Carvalho, A. J. F.; Balogh, D. T.; Oliveira,

O. N.; von Seggern, H.; Faria, R. F. Chem. Phys. Lett. 2005, 408, 31.(19) Aoki, A.; Maeda, S. N.; Kawai, Y.; Tanaka, T.; Miyashita, T. Chem. Lett.

2005, 34, 1566.(20) Kim, Y. E.; Park, H.; Kim, J. J. Appl. Phys. Lett. 1996, 69, 599.(21) Jung, G. Y.; Pearson, C. L.; Horsburgh, E.; Samuel, I. D. W.; Monkman,

A. P.; Petty, M. C. J. Appl. Phys. D 2000, 33, 1029.(22) Parker, I. D. J. Appl. Phys. 1994, 75, 1656.(23) Breeze, A. J.; Schlesinger, Z.; Carter, S. A.; Brock, P. J. Phys. ReV. B.

2001, 64, 125205.(24) Kavan, L.; Graetzel, M. Electrochim. Acta 1995, 40, 643.

(25) Nazeeruddin, M. K.; Humphry-Baker, R.; Liska, P.; Graetzel, M. J. Phys.Chem. B 2003, 107, 8981.

(26) Rudmann, H.; Shimada, S.; Rubner, M. F. J. Am. Chem. Soc. 2002, 124,4918.

Figure 2. Chemical structure of bis(4,4!-tridecyl-2,2!-bipyridine)-(4,4!-dicarboxy-2,2!-bipyridine) ruthenium(II)-bis(chloride) (N965).

80 Langmuir, Vol. 25, No. 1, 2009 Letters

pH=11

E=0.84 V SHE

RuII*/RuI

RuII/RuI

TCNQ/TCNQ–

E=0.29 V SHE

E=–1.26 V SHE

Tin porphyrinSnCP - Analog of CoAP but with a carboxy group to anchor on IrO2


Oxygen reduction

E o⎡⎣ ⎤⎦SHE

0.57 O2/H2O4e–

e–

Cat-Ox:NiO nanoparticles adsorbedBandgap=3.55 eV

NiO






























pH=11 NiO

0.4- 0.6 VB

E=0.84 V SHE

RuII*/RuI

RuII/RuI

E=–1.26 V SHE

of photocurrent generation, we now use this material to build asandwich-type DSSC and study the effect of the redox coupleI3-/I- on the photodinduced dynamics of the system and how

it relates to the photocurrent generation (Figure 1). It is worthnoting that, to the best or our knowledge, this and our previouswork with a dyad sensitizer1 are the first studies in whichpolychromatic transient absorption is used to study the ultrafastdynamics of dye-sensitized mesoporous semiconductors in thepresence of redox active electrolytes.

2. Experimental Section2.1. Samples. Nanostructured NiO films were prepared on

conducting glass (for the solar cell study) and microscope glass(Menzel glass; for the time-resolved studies) according to theprocedure described in ref 16.16 The films were 1-3 µm thickand gray. The color of the films was attributed to partialoxidation of the semiconductor during the sintering. Dye-sensitization of the NiO films was carried out by soaking thefilm in an ethanol (Kemetyl) solution of coumarin 343 (5 ×10-4 M) overnight. The sensitized films were then rinsed withethanol and dried at room temperature. After dye loading, thefilms were of a bright orange color.

Electrolytes were made using NaI (Aldrich), I2 (Merck), andpropylene carbonate (Aldrich). Those samples where the filmsare in contact with the electrolyte were prepared in the following

way. A drop of electrolyte was poured on the film surface. Thefilm was covered with a thin glass cover slide. Because of thecover, the electrolyte drop spread over the surface. The amountof solution was such that the glass cover slide adhered to thefilm surface by capillarity. From the difference in optical densitywith and without electrolyte at 362 nm (absorption maximumfor I3

-) and assuming that I3- is formed quantitatively from I2

and I- (which is present in large excess), it could roughly beestimated that the thickness of the electrolyte layer was about20 µm.

Coumarin 343 was purchased from Aldrich and was used asreceived for dye sensitization and for femtosecond transientabsorbance measurements in solution. All the solvents were ofthe highest commercially available purity and were used assupplied.

2.2. Measurements. Absorption spectra were recorded on aHewlett-Packard HP 8453. IPCE was measured in a setupdescribed previously.17 The photoinduced absorption setup hasbeen described elsewhere.18 A blue light-emitting diode (LuxeonStar 1 W, royal blue) was electronically modulated (on/off) toexcite the sample.

Femtosecond transient absorption measurements were carriedout with two different femtosecond laser systems. One of thesystems is described in ref 9 and consists of a 1-kHz regenerativeamplifier (Quantronix) pumped by a Q-switched frequencydoubled Nd:YLF laser (Quantronix) and seeded by a mode-locked Ti:sapphire oscillator (Mira, Coherent), the latter pumpedby a CW argon-ion laser (Coherent). The other system consistsof a 1-kHz regenerative amplifier (Legend HE, Coherent)pumped by a Q-switched frequency-doubled Nd:YLF laser(Evolution, Coherent) and seeded by a mode-locked Ti:sapphireoscillator (Vitesse, Coherent). The details of the optical setupand detector system, identical for both laser systems, aredescribed in ref 9. Briefly, 422-nm laser pulses of energybetween 0.4-0.8 µJ, obtained by sum frequency generation ofthe output of an optical parameter amplifier, were used as pump.The probe beam consisted of a white light continuum generatedby focusing the 800-nm fundamental output on a moving CaF2

plate. The sample was mounted on a holder which moved upand down with a frequency of about 1 Hz. All measurementswere carried out at the magic angle polarization of pump andprobe.

The spectra are the average of 5-15 scans with 500-1500shots at each time step, depending on the quality of the signal.The absorbance of the liquid samples was about 0.2 and that ofthe sensitized films about 1 at the excitation wavelength. Byconvolution of the signal with a Gaussian pulse, the instrumentresponse function, measured as the full width at half-maximum(fwhm) of the Gaussian, was obtained. On film samples, thefwhm was estimated to be about 170 fs at 360 nm, 150 fs at450 nm, and 120 at 600 nm. Measurements on liquid samples(1 mm quartz cell) gave ∼20% larger fwhm values.

3. Results and Discussion

To obtain a better understanding on the effect of the redoxcouple I3

-/I- on the dynamics of the sensitized NiO films, wehave used different electrolyte mixtures where we have variedthe total concentration of ions and/or the relative concentrationsof I3

- and I- in the propylene carbonate solvent (see Table 1).Hereafter, we will refer to the coumarin sensitized films in theabsence of electrolyte as C343|NiO. When in contact withelectrolyte, the sensitized films will be named as C343-dil|NiO,C343-el2|NiO, C343-el3|NiO, C343-el4|NiO, or C343-el5|NiO,depending on which electrolyte mixture was used. Finally, we

Figure 1. (A) Schematic representation of the electron transferprocesses thought to occur in a dye-sensitized p-type semiconductorin contact with a redox active electrolyte. First, an electron is transferredfrom the valence band (VB) of the p-type semiconductor (NiO) to theexcited dye (C343). Second, an electron is transferred from the radicalanion of the dye to the oxidant species of the redox couple, regeneratingthe sensitizer. Such a system can be used as active electrode,photocathode, in a DSSC. (B) Some relevant values are: reductionpotential of C343 in MeCN, E1/2(C343/C343-) ) -1.2 V vs NHE, 37

energy of the first excited singlet state of C343 in MeCN (calculatedfrom absorption and fluorescence spectra), ES1(C343) ) 2.6 eV, energyof the first triplet state of C343 in MeCN (estimated from theliterature,28,29 ET1(C343) ≈ 1.6 eV, valence band (VB) edge ofnanostructured NiO measured in water at pH ) 6.8, 16 EVB(NiO) )0.4 V vs NHE, and the redox potential of the electrolyte, E(I3

-/I-) )0.44 V vs NHE.38 Literature redox potentials measured vs differentreference electrodes have been converted to NHE to facilitate thecomparison.

Coumarin 343-NiO Films in DSSCs J. Phys. Chem. C, Vol. 112, No. 25, 2008 9531TCNQ/TCNQ–

E=0.29 V SHE


Oxygen reduction

E o⎡⎣ ⎤⎦SHE

1.17O2/H2O

4e– e–

Cat-Ox:TiO2 nanoparticles adsorbedBandgap=3.2 eV

TiO2






























pH=1

E=1.26 V SHE

RuIII/RuII

RuIII/RuII*

Fc+/Fc

E=0.64 V SHE

E=–0.86 V SHE

TiO2

ZnTPPC

enhancement of light absorption, whereas malonic acidgroup facilitates stronger binding to the semiconductorsurface with a consequent improvement in the electroniccoupling of the dye. With these key modifications (malo-nic acid anchoring groups, extendedp-conjugation usingethenyl-type spacer units), a newgeneration of porphyrinphotosensitizers has been designed for DSCs that give

record solar-to-electrical conversion efficiencies of over7%.

The Scheme 2 shows a possible extension of the basicporphyrin core in the design of porphyrins for use inDSCs. In principle, modifications of the design of aporphyrin sensitizer are based on a P–B–A structure, inwhich B represents a p-conjugation bridge serving as aspacer between the porphyrin light-harvesting center Pand the carboxyl anchoring group A. A DSC device usingporphyrin sensitizers with the B–A unit functionalized atthe b-position is reported to give the highest cell per-formance as great as h = 7.1%; the meso-substituted por-phyrins gave smaller h values. Liu et al. examined [35,36]a series of porphyrin derivatives of the type D-P-B-Astructure. YD11, YD12, and YD13 have the same diary-lamino substituent as in YD11 but with the phenyl groupin B being replaced by naphthalene and anthracene,respectively. Both YD11-sensitized and YD12-sensitizedsolar cells exhibit excellent cell performances (n = 6.5–6.7%), comparable to that of N719 dye measured undersimilar conditions.

It was mentioned earlier that, through appropriate sol–gel hydrolysis and sintering procedures it is possible toprepare mesoporous Titania layers that are opticallytranslucent to visible light. With such films it is possibleto use different colored dyes to make dye solar cells indifferent colors—a feature that has potential advantages

Artificial photosynthesis: biomimetic approaches to solar energy conversion and storage Kalyanasundaram and Graetzel 303

Scheme 2

Extension of the basic porphyrin chromophoric unit for use in dye-sensitized solar cells.

Figure 3

Dye-sensitized solar cell based on metalloporphyrin as thephotosensitizer.

www.sciencedirect.com Current Opinion in Biotechnology 2010, 21:298–310


Proton reduction

H+/H2

E o⎡⎣ ⎤⎦SHE

Cat-Red 2e–

Cat-Red:Co(II) porphyrin

Organic Photosystem:Ru(bpy)32+

MoS2

!Saturday, September 18, 2010

Conclusion

Reactions in biphasic systems : Can we do better than Nature ?

The 21st century challenges : Energy & Food


electrochemistry: from soft interfaces to bioanalytics · 2018. 8. 16. · ens lyon...

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