Electrochemical Generation of Nanostructures at the Liquid-Liquid
Interface
Robert A.W. Dryfe
School of Chemistry,
Univ. of Manchester (U.K.)
robert.dryfe@manchester.ac.uk
Leiden, Nov. 2008
Liquid/Liquid Interfaces in catalysis
• Widely used: bi-phasic system, allows for
ease of separation of catalysis from reactant
mixture.
• Electrochemical investigations of phasetransfer catalysis (Schiffrin 1988 [1], Girault
1994 [2])
• Water does not have to be one of the
phases = “Fluorous biphase catalysis”
(Horvath 1994) [3]
• Stable room-temperature ionic liquids:
•
(Ballantyne 2008 [4])
Leiden, Nov. 2008
H3DA TPBF3 ethylmethylimidazolium ethylsulfate
(EMIM EtSO4) interface
Liquid/Liquid Interfaces:
electro-catalyst generation
• Reduction of solution phase
Mn+:
• Heterogeneous ET (surface
of electronic conductor)
i
ii
n+
A
A
e-
solution
Phase
electrode
Phase
solution
Phase
electrode
Phase
An+ + ne-
i
ii
An+
• Homogeneous ET
(nanoparticle preparation)
A
e-
Leiden, Nov. 2008
solution
Phase
solution
Phase
An+ + D
Dn+
D
• Heterogeneous ET
(aq/organic interface) –
with/without potential control
A
i
Dn+ + A
ii
n+
A
A
e-
D
Aq
Phase
Org
Phase
Aq
Phase
Org
Phase
Dn+
An+ + D
Dn+ + A
Liquid/Liquid Interfaces:
electro-catalytic reactions
• Questions:
•
Can the catalyst be used in situ - for catalysis of
processes at liquid-liquid phase boundaries?
• If so, could catalyst density be controlled (Langmuir
trough approach) to optimise reactivity?
• Or can catalyst be removed and immobilised on a
(conventional) electrode?
Leiden, Nov. 2008
{Liquid-liquid Electrochemistry 1:
Distribution potential}
• Each ion: distribution equilibrium at the organic/water interface
• Define standard Galvani potential of transfer:
worgi0 
worgGi0
• Vary potential with common-ion
•
zi F
ratio of ion concentration in each phase (maintained by
hydrophilic/hydrophobic counter-ions) “poises” potential
RT aiorg
 i    
ln w
zi F
ai
w
org
w
org
0
i
• (Nernst-Donnan equilibrium )
•
- ion transfer/electron transfer – particularly for SECM @ L/L.
Leiden, Nov. 2008
{Liquid-liquid Interfaces 2:
Polarised Interfaces}
• External polarisation of L/L interface (both phases contain
electrolyte):
• Electrolytes = AX(aq) and CY(org), the following inequalities are met:
•
worg X0   0
•
worgC0   0
also:
+
and
A CV showing the transfer of Tetrapropylammonium in the
absence of a reducing agent
150
100
50
i/µA
0
-50
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
-100
-150
-200
-250
-300
worg
/V
E/V
Leiden, Nov. 2008
wor  A0   0
worgY0  0
Structure of L/L interface
• Essentially sharp, even down to molecular scale – nm-scale
transition from phase 1 to phase 2.
• Interfacial fluctuations (capillary waves):
• Competition between thermal motion and interfacial tension
• Appear to extend down to molecular scale) = nm scale amplitude
• Experimental probes: X-ray scattering, non-linear optical
spectroscopy (SFG, SHG), (Schlossman, 2000 [5]), (Richmond
2001 [6]).
• => Smooth, reproducible interface.
Leiden, Nov. 2008
Modify Sharp (but fluctuating) interface?
•
Catalysis – introduction of metal
(nano-)particles
•
Result: electro-catalytic processes
at interface with only ionic
Pt C.E.
contacts.
• “In order to study the
electrochemical properties of
nanoparticle… we need to
attach them to an electrode
surface” – DJ Schiffrin, this
week.
• (1) “Synthesise, then fix them”
• (2) “in situ growth.”
Leiden, Nov. 2008
R.E.
+e-
-e-
-e-
+e-
Approaches 1 vs. 2 at L/L interface
i
ii
n+
A
• Source of particles?
A
e-
– (i) Assembled at
interface (particles =
surfactants)
– (ii) Grown at interface
(either (a)
spontaneous
deposition or (b)
electrodeposition).
solution
Phase
solution
Phase
An+ + D
n+
D
D
Dn+ + A
Then spontaneous assembly
(adsorption) at interface
i
ii
n+
A
A
e-
D
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Aq
Phase
Org
Phase
Aq
Phase
Org
Phase
Dn+
An+ + D
Dn+ + A
(i) Assembly of (pre-formed) particles at L/L
interfaces
• Method: form hydrosol (organo-sol), particles
adsorb interface on introduction of organic
(aqueous) phase.
• Particles are surfactants, if favourable
contact angle,q.
• Desorption energy given by:
E  r 2 o / w (1  cos q ) 2
• Particles of given type, will be displaced by
those with larger radius (r):
• Size segregation effect demonstrated for
CdSe (Russell, 2003 [7]).
Leiden, Nov. 2008
(i) Assembly of (pre-formed) particles at L/L
interfaces - continued
• Other terms in equation:
•
E  r 2 o / w (1  cos q ) 2
q can be varied by changing surface
chemistry (Vanmaekelbergh, 2003 [8]) –
induce assembly of Au NPs by addition
of ethanol – contact angle tends 90o.
• Residual surface charge, Au NPs
attracted to/from polarised L/L interface
– see Figure, from (Fermin, 2004 [9])
• Lippmann equation, interfacial tension
is function of applied potential
  o / w
Q   w
  o
w
Leiden, Nov. 2008


T , p
Ordering of insulating particles at L/L
interfaces
• System
– 1.6mm SiO2 particles (Duke
Sci. Corp., USA).
• Hydrophobic coating dichlorodimethylsilane.
– Non-aqueous phase
• Octane (e = 2.0) or
Octanone (e = 10.3).
• Suspend at water/org interface
Dried: close packing
• (Campbell/Dryfe 2007, but
after Nikolaides, 2002 [10])
Leiden, Nov. 2008
Spontaneous ordering of SiO2
Use image analysis to identify
Field of view: 190 microns x individual particle positions:
radial distribution function
143 microns:
found.
- metallic particles, more polar
phases?
Leiden, Nov. 2008
(ii – a) In situ growth of particles at L/L
interfaces: spontaneous chemical reduction
• Faraday (1857 [11]): formation of colloidal Au
at L/L (water/CS2) interface
• “dark flocculent deposits”, metal in “a fine
state of division”.
• General problem of particle formation at L/L
interface is prevention of aggregation:
•
e.g. Au deposition @ water/1,2dichloroethane interface, fractal structures
form: image statistics, growth laws for
aggregation process (scale bar = 10 microns)
Leiden, Nov. 2008
Control deposit aggregation
• (a) Template diameter <
“intrinsic” particle diameter
(TEM: Pt deposition in zeolite
Y)
– Electrodeposition
• (b) Presence of ligands in
interfacial system (TEM: Au
deposition in presence of
phosphines)
• - Spontaneous deposition
Leiden, Nov. 2008
Stabilisation: surface chemistry
• Ideal case: modify surfaces to prevent
aggregation, but retain catalytic activity.
• Brust/Schiffrin (1994, [12]) (+ Faraday?): thiol
stabilisation of Au formed by two-phase
reduction
• Hutchison (2000 [13]), Rao (2003 [14]) (+
Faraday?) : phosphine ligands for stabilisation
of Au formed at L/L interface.
•
• Question: for Au deposition, can process (i) =
assembly of particles at L/L be related to
process (ii) = in situ L/L formation?
Leiden, Nov. 2008
Au formation at L/L interface
•
Au NPs formed at interface,
• TEM suggests particle size
regular, density increases with
time.
1.5 hrs
24 hrs
Leiden, Nov. 2008
Comparison of (i) assembly vs. (ii) formation
• Works – i.e. electron microscopy,
xrd
i
A
and xps suggest can get similar
(ca 2A
nm) Au NP from routes (i) and (ii) eif wesolution
Phase
use the same reducing agent.
ii
i
n+
-
solution
Phase
An+ + D
Dn+
D
i
Dn+ + A
ii
n+
A
A
e-
D
Leiden, Nov. 2008
Aq
Phase
Org
Phase
Aq
Phase
Org
Phase
Dn+
An+ + D
Dn+ + A
The characterisation problem
• Deposit characterisation:
ex situ, and (normally)
vacuum based methods
• TEM, SEM, XPS –
particle distribution lost.
• Reactive systems: ebeam/x ray damage?
• Dryfe/Campbell 2008
Leiden, Nov. 2008
gives……..
In situ deposit characterisation: gel or freeze
interface
• Deposit Au at gel/organic
interface: thickness (600 nm)
• Approach (ii), deposit Au at
L/L interface (org = acrylate
and photo-initiator) = photocure interface.
• (after Benkoski 2007,
approach (i) [15])
• Aim: “freeze” structure of
deposit – aggregate of ca 200
nm particles. Dryfe/Ho 2008
Leiden, Nov. 2008
In situ deposit characterisation: alternative
techniques (1)
• Structure of “neat” L/L
interface: x-ray scattering, nonlinear spectroscopy.
• Both recently applied to NP
assembly/formation at L/L
interface.
• Former: e- density profile
attributed to cluster (d = 18
nm) of 1.2 nm NPs.
• Approach (ii)
From Sanyal (2008 [16])
Leiden, Nov. 2008
In situ deposit characterisation: alternative
techniques (2)
• Second-harmonic generation
from polarised water/octanone
interface, for Au NPs
assembled at interface (ie
approach (i)),
• Short time-scales, reversible
particle assembly
• Longer time-scales,
irregularities in SHG response
attributed to NP aggregation.
Leiden, Nov. 2008
From Galletto
(2007 [17]).
(ii – b) In situ growth of particles at L/L
interfaces: electrochemical reduction
• Motivation: apply variable potential difference (4-electrode
methodology):
RT  aRw1aOorg2 
w
0
0
 org  E2  E1 
ln  w org 
F  aO1aR 2 
• Study electrochemical growth in absence of solid substrate:
– M. Guainazzi (1975 [18]) – Cu, Ag
– Schiffrin/Kontturi, (1996 [19]) (Au, Pd)
i
ii
n+
A
e-
– Unwin, (2003, [20]) - (Ag)
– Cunnane, (1998,.[21]) (polymers)
– Dryfe, (2006, [22]) (review).
A
D
Aq
Phase
Org
Phase
Aq
Phase
Org
Phase
Dn+
An+ + D
Dn+ + A
• Advantage: Analysis of current response - information on growth.
Leiden, Nov. 2008
What is known at present?
• Deposit “units” nm scale, adsorb,
tend to aggregate.
• (TEM of Pd, scale bar = 100 nm)
• Replace single interface with
array of micron scale (or smaller)
interfaces = template.
•
-alumina as template, 200 nm
diameter pores (SEM of Pd,
scale bar = 100 nm)
Leiden, Nov. 2008
Nucleation/Growth: Voltammetry
Electrolytic cell:
Mn+(1) + nR(2) → M(s) + nO+(?)
Cell 1
x mM (NH4)2 PdCl4
Where Mn+ = PdCl42−,
R = n-BuFeCp2.
Ag AgCl
y mM BuFc
x mM TPACl
20 mM
50x mM LiCl
1 mM BTPPACl
10 mM LiCl
AgCl
BTPPATPBF 20
A CV showing the transfer of Tetrapropylammonium + in the
presence of a reducing agent
E0 ≈ 0.3 V
120
100
80
60
40
i/µA
Insufficient for spontaneous
reaction: extra η ≈ 0.2 V
needed.
N.B. Irreversible deposition
20
0
-0.1
-20 0
0.1
0.2
0.3
0.4
-40
-60
-80
w
E/V
org / V
Leiden, Nov. 2008
0.5
0.6
0.7
0.8
Ag
Chronoamperometry
• Interfacial Pd depn. Step
potential, increasing h.
All L/L
500
450
400
•
350
i/μA
• Approximate treatment, use of
excess (40-fold) of electron
donor (org): metal precursor
(aq).
200
0.6 V
0.65 V
150
100
50
0
0
5
10
15
Leiden, Nov. 2008
25
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.5
1
1.5
t/tmax
• t > tmax does not follow Cottrell
20
t/s
Apply “classical” models to Pd
deposition @ L/L.
• Behaviour intermediate (prog blue vs. instantaneous models
- pink),
0.5 V
0.55 V
250
i/imax
•
0.4 V
0.45 V
300
2
2.5
3
Analysis of chronoamperometry
• Heerman/Tarallo ≈ Mirkin/Nilov
models [23, 24]:
j  zFDc
  1
e  At
 At 
1/ 2
Applied
overpotential/
V
1
Dt 
1/ 2
At1 / 2

 e d
0
1  exp  qN Dt 


1/ 2 1/ 2
0
 2MDc 

q  2 
  
1/ 2
t


 1  e  At
  1  
 At



Nucleation
Rate constant/
s-1
Nucleation
saturation
density /cm-2
Diffusion
Coefficient/
cm2 s-1
[BuFc] /
mM
0.47
0.29
10063
7.6×10-6
20
0.52
0.64
11589
9.8 × 10-6
20
0.57
0.76
8349
2.5 × 10-5
20
0.62
0.54
11526
4.1 × 10-5
20
Leiden, Nov. 2008
Extending model: 4th parameter
-4
1.6x10
-4
•
1.4x10
Cell: Cell 1
-4
1.2x10
x mM TPACl
50x mM LiCl
1 mM BTPPACl
20 mM
10 mM LiCl
BTPPATPBF 20
AgCl
-4
-2
Ag AgCl
y mM BuFc
Ag1.0x10
J/ A cm
x mM (NH4)2 PdCl4
-5
8.0x10
-5
6.0x10
•
Co-evolution of hydrogen
•
Palladium surface grows, acts as
catalyst.
•
Deposition (almost) insensitive to
applied potential: implies zero
critical cluster!
Leiden, Nov. 2008
5
10
15
20
time/ s
-4
2.5x10
-4
2.0x10
-2
Proton reduction rate included as
4th parameter (after Palomar, 2005
[25]): improved fit, but no direct
evidence for hydrogen evolution.
0
J/ A cm
•
-5
4.0x10
-4
1.5x10
-4
1.0x10
-5
5.0x10
0
10
20
30
time/ s
40
Competitive reactions
• pH dependence of metal
deposition?
• However, ferrocene oxidation
is coupled to H+ transfer (H2O2
generation)
• Nernst-Donnan equilibrium
dictates interfacial potential,
hence extent of H+ transfer.
(from Su, Angew. Chem, 2008
[26])
Leiden, Nov. 2008
Potential dependence of particle size
• High resolution TEM of Pd, deposition for 20 s at L/L.
 = 0.5 V
Relative number of particles
30
(upper), down
to 0.4 V (lower)
– higher h:
higher mean
particle size.
25
20
15
10
5
0
3.5-4
4-4.5
4.5-5
5-5.5
5.5-6
6-6.5
5.5
particle size (nm)
5
30
Particle size (nm)
Relative number of particles
35
25
20
15
10
5
0
2-2.5
2.5-3
3-3.5
3.5-4
4-4.5
4.5-5
5-5.5
4.5
4
3.5
particle siz e (nm)
0.4
Leiden, Nov. 2008
0.425
0.45
0.475
Overpotential (V)
0.5
In situ electrocatalysis at L/L
• Photo-catalytic interfacial
electron transfer,
mediated by Pd
deposited in situ.
• (from Lahtinen,
Electrochem Comm,
2000 [27])
• Complex system: flow
based approach ?
Leiden, Nov. 2008
Ex situ Electrocatalysis
• Au-phosphine stabilised
NPs formed at L/L
interface, transferred by
adsorption on to glassy
carbon surface:
• Response of GC to
formaldehyde oxidation
(before/after Au NP
adsorption) is shown:
• Electrocatalytic activity of
materials. (Luo/Dryfe,
2008)
Leiden, Nov. 2008
Conclusions
•
L/L interface offers a ready “contact-less” route to the:
•
(i) assembly of (catalytically active) particles and
•
(ii) to the growth of (catalytically active) particles, the latter either by
spontaneous or electrochemical approaches.
•
Issues -
Deposit geometry  conditions
– Applicability of “classical” deposition models
•
- difficulty/lack of applicability of “standard” nano-scale characterisation
techniques
•
Nano-scale morphology not dictated by strong substrate-deposit attraction
but strong substrate(1)-substrate(2) repulsion.
•
Regularity of particle structure (before aggregation) – uniform flux to each
particles?
Leiden, Nov. 2008
Suggestions for Future Work
• Catalytic production of H2O2 at the L/L interface
• Photo-catalytic reduction (H2, CO2??) at this
interface
• Does one of the phases have to be H2O?
• Catalysis as fn(, ) ?
Leiden, Nov. 2008
References (1/2)
•
•
•
•
•
•
•
•
•
•
•
•
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