Electrochemical%20Generation%20of%20Nano-structures%20at%20the%20Liquid-Liquid%20Interface

1
Electrochemical Generation of Nano-structures at
the Liquid-Liquid Interface

• Robert A.W. Dryfe
• School of Chemistry,
• Univ. of Manchester (U.K.)
• robert.dryfe_at_manchester.ac.uk

2
Liquid/Liquid Interfaces in catalysis

• Widely used bi-phasic system, allows for ease of
  separation of catalysis from reactant mixture.
• Electrochemical investigations of phase-transfer
  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)

H3DA TPBF3 ethylmethy-limidazolium ethylsulfate
(EMIM EtSO4) interface
3
Liquid/Liquid Interfaces electro-catalyst
generation

• Reduction of solution phase Mn
• Heterogeneous ET (surface of electronic
  conductor)
• Homogeneous ET (nanoparticle preparation)
• Heterogeneous ET (aq/organic interface)
  with/without potential control

4
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?

5
Liquid-liquid Electrochemistry 1Distribution
potential

• Each ion distribution equilibrium at the
  organic/water interface
• Define standard Galvani potential of transfer
• Vary potential with common-ion
• ratio of ion concentration in each phase
  (maintained by hydrophilic/hydrophobic
  counter-ions) poises potential
• (Nernst-Donnan equilibrium )
• - ion transfer/electron transfer particularly
  for SECM _at_ L/L.

6
Liquid-liquid Interfaces 2Polarised Interfaces

• External polarisation of L/L interface (both
  phases contain electrolyte)
• Electrolytes AX(aq) and CY(org), the following
  inequalities are met
• also
• and

7
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).
• gt Smooth, reproducible interface.

8
Modify Sharp (but fluctuating) interface?

• Catalysis introduction of metal
  (nano-)particles
• Result electro-catalytic processes at interface
  with only ionic 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.

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Approaches 1 vs. 2 at L/L interface

• Source of particles?
• (i) Assembled at interface (particles
  surfactants)
• (ii) Grown at interface (either (a) spontaneous
  deposition or (b) electrodeposition).

Then spontaneous assembly (adsorption) at
interface
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(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
• Particles of given type, will be displaced by
  those with larger radius (r)
• Size segregation effect demonstrated for CdSe
  (Russell, 2003 7).

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(i) Assembly of (pre-formed) particles at L/L
interfaces - continued

• Other terms in equation
• 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

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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
• (Campbell/Dryfe 2007, but after Nikolaides, 2002
  10)

Dried close packing
13
Spontaneous ordering of SiO2
Use image analysis to identify individual
particle positions radial distribution function
found. - metallic particles, more polar
phases?

• Field of view 190 microns x 143 microns

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(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 _at_ water/1,2-dichloroethane
  interface, fractal structures form image
  statistics, growth laws for aggregation process
  (scale bar 10 microns)

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Control deposit aggregation

• (a) Template diameter lt 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

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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?

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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
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Comparison of (i) assembly vs. (ii) formation

• Works i.e. electron microscopy, xrd and xps
  suggest can get similar (ca 2 nm) Au NP from
  routes (i) and (ii) if we use the same reducing
  agent.

i
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The characterisation problem

• Deposit characterisation ex situ, and (normally)
  vacuum based methods
• TEM, SEM, XPS particle distribution lost.
• Reactive systems e- beam/x ray damage?
• Dryfe/Campbell 2008

gives..
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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) photo-cure
  interface.
• (after Benkoski 2007, approach (i) 15)
• Aim freeze structure of deposit aggregate of
  ca 200 nm particles. Dryfe/Ho 2008

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In situ deposit characterisation alternative
techniques (1)

• Structure of neat L/L interface x-ray
  scattering, non-linear 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)
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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.

From Galletto (2007 17).
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(ii b) In situ growth of particles at L/L
interfaces electrochemical reduction

• Motivation apply variable potential difference
  (4-electrode methodology)
• Study electrochemical growth in absence of solid
  substrate
• M. Guainazzi (1975 18) Cu, Ag
• Schiffrin/Kontturi, (1996 19) (Au, Pd)
• Unwin, (2003, 20) - (Ag)
• Cunnane, (1998,.21) (polymers)
• Dryfe, (2006, 22) (review).
• Advantage Analysis of current response -
  information on growth.

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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.
• g-alumina as template, 200 nm diameter pores
  (SEM of Pd, scale bar 100 nm)

25
Nucleation/Growth Voltammetry

• Electrolytic cell
• Where Mn PdCl42-,
• R n-BuFeCp2.
• DE0 0.3 V
• Insufficient for spontaneous reaction extra ?
  0.2 V needed.
• N.B. Irreversible deposition

Mn(1) nR(2) ? M(s) nO(?)
26
Chronoamperometry

• Interfacial Pd depn. Step potential, increasing
  h.
• Approximate treatment, use of excess (40-fold) of
  electron donor (org) metal precursor (aq).
• Apply classical models to Pd deposition _at_ L/L.
• Behaviour intermediate (prog - blue vs.
  instantaneous models - pink),
• t gt tmax does not follow Cottrell

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Analysis of chronoamperometry

• Heerman/Tarallo Mirkin/Nilov models 23, 24

Applied overpotential/ V Nucleation Rate constant/ s-1 Nucleation saturation density /cm-2 Diffusion Coefficient/ cm2 s-1 BuFc / mM
0.47 0.29 10063 7.610-6 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
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Extending model 4th parameter

• Cell
• Co-evolution of hydrogen
• Palladium surface grows, acts as catalyst.
• Proton reduction rate included as 4th parameter
  (after Palomar, 2005 25) improved fit, but no
  direct evidence for hydrogen evolution.
• Deposition (almost) insensitive to applied
  potential implies zero critical cluster!

29
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)

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Potential dependence of particle size

• High resolution TEM of Pd, deposition for 20 s at
  L/L.

Df 0.5 V (upper), down to 0.4 V (lower)
higher h higher mean particle size.
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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 ?

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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)

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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?

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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(D?, p) ?

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