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

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

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Title: 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 1 Distribution
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 2 Polarised 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.

9
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
10
(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).

11
(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

12
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

14
(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)

15
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

16
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?

17
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
18
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
19
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..
20
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

21
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)
22
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).
23
(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.

24
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

27
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
28
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)

30
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.
31
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 ?

32
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)

33
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?

34
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) ?

35
References (1/2)
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    G. Geblewicz, J. Electroanal. Chem. 247, 203
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  • 2. S.N. Tan, R.A.W. Dryfe and H.H. Girault,
    Helv. Chim. Acta, 77, 231 (1994)
  • 3. I.T. Horvath and J. Rabai, Science, 266, 72
    (1994)
  • 4. A.D. Ballantyne, A.K. Brisdon and R.A.W.
    Dryfe, Chem. Comm., 4980. 5. D.M. Mitrinovic,
    A.M. Tikhonov, M. Li, Z.Q. Huang and M.L.
    Schlossman, Phys. Rev. Lett. 85, 582 (2000).
  • 6. L.F. Scatena, M.G. Brown and G.L. Richmond,
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    H. Hoffmannova, P. Krtil, Z. Samec, J. Amer.
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    Dinsmore, M.P. Brenner, C. Gay and D.A. Weitz,
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36
References (2/2)
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