Electrochemical Generation of Nanostructures at the Liquid-Liquid Interface

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 H 3 DA TPBF 3 ethylmethylimidazolium ethylsulfate (EMIM Et. SO 4) interface

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 Leiden, Nov. 2008

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: • 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 @ 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: • also: • and Leiden, Nov. 2008

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 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

Approaches 1 vs. 2 at L/L interface • Source of particles? – (i) Assembled at interface (particles = surfactants) Then spontaneous assembly (adsorption) at interface – (ii) Grown at interface (either (a) spontaneous deposition or (b) electrodeposition). Leiden, Nov. 2008

(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 Cd. Se (Russell, 2003 [7]). Leiden, Nov. 2008

(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 90 o. • 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 Leiden, Nov. 2008

Ordering of insulating particles at L/L interfaces • System – 1. 6 mm Si. O 2 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 Si. O 2 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/CS 2) 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, 2 dichloroethane 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 and xps suggest can get similar (ca 2 nm) Au NP from routes (i) and (ii) if we use the same reducing agent. Leiden, Nov. 2008 i

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): • 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. 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. • g-alumina as template, 200 nm diameter pores (SEM of Pd, scale bar = 100 nm) Leiden, Nov. 2008

Nucleation/Growth: Voltammetry Electrolytic cell: Mn+(1) + n. R(2) → M(s) + n. O+(? ) Where Mn+ = Pd. Cl 42−, R = n-Bu. Fe. Cp 2. DE 0 ≈ 0. 3 V Insufficient for spontaneous reaction: extra η ≈ 0. 2 V needed. N. B. Irreversible deposition Leiden, Nov. 2008

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 @ L/L. • Behaviour intermediate (prog blue vs. instantaneous models - pink), • t > tmax does not follow Cottrell Leiden, Nov. 2008

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/ cm 2 s-1 [Bu. Fc] / m. M 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: 4 th parameter • Cell: • Co-evolution of hydrogen • Palladium surface grows, acts as catalyst. • Proton reduction rate included as 4 th parameter (after Palomar, 2005 [25]): improved fit, but no direct evidence for hydrogen evolution. • Deposition (almost) insensitive to applied potential: implies zero critical cluster! Leiden, Nov. 2008

Competitive reactions • p. H dependence of metal deposition? • However, ferrocene oxidation is coupled to H+ transfer (H 2 O 2 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. Df = 0. 5 V (upper), down to 0. 4 V (lower) – higher h: higher mean particle size. Leiden, Nov. 2008

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 H 2 O 2 at the L/L interface • Photo-catalytic reduction (H 2, CO 2? ? ) at this interface • Does one of the phases have to be H 2 O? • Catalysis as fn(D , p) ? Leiden, Nov. 2008

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