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

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  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@manchester.ac.uk Leiden, Nov. 2008

  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 Leiden, Nov. 2008

  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 Leiden, Nov. 2008

  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? Leiden, Nov. 2008

  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 @ L/L. Leiden, Nov. 2008

  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 Leiden, Nov. 2008

  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]). • => Smooth, reproducible interface. Leiden, Nov. 2008

  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.” Leiden, Nov. 2008

  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 Leiden, Nov. 2008

  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]). Leiden, Nov. 2008

  11. (i) Assembly of (pre-formed) particles at L/L interfaces - continued • Other terms in equation: • q can bevaried 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 Leiden, Nov. 2008

  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 Leiden, Nov. 2008

  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: Leiden, Nov. 2008

  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 @ water/1,2-dichloroethane interface, fractal structures form: image statistics, growth laws for aggregation process (scale bar = 10 microns) Leiden, Nov. 2008

  15. (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 Control deposit aggregation Leiden, Nov. 2008

  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? Leiden, Nov. 2008

  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 Leiden, Nov. 2008

  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 Leiden, Nov. 2008

  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…….. Leiden, Nov. 2008

  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 Leiden, Nov. 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]) Leiden, Nov. 2008

  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]). Leiden, Nov. 2008

  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. Leiden, Nov. 2008

  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) Leiden, Nov. 2008

  25. Nucleation/Growth: Voltammetry Electrolytic cell: Where Mn+ = PdCl42−, R = n-BuFeCp2. DE0≈ 0.3 V Insufficient for spontaneous reaction: extra η≈ 0.2 V needed. N.B. Irreversible deposition Mn+(1) + nR(2) → M(s) + nO+(?) Leiden, Nov. 2008

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

  27. Analysis of chronoamperometry • Heerman/Tarallo ≈ Mirkin/Nilov models [23, 24]: Leiden, Nov. 2008

  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! Leiden, Nov. 2008

  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]) Leiden, Nov. 2008

  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. Leiden, Nov. 2008

  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 ? Leiden, Nov. 2008

  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) Leiden, Nov. 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? Leiden, Nov. 2008

  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) ? Leiden, Nov. 2008

  35. References (1/2) • 1. V.J. Cunnane, D.J. Schiffrin, C. Beltran and G. Geblewicz, J. Electroanal. Chem. 247, 203 (1988). • 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, Science 292, 908 (2001). • 7. Y. Lin, H. Skaff, T. Emrick, A.D. Dinsmore and T.P. Russell, Science, 299, 226 (2003). • 8. F. Reincke, S.G. Hickey, W.K. Kegel, and D. Vanmaekelbergh, Angew Chem. Int. Ed., 43, 458 (2004). • 9. B. Su, J.P. Abid, D.J. Fermín, H.H. Girault, H. Hoffmannova, P. Krtil, Z. Samec, J. Amer. Chem. Soc. 126, 915 (2004). • 10. M.G. Nikolaides, A.R. Bausch, M.F. Hsu, A.D. Dinsmore, M.P. Brenner, C. Gay and D.A. Weitz, Nature420, 299 (2002). • 11. M. Faraday, Philos. Trans. I, 147, 145 (1857). • 12. M. Brust, M. Walker, D.J. Schiffrin and R. Whyman, J. Chem. Soc. Chem. Comm., 801 (1994). • 13. W.W. Weare, S.M. Reed, M.G. Warner and J.E. Hutchison, J. Amer. Chem. Soc., 122, 12890 (2000). Leiden, Nov. 2008

  36. References (2/2) • 14. C.N.R. Rao, G.U. Kulkarni, P.J. Thomas, V.V. Agrawal and P. Saravanan, J. Phys. Chem. B, 107, 7391 (2003). • 15. J.J. Benkoski, R.L. Jones, J.F. Douglas and A. Karim, Langmuir, 23, 3530 (2007). • 16. M.J. Sanyal, V.V. Agrawal, M.K. Bera, K.P. Kalyanikutty, J. Daillant, C. Blot, S. Kubowicz, O. Komovalov and C.N.R. Rao, J. Phys. Chem. C, 112, 1739 (2008). • 17. P. Galletto, H.H. Girault, C. Gomis-Bas, D.J. Schiffrin, R. Antoine, M. Broyer and P.F. Brevet, J. Phys. Cond. Matt. 19, 375108(2007). • 18 M. Guainazzi, G. Silvestri and G. Serravalle, J. Chem. Soc. Chem. Commun.,200 (1975). • 19 Y. Cheng and D.J. Schiffrin, J. Chem. Soc. Farad. Trans., 92, 3865 (1996). • 20. J.D. Guo, T. Tokimoto, R. Othman and P.R. Unwin, Electrochem. Comm., 5, 1005 (2003). • 21 V.J. Cunnane and U Evans, Chem. Comm., 2163 (1998). • 22. R.A.W. Dryfe, Phys. Chem. Chem. Phys. 8, 1869, (2006). • 23. L. Heermann and M. Tarallo, J. Electroanal. Chem., 470, 70 (1999). • 24. M.V. Mirkin and E. Nilov, J. Electroanal. Chem., 283, 35 (1990). • 25. M. Palomar-Pardave, B.R. Scharifker, E.M. Arce and M. Romero-Romo, Electrochim. Acta, 50, 4736 (2005). • 26. B. Su, R. Partovi Nia, F. Li, M. Hojeij, M. Prudent, C. Corminboeuf, Z. Samec and H.H. Girault, Angew. Chem. Int. Ed., 47, 4675 (2008). • 27. R.M. Lahtinen, D.J. Fermín, H. Jensen, K. Kontturi and H.H. Girault, Electrochem. Comm. 2, 230 (2000). Leiden, Nov. 2008