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Univ. of Manchester (U.K.)

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1 Univ. of Manchester (U.K.)
Electrochemical Generation of Nano-structures at the Liquid-Liquid Interface Robert A.W. Dryfe School of Chemistry, Univ. of Manchester (U.K.) 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 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 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 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 water/1,2-dichloroethane interface, fractal structures form: image statistics, growth laws for aggregation process (scale bar = 10 microns) Leiden, Nov. 2008

15 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

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 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]: 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 7.6×10-6 20 0.52 0.64 11589 9.8 × 10-6 0.57 0.76 8349 2.5 × 10-5 0.62 0.54 11526 4.1 × 10-5 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., 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, Nature 420, 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, (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, (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, (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, (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

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