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Public Defense "Assemblies of gold nanoparticles at liquid-liquid interfaces: from liquid optics to electrocatalysis"
- 1. Evgeny Smirnov Supervisor: Prof. Hubert H. Girault Assemblies of Gold Nanoparticles at Liquid-Liquid Interfaces: from Liquid Optics to Electrocatalysis EPFL, Lausanne, 25.01.2017
- 2. Outline Overview: Liquid-Liquid Interfaces and Plasmonic Nanoparticles Chapter 3-5: Self-Assembly of Nanoparticles at Liquid-Liquid Interfaces Chapter 6-8: Electrochemical Investigation of Nanofilms and Redox Electrocatalysis Chapter 9: Perspectives General Conclusions 2 IntroductionOverview Main Results Conclusions
- 3. Overview: Liquid-Liquid Interfaces (LLIs) 3 IntroductionOverview Main Results Conclusions Liquid-Liquid or Liquid-Air Interfaces (LLI or LAI) provide: • a defect free platform in comparison with widely used BUT energetically heterogeneous liquid-solid and air- solid interfaces… • …with self-healing nature and usually high transparency… • …as well as infinite mechanical flexibility… • …to assemble various types of subnano-, nano- and even microobjects(!), such as molecules, interfacial complexes and nano- and micro- solid or soft particles. Therefore, LLI is almost a perfect system to make, study and manipulate with nanoparticle films. http://simple.wikipedia.org/wiki/Cell_membrane
- 4. Overview: Plasmonic Nanoparticles 4 IntroductionOverview Main Results Conclusions D. Schaming et al., Gold Bulletin, 46 (4), 2013 T 4 3Na citr+ + +AuNH P …lAuCl NaC
- 5. Self-Assembly at LLIs: MELLDs 5 Main Results Conclusions Smirnov et al. ACSNano, 8, 9471 (2014)IntroductionOverview Main motivation: Smart mirrors and filters at LLIs driven by an external field (electric or magnetic) and understanding the self-assembly at LLIs Main problems of self-assembly: Relatively high potential barrier, which prevents NPs to adsorb at LLIs Obtained films are usually brittle and form cracks easily Some methods require expensive and time consuming functionalization of NP Flatte, Kornyshev,& Urbakh, PNAS, 105, 18212 (2008) Dery, Borra & Ritcey, Chem.Mat., 20, 6420 (2008) Ritcey & Borra, ChemPhysChem, 11, 981 (2010) TTF TTF•+O R Organic solvent only Both org. and water
- 6. Self-Assembly at LLIs: It’s Alive!!! 6 Main Results Conclusions https://youtu.be/IJC2K1O_22MIntroductionOverview
- 7. Self-Assembly at LLIs: Proposed Mechanism 7 Main Results Conclusions Smirnov et al. ACSNano, 8, 9471 (2014)IntroductionOverview
- 8. Self-Assembly at LLIs: Mechanism in a Nutshell 8 Main Results ConclusionsIntroductionOverview Without TTF With TTF TTF molecules
- 9. Self-Assembly at LLIs: MELLDs 9 Main Results Conclusions Smirnov et al. ACSNano, 8, 9471 (2014)IntroductionOverview Complete removal of citrate@NPs (from 10 nm to 80 nm) from the water phase Arbitrary surface coverage, multilayers and self-healing nature: Particle Connectivity & Mechanical Properties Universality for water-organic interfaces (>10 solvents were tested)
- 10. Self-Assembly at LLIs: In-situ Study of Optical Properties 10 Main Results ConclusionsIntroductionOverview Scattered light Extinction = A + S Incident light Reflected light Absorbance
- 11. Self-Assembly at LLIs: In-situ Study of Optical Properties 11 Main Results Conclusions Main questions: A typical procedure is ex-situ testing of nanofilm Optical properties vs AuNPs content? Where is the border between mirrors and filters? How does the solvent nature affect optical properties? IntroductionOverview 12 nm AuNPs: Dropping off of the reflectance ~ the morphology of nanofilm (buckling and wrinkles), but continuous growth of extinction (filtering applications). Also, transformation from 2D to 3D regimes was found, confirmed by both in situ and ex situ methods. AuNPs content increasing / ML Smirnov et al. Nanoscale, 8, 7723 (2016) Filters – small NPs (below 15-20 nm), two layers of 12 nm particles absorb up to 90% of the light Mirrors – large NPs (above 25 nm), a single layer of 38 nm particles reflects more than 50% of the light ML – hexagonally close-packed MonoLayer of AuNPs 38 nm AuNPs:
- 12. Self-Assembly at LLIs: Low Interfacial Tension 12 Main Results ConclusionsIntroductionOverview Without TTF: Self-assembly at LLIs with low interfacial tension is another promising way toward cheap, large scale nanofilms. With TTF: Concentration of nanoparticles in the organic phase– AuNPs Water Oil (PC) + TTF High γw/o Water Oil Without TTF = hydrophilic NPs With TTF = less hydrophilic NPs Incident light Reflective surface Water Oil Considered previously Smirnov et al. submitted to ChemComm Incident light Low γw/o Reflective surface
- 13. Back to the Overview: LLIs and Electrochemistry 13 http://simple.wikipedia.org/wiki/Cell_membrane o o w w 0 o 1/2 o w w ln ln 2 i i i i ii i DRT RT z F z F D Main Results ConclusionsIntroductionOverview
- 14. How to Transfer Nanofilm into 4-electrode Cell? Main motivation: Study of ion and electron transfer reactions in the presence of nanoparticle film for further applications (drug delivery, interfacial catalysis etc.) Main problems: Contamination of aqueous phase by AuNPs Large volumes of alcohol affects organic phase Large capacitive (background) current Unknown amount of AuNPs adsorbed at the interface 14 Main Results ConclusionsIntroductionOverview Su, B. Assembly and reactivity of nanoparticles at liquid/liquid interfaces (2006) Younan et al., Electrochem. Commun., 12, 912 (2010)
- 15. How to Transfer Nanofilm into 4-electrode Cell? A syringe pump with a capillary allows: delivering of gold nanoparticles solution directly to the ITIES with… negligible amount of the AuNP solution in the bulk, whereas… using pre-concentrated nanoparticles solution prevents pollution of electrochemical system by hindering ions and molecules such as citrate and methanol. But! The uniform coverage is only 0.5 ML. Smirnov et al. ACSNano, 9 (6), 6565 (2015)15 Main Results ConclusionsIntroductionOverview
- 16. Ion Transfer Through a Nanofilm 16 Main Results ConclusionsIntroductionOverview Low capacitive current AuNP film is transparent for ion transfer with semi-infinite diffusion profile! Possible morphologies Diffusion profiles Smirnov et al. ACSNano, 9 (6), 6565 (2015) Top view Side view
- 17. Charging of AuNP Films by Electron Donors 17 Main Results ConclusionsIntroductionOverview Fc TTF Ion-transfer peaks of TTF+ and Fc+ are the sign of the electron transfer from the donors to an AuNP in the nanofilm.
- 18. Oxygen Reduction by DMFc at Neutral pH • Overpotential is reduced from ca. +500 mV down to ca. +100 mV • H2O2 concentration from titration 0.10 mM, from amount of DMFc+ - 0.9 mM. Thus, yield of H2O2 is about 22%. 18 Main Results Conclusions Smirnov et al. ElectroChem. Acta, 197, 362 (2016) Shaking Flask Experiment IntroductionOverview CVs at ITIES Aerobic Anaerobic
- 19. Perspectives 19 Main Results ConclusionsIntroductionOverview Colloidosomes Large Liquid Mirrors SERS and ElChem-SERS Self-terminating Welding Marangoni Shutters
- 20. A new method of AuNP self-assembly has been proposed. It bases on “mild” charging of NP core by TTF and leads to self-healing MeLLDs. In the case of low interfacial tension TTF promotes extraction of NPs into the organic phase Optical properties of nanofilms has been studied in situ and showed unexpected behavior such as decreasing reflectance with increased extinction A syringe pump method has been developed to transfer AuNP nanofilm into 4-electrode cell. It resulted in a lustrous nanofilm, preventing pollution of both phases and improving performance of the cell Nanoparticles in the film can be charged by an electron donor in one phase and discharge by an electron acceptor in another. This property was successfully used to perform oxygen reduction by DMFc at neutral pH General Conclusions 20 Main Results ConclusionsIntroductionOverview
- 21. Before party like a Russian A short discussion…
Editor's Notes
- The message: LLI is almost perfect system to create, study and manipulate with nanoparticle films.
- Today my presentation will be divided into three big blocks: I will start with a short overview why I’m interested in using LLIs for self-assembly Then I will continue on different aspects of self-assembly of gold nanoparticles at LLIs And next, electrochemical investigation of nanofilms will be presented Finally, if I have enough time I would like to present and discuss in a nutshell Perspectives of the current work.
- Interfaces are surrounding us. If I tell you oil-water interface, maybe, you will remember something like that – oil drops on the surface of soup. However, two back-to-back LLIs is a basement of our body – cell membranes. The typical thickness of a single LLI is less than 1 nm, 100 000 times thinner than you hair. Why are LLIs so special? The message: LLI is almost perfect system to make, study and manipulate with nanoparticle films.
- If we consider a metal NP with weakly bounded electrons, these electrons will follow an external electro-magnetic field – light – and shift in one direction, leaving positively charged ions. This will create a dipole moment and returning force, brining electrons back. If we punch those electrons in resonance mode, the abnormal absorption devoted to plasmonic NP will appear in the spectrum. This plasmonic effect is size-depended. To obtain such nanoparticles we used pretty simple synthetic procedure – heating up yellowish gold with transparent reductant. And you can distinguish gold nanoparticle solution from raspberry jus only with a laser pointer.
- The main motivation for this part devoted to self-assembly of nanoparticles was developing smart mirrors and filters and understanding the self-assembly process at LLIs. However, there are several problems with self-assembly… To overcome these problems we proposed a new method to self-assembly citrate-stabilized AuNPs at LLIs – interfacial assembly of gold nanoparticles promoted by TTF molecules. TTF has 4 supfurs!!!, so sticks at the gold surface, as well it plays a role of “glue” sticking nanoparticles together due to π-π-interactions. We found that nanoparticles assembled at LLI – here for example, at DiChloroEthane-water interface – upon vigorous shaking of two presented here solutions. TTF can be easily oxidized forming stable ion-radicals TTF+ and donate electrons to a AuNP. The TTF itself is mainly soluble in organic phase, whereas TTF+ is soluble in both the aqueous and organic phases. Thus, TTF react with AuNP in both phases and entrapped them.
- Video
- The mechanism is rather complicated and presented on this slide, but I will explain it in a nut shell…
- A simplified explanation – I will need your imagination! Imagine that gold nanoparticle is a brick. Without TTF it could aggregate with other bricks and form something like that (black precipitate at the bottom of the flask). However, if our bricks are functionalized with click-junction like in LEGO bricks, we will manage to assemble such bricks into the wall. But those functionalized particles will self-assemble and self-heal holes, crack inside of the wall.
- These nanofilms and preparation method possess several key-features: Arbitrary surface coverage to make sub-monolayer as well as multilayer interfacial films Complete removal of nanoparticles from the aqueous phase (in other word, we know amount of particles at the interface) University for w/o interfaces (we tested this method with more than 10 solvents), therefore, the suggested method is quite general Particles in the nanofilm are connected with each other by TTF-molecules and provide certain mechanical properties. For example, formation of wrinkles rather than cracks. For further investigation of optical properties we used these two features of the obtained nanofilms.
- Interactions of the light with any object can be described as absorbance, scattering and a special case of scattering – reflectance. At the end, we have extinction which is by definition is a sum of absorbance and scattered light. We developed a technique to characterize our films in extinction and reflectance modes using an integrating sphere that helps to collect all signal form the sample.
- Usually, to study optical properties of interfacial films, they are transferred onto a solid substrate, which may influence these properties. We developed in-situ procedure with an integrating sphere to investigate optical responses in extinction and reflectance modes. There are typical spectral data that we obtained. Another interesting question was how optical properties were varying with AuNPs content? Our intuition suggests that the more AuNPs are placed at the interface, the shinier it will be. However, as we showed it didn’t work like that. At the beginning increasing NP content led to linear increase of extinction and reflectance from the film. However, at some point this linear dependence was broken: extinction changed the slope and reflectance went down. The knee was observed at 0.6-0.7 hexagonally close-packed monolayer, which is marked as ML. We related that to the film morphology changes such as buckling and wrinkles formation. More practical question: where is the border between mirrors and filters? Now we can firmly say that the line between these two types of applications are about 20-25 nm. Obtained findings are novel challenges for groups who work on simulations and modelling of smart mirrors, as they should take into account size of nanoparticles (20 nm and more) as well as surface coverage to avoid fading.
- We have already considered water-oil interface with relatively high interfacial tension, like water-DCE. Without TTF hydrophilic AuNPs cannot assemble at such LLIs; addition of TTF leads to self-assembly into a lustrous, reflective interfacial nanofilm. Nevertheless, in the case of low interfacial tension, for example, water-propylene carbonate system, citrate covered AuNPs were assembled into a lustrous film even without TTF and the presence of TTF allowed them to be extracted into the organic phase. Thus, this method is definitely useful for cheap and large scale preparation of nanofilms, whereas using of TTF allows concentrating of nanoparticles into the oil phase.
- And again coming back to the Overview. As I said LLI is a part of any biological object, with methods of electrochemistry we may study charge transfer events at such interfaces (it could be ion or electrons). In this picture you may see a typical electrochemical cell where the aqueous phase is blue and the organic phase is red. Of course, to make both liquid phases conductive we must add some electrolytes – LiCl and BATB. ITIES states for Interface between two immiscible electrolyte solutions. Here, typical cyclic voltamogramms are represented. In the middle of the potential window an interface is polarized, but ions do not cross the interface, whereas at some potentials they may cross and we see increase of the current.
- Besides self-assembly process and optical properties of nanoparticle film, we also studied ion-permittivity of such interfacial films and their role in interfacial electron transfer reactions. The main problem we faced here was how to transfer nanofilm into 4-electrode electrochemical cell? Many of previous methods, including methanol assisted deposition, led to contamination… For example, there is an image of electrochemical cell taken from previous publications: aqueous phase contains AuNPs, DCE phase is muddy due to large alcohol content…
- We developed a method, which is also based on alcohol promoted self-assembly, to deliver concentrated solution of nanoparticles directly to LLI with a capillary, as shown on figures. This procedure prevented pollution of both phases and led to a similar type of nanofilms as considered before. Pollutants may be nanoparticles, alcohol, hindering ions or something else. However, there is only one limitation that we had to work with roughly half monolayer films.
- To show ion-permittivity of such nanofilms, we studied ion-transfer of standard ions such as TMA+, which are used to calibrate the potential window. As we can see, addition of the nanofilm at the electrified interface did not increase significantly the capacitive current. Ions were crossing the film without remarkable changes in CV. The nanofilm morphology is a mixture between network of particle and separated islands. It contains denser regions and voids. A single void gave a spherical diffusion profile, whereas their overlapping resulted in semi-infinite diffusion profile. Thus, to conclude, obtained films are transparent for ion transfer with semi-infinite diffusion profile.
- Further, we investigated what happened if we add electron-donor molecules in the organic phase. As I said before, we believed that TTF donated electrons to gold nanoparticles. However, there was no direct proof of these events, as the quantity of TTF+ was too low. Nevertheless, we managed to do that in the electrochemical cell, because all oxidized TTF was located in the vicinity of the interface. Thus, TTF+ molecules were transferred upon cyclic (large peak in the middle of the window). It wasn’t an artifact, as similar results were achieved with other electron donor molecules such as Fc and DMFc. So, what happened if we substitute Fc with DMFc, in other words, much stronger electron donor?
- As I promised I came back to the case of DMFc. Interaction of nanoparticles with DMFc led to an irreversible reaction, which can be seen as an onset of an electrochemical wave in the middle of the window. Remarkably, in anaerobic conditions this wave disappeared that means that oxygen was involved in this electrocatalytic process. One of the possible products is H2O2. We performed a shaking flask experiment with chemical polarization of the interface by distribution of a common ion. Results of that experiment revealed high concentration of DMFc+ and H2O2. The yield of H2O2 was 22%. Once again, firstly, nanoparticles were charged by DMFc and Fermi level was fixed by DMFc redox couple, then slightly positive Galvani potential difference is required to overcome a potential barrier and start oxygen reduction on the surface of gold nanoparticles.
- Now, let me say a couple of words about perspectives. There are several areas where presented methods, materials and findings may be applied. Colloidosomes Large Liquid Mirrors – for example, for telescopes SERS and Electrochemical SERS Self-terminating welding (melting point of gold - 1,064) Marangoni shutters at LLI
- To conclude…