Ionic Liquids: From Knowledge to Application

Chapter 12

Downloaded by COLUMBIA UNIV on June 29, 2012 | http://pubs.acs.org Publication Date (Web): December 18, 2009 | doi: 10.1021/bk-2009-1030.ch012

Nanoparticle Synthesis in Ionic Liquids Anja-Verena Mudring, Tarek Alammar, Tobias Bäcker, and Kai Richter Anorganische Chemie I – Festkörperchemie und Materialien, Ruhr-Universität Bochum, D-44780 Bochum, Germany

Ionic liquids are able to offer outstanding properties as media for the synthesis of nanoparticles. The low surface tension of many ionic liquids leads to high nucleation rates and, in consequence, to small particles. The ionic liquid itself can act as an electronic as well as a steric stabiliser and depress particle growth. As highly structured liquids, ionic liquids have a strong effect on the morphology of the particles formed. Three synthetic techniques make special use of the unique properties that ionic liquids offer when compared to conventional VOCs (volatile organic solvents). Firstly, direct microwave synthesis can be used because of the ionic character and high polarisability of the synthetic medium. Secondly, physical vapour deposition (PVD) under high vacuum becomes possible due to the low vapour pressure of some ionic liquids. In order to make full use of the possibilities that ionic liquids offer we have designed a set of reducing ionic liquids which can be used as direct reaction partners for the generation of metal nanoparticles. Thirdly, sonochemistry has proven an especially powerful route towards oxidic nanomaterials.

© 2009 American Chemical Society In Ionic Liquids: From Knowledge to Application; Plechkova, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

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Nanoparticles and Ionic Liquids


Bringing materials to the nanoscale has been occupying chemists and materials scientists, as well as engineers, for quite some time (1). When a material is brought to the nanoscale, its chemical and physical properties can change significantly. These size-dependent properties make nanostructures of various compounds extraordinarily valuable. In recent years, ionic liquids have also been discovered to be excellent media for the preparation and stabilisation of nanoparticles (2). Even though this kind of nanochemistry in ionic liquids is still in its infancy, many signs imply that it is an important emerging field with a huge potential, because ionic liquids offer many advantages for the synthesis of inorganic nanomaterials: 1. 2.

3. 4.

5. 6.

7.

8. 9.

Ionic liquids can be designed to be good solvents for inorganic salts which are the starting materials to nanoparticle synthesis. Ionic liquids can be chosen in such a way that inorganic syntheses with polar/ionic starting materials can be carried out under ambient and anhydrous or water-poor conditions, thereby suppressing the undesired formation of hydroxides or oxyhydrates. Although polar, ionic liquids can have low interfacial tensions. Since low interfacial tensions result in high nucleation rates, very small particles can be generated which undergo Ostwald ripening to a small degree. As they consist of cations and anions, ionic liquids can form a protective electrostatic shell around nanoparticles to prevent agglomeration. In addition, the resemblance of many ionic liquids to well-known surface active substances is clear and ionic liquids may, in addition, stabilise nanoparticles through coordination via the cation or anion (ionic or covalent bonds). In particular, cations and anions with long or bulky alkyl chains can also sterically stabilise nanoparticles in solution. High dispersion force components of the surface tension enhance the differences between the interfacial energies of different crystal faces, and thus morphology control is ensured in these media. Ionic liquids may form extended hydrogen-bonded systems in the liquid state; in this sense, ionic liquids may be regarded “supramolecular” solvents. This special quality can be utilised in the synthesis of extended ordered nanoscale structures, or for morphology control through the templating effect of various ionic liquids. Properties and property combinations of ionic liquids can be tuned through the respective cation/anion combination, for example hydrophilic/hydrophobic nature, gas solubilities, the extent of hydrogen bonding, or mesogenic character. Ionic liquids can become a reactive agent, e.g. hydroxyl-functionalised ionic liquids can act both as a reducing agent, solvent and nanoparticle stabiliser. Depending on the chosen ionic liquid, it is expected that the synthesised nanoparticles can be tailored to be either soluble in water (high charge concentration, polar functionalities like carboxyl groups, etc.) or be hydrophobic solvents (long alkyl chains, perfluorinated moieties).

In Ionic Liquids: From Knowledge to Application; Plechkova, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

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10. The extremely low vapour pressure and non-flammability of most ionic liquids makes reactions at elevated temperatures and under vacuum possible and safe, which are impossible to conduct in conventional solvents. 11. Although ionic liquids are not generally green – and one has to be aware that there exist about 1018 possible cation/anion combinations for potential ionic liquids (3) – they can be designed in such a way that the twelve principles of green chemistry developed by Anastas and Warner (4) are followed. Relevant green aspects include not only the volatility, nonflammability, non-corrosiveness and low toxicity, but, most importantly, the use of any auxiliary substances such as solvents, separation agents, etc., should be unnecessary. The developments in the use of ionic liquids as reaction media for inorganic nanomaterials have so far mainly focussed on (1) the intrinsic high charge and polarisability of the ionic liquid to create electrostatic and steric stabilisation for nanoparticles and to favour phase transfer of the nanoparticles from water to water-immiscible solvents, and (2) using the pre-organised structure of the ionic liquid as a template for porous inorganic nanomaterials. Transition metal nanoparticles, such as iridium, rhodium, palladium, platinum and gold nanoparticles, have been generated by standard reduction methods, where the ionic liquid acts both, as a (co-)solvent and stabiliser for these nanoparticles (5). Moreover, alloy nanoparticles, such as CoPt3, have been prepared via thermolysis in ionic liquids (6). However, in the majority of cases, ionic liquids were used together with co-solvents which significantly alter the properties of the solvent compared to the neat ionic liquid. In our work, we strive for make full use of the unique possibilities that ionic liquids offer compared to conventional volatile organic solvents, and aim at carrying out the reactions in neat ionic liquids with the least possible number of additional chemicals. This reduces the number of parameters that influence the reaction and gives an understanding of the reaction mechanism, and not only allows fine-tuning of the reaction parameters (as well as the ionic liquid itself), but also permits formation of purer, less contaminated compounds.

Nanoparticle Synthesis via Physical Vapour Deposition As many ionic liquids have negligible vapour pressure, they can be handled under high vacuum conditions, even at elevated temperatures. This allows employment of PVP (physical vapour deposition) methods to prepare nanoparticles (7). The experimental setup that we use in our laboratory is a commercial evaporation apparatus (Torrovap TVP 800) which consists essentially of a rotating reaction flask in which either a resistive or electron beam evaporation source can be mounted (Figure 1). The system is based on the experimental experience of Timms, Skell, Klabunde and Green for the synthesis of low-valent main-group halides and transition metal arene complexes, and the work of Ozin, Burdett and Turner in matrix cryochemistry (8). The reaction chamber is maintained under high vacuum by a high speed pump assembly


180 (rotary oil pump combined with an oil diffusion pump). A quantity of the respective ionic liquid is introduced into the rotating reaction vessel, thereby creating a liquid film over the inside wall of the flask. evaporation source (resistive/e-gun)

transfer tube gate valve

rotary seal rotary reaction flask safety hood cooling bath (if required)


diffusion pump

mechanical pump

Figure 1. Experimental setup for physical vapour deposition of substrates into ionic liquids. After the system has been evacuated, a metal, an intermetallic phase, or a metal salt is evaporated either via resistive heating from a molybdenum or tungsten container (300 A source) or via an electron gun (6.0 kW) and condensed into the ionic liquid film. Resistive heating is used for evaporation in the temperature regime of 25-2000 °C, whereas electron beam vaporisation is best suited for the high temperature region (1000-2500 °C). Evaporation via an electron beam has the advantage that it is a containerless method and can be applied whenever the reaction of the materials to be evaporated with the container might become a problem. The electron beam can be focused on the central portion of the sample, and only the inner portion of the sample is first melted and then evaporated. This method can be used for large scale vaporisation (kg h-1) of metal halides, carbides, or oxides, as well as refractory metals and their alloys. Fortunately, many physicochemical and physical data (such as evaporation conditions, boiling points under vacuum or gas phase compositions) are wellknown from matrix-isolation spectroscopic studies. A microbalance quartz crystal is used to monitor the progress of evaporation. The naked particles react with ionic liquid ions faster than they react with each other to form a bulk product. As the ionic liquid is present in large quantities, formation of larger particles can be prevented by blocking the growth and by disturbing the film formation through flask rotation. The resulting reaction product can then be transferred anaerobically from the reaction flask for subsequent examination and experiments. To our knowledge, this described PVP method has never been applied to ionic liquids. Figure 2 shows a TEM (transmission electron microscopy) micrograph of copper particles in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate, [C4mim][PF6], as a typical result of metal evaporation experiments. These particles have been obtained by evaporating copper under thermodynamic equilibrium in high vacuum into the ionic liquid. The particles have a narrow size distribution of around 3 nm. Interestingly, the single copper


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particles form a quite regular assembly on the TEM grid. Frequently, areas which are strongly reminiscent of a 2D-ordered close-packing of spheres can be detected. However, the copper particles never come closer than 20 nm. This might be indicative of the formation of charged, structured ionic liquid regions around the particle that lead to repulsion of the single nanoparticles.

20 Å

Figure 2. Transmission electron microscopic micrograph of copper particles in [C4mim][PF6] synthesised via physical vapour deposition. It is not only possible to deposit the naked metal into an ionic liquid, but metal nanoparticles can also deposited onto materials dispersed in the ionic liquid. The formation of Cu/ZnO nanocomposites may serve as an example. First, ZnO nanorods were dispersed in [C4mim][NTf2] (1-butyl-3methylimidazolium bis{(trifluoromethyl)sulfonyl}amide and then copper was evaporated into this dispersion similar to the experiment described above. Astonishingly, TEM investigations (Figure 3) show only a few free standing copper particles in the solution. Most of the particles are found on the surface of the ZnO nanorods. Again, the size distribution is quite narrow, around 3-4 nm. We are currently exploring the nature of the Cu/ZnO interactions.

Figure 3. Transmission electron micrograph of a sample where copper was evaporated into a dispersion of ZnO nanorods in [C4mim][NTf2].


182 In summary, PVD is an extremely powerful way for synthesising neat compounds compared to conventional nanoparticle preparation in solution. Firstly, no other stabilising agents other than the ionic liquid itself are needed. Secondly, no other reactants, such as a reducing agent (or its reaction product), will be present in solution.


Microwave Synthesis From the perspective of microwave chemistry, one of the key advantages of ionic liquids is the presence of large ions with high polarisability and conductivity. Therefore, ionic liquids are good media for absorbing microwaves, leading to high heating rates which results in a high formation rate of nuclei (nanoparticles!) (9). Short reaction times ranging from seconds to a few minutes can be achieved. This gives ionic liquids an additional advantage over conventional solvents in the synthesis of inorganic nanomaterials. In contrast to the conventional high temperature reactions which are often employed for the synthesis of inorganic materials and which preclude the formation of metastable and low-temperature phases, the temperature impact can be carefully controlled via microwave irradiation – microwave synthesis at subambient temperatures can even be achieved. Control over particle size can be achieved by reaction temperature and time as well as reactant concentration and choice of the ionic liquid. For our experiments, we have use a CEM Discover microwave system. It has a circular single mode cavity which allows focussing the microwaves on the reactants in such a way that the sample is in a homogeneous, highly dense microwave field. In consequence, heating spreads uniformly through the sample which should lead to a narrow nanoparticle size distribution. Ionic liquids can even be designed to act as a metal reductant (10). In order to access nanostructured coin metals, and a set of different highly reducing ionic liquids was synthesised based on the choline cation and its derivatives in combination with methanoate or bis{(trifluoromethyl)sulfonyl}amide as the counter anion (Figure 4). Potentially, the hydroxyl group, as well as the methanoate anion, can act as reductant.



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Figure 4. Ionic liquids as reducing agents for coinage metal salts. 2-Hydroxyethylammonium methanoate proved to be the most powerful reducing agent amongst the investigated ionic liquids, as it is able to reduce copper, silver and gold salts to the metal. Figure 5 shows the result of heating a solution of copper(II) pentane-2,4-dionate in 2-hydroxyethylammonium methanoate for five minutes at 80 °C by microwave irradiation under argon. A brownish-yellow solution of a copper colloid is formed; as soon as the sample is exposed to air, the copper particles are oxidised, which can be monitored by the colour change to green.

Figure 5. Synthesis of a colloidal copper solution from copper(II) pentane-2,4dionate and 2-hydroxyethylammonium methanoate by microwave irradiation.


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Similar reaction procedures with silver(I) (Ag[NO3]) and gold(III) (K[AuCl4]) salt solutions resulted (for silver) in the formation of large monoliths with a high surface area (11) and (for gold) in the formation of films. Analysis of the reaction by-products showed that the methanoate anion acts as the reducing agent in all of these reactions. However, if 2-hydroxyethyl-N,N,Ntrimethylammonium bis{(trifluoromethyl)sulfonyl}amide is used as a reducing agent for silver and gold salts, the formation of small particles could be achieved. Alas, as can be grasped from the transmission electron micrographs, the particle size distribution in the case of silver is wide. For the more reactive gold, the formation of large particles which might result form the coalescence of smaller particles under the reaction conditions, is observed.

Figure 6. Silver (left) and gold (right) particles derived from microwave synthesis in 2-hydroxyethyl-N,N,N-trimethylammonium bis{(trifluoromethyl)sulfonyl}amide.

Ultrasound Synthesis Sonochemical synthesis can be an alternative means to the above mentioned synthetic methods. It has been used in the preparation of many materials such as metal, oxide, sulfide, and carbide nanoparticles, and has recently become popular in combination with ionic liquids as the reaction medium (12). However, for the sonochemical synthesis of ZnO nanostructures, so far only dilute aqueous solutions of ionic liquids were used, which did not use the ionic liquid as a solvent and reaction medium, but rather as a stabiliser and surface active substance (13). However, direct synthesis of ZnO nanorods from Zn(CH3COO)2.2H2O and NaOH in the neat ionic liquid [C4mim][NTf2] can be achieved without further use of organic solvents, water, surfactants or templates, by irradiating the reaction mixture with ultrasound in a conventional ultrasonic bath (USC200T, VWR International; 45 KHz and 60 W) for 12h (14). The selected area electron diffraction (SAED) pattern in Figure 7 (right) shows unambigiously the formation of crystalline, hexagonal ZnO. The TEM images (Figure 7) reveal that the sample is composed of crystalline nanorods of about 20 nm in diameter and 50-100 nm in length.


185 200 110 103 102 101 002 100


Figure 7. TEM images of ZnO powder samples (left and middle) and an SAED diffraction pattern of a particle (right).

The UV-Vis absorption spectrum of the ZnO nanorods shows an absorption peak at about 365 nm from which a band gap of 3.31 eV can be derived. This is smaller than that of the bulk material (15). The room temperature photoluminescence spectrum of the ZnO nanorods shows the typical strong green-yellow emission with the peak maximum at 563 nm. Dinitrogen adsorption-desorption measurements give a specific surface area of a typical ZnO nanorod sample of 49.93 m2 g-1. Nanostructured lanthanide(III) oxides can be easily synthesised by a similar reaction procedure from their (hydrous) ethanoates. Figure 8 shows a typical TEM micrograph of as-prepared Tb2O3 nanospindles. The particles are about 40 nm in diameter and 200-800 nm in length.

Figure 8. TEM image of Tb2O3 nanospindles.


186 Figure 9 shows an emission spectrum of the Tb2O3 nanospindles upon excitation with λex=395 nm. The narrow band can be assigned to the typical intraconfigurational f-f transitions (16). The most intense peak is located at 543 nm corresponding to the 5D4 → 7F5 transition. Due to the good colour purity of the obtained samples, this synthetic route may prove to be valuable for the easy and fast preparation of phosphors.

5

Intensity (a.u.)


5

D

7 4

F

D

7

4

F

Emission

5

6

5

D

7 4

F

4

5

450

500

550

600

D

7 4

F

3

650

W l th/ nm ( ) Wavelength

Figure 9. Photoluminescence spectrum of Tb2O3 nanospindles.

Green Aspects of Nanoparticle Synthesis in Ionic Liquids The three different techniques presented here are fast and efficient ways to synthesise nanomaterials from ionic liquids. All three techniques make use of the unique properties ionic liquids can offer as reaction media. As molten salts, they not only provide an excellent environment for the synthesis and stabilisation of nanoparticles (as mentioned before), but it is possible to work at ambient temperature under high vacuum due to the virtually non-existent vapour pressure of some ionic liquids. As salts composed of highly polarisable ions, ionic liquids couple in an excellent way to microwave radiation and finally, as chemically robust media of tuneable viscosity, sonochemistry is possible. In addition, it has been shown that ionic liquids can be designed in such a way that they serve not only as a reaction medium but also as a reaction partner. Altogether, it can be stated that nanoparticle syntheses comply with many of the twelve principles of Green Chemistry, as summarised by Anastas and Warner (4). The synthetic routes are atom and energy efficient: due to the fewer reaction components involved, they are much cleaner and less waste is produced. The ionic liquids presented here are indeed inherently safer materials for nanoparticle synthesis.


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Acknowledgements This research was supported by the Deutsche Forschungsgemeinschaft within the framework of the collaborative research program SFB 558 “Substrate-catalyst interaction in heterogeneous catalysis” and the European Research Starting Grant “EMIL – Exceptional Materials from Ionic Liquids”. AVM thanks the Fonds der Chemischen Industrie for a ChemiedozentenStipendium.


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Ionic Liquids: From Knowledge to Application

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