Małgorzata
Swadźba-Kwaśny
*,
Léa
Chancelier
,
Shieling
Ng
,
Haresh G.
Manyar
,
Christopher
Hardacre
and
Peter
Nockemann
*
QUILL/School of Chemistry and Chemical Engineering, The Queen's University of Belfast, Belfast, BT9 5AG, United Kingdom. E-mail: m.swadzba-kwasny@qub.ac.uk; p.nockemann@qub.ac.uk
First published on 18th November 2011
Two stable nanofluids comprising of mixed valent copper(I,II) oxide clusters (<1 nm) suspended in 1-butyl-3-methylimidazolium acetate, [C4mim][OAc], and copper(II) oxide nanoparticles (<50 nm) suspended in trioctyl(dodecyl)phosphonium acetate, [P8 8 8 12][OAc], were synthesised in a facile one-pot reaction from solutions of copper(II) acetate hydrate in the corresponding ionic liquids. Formation of the nanostructures was studied using 13C NMR spectroscopy and differential scanning calorimetry (DSC). From a solution of Cu(OAc)2 in 1-ethyl-3-methylimidazolium acetate, [C2mim][OAc], crystals were obtained that revealed the structure of [C2mim][Cu3(OAc)5(OH)2(H2O)]·H2O, indicating the formation of copper hydroxo-clusters in the course of the reaction. Synthesised nanostructures were studied using transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Physical properties of the prepared IL-nanofluids were examined using IR and UV-VIS spectroscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and densitometry.
Ionic liquids (ILs) are compounds comprised exclusively of ions, usually a bulky organic cation and an organic or inorganic anion.9 Because of their entirely ionic nature, they commonly have a very large liquidus range and an extremely low vapour pressure. Since there are millions of possible anion/cation combinations,10 the properties of ionic liquids can be tailored, whereupon they are also referred to as ‘designer solvents’.11
Ionic liquids have been widely applied as electrolytes, due to their high ionic conductivity and a large electrochemical window,12 and as solvents and/or catalysts, owing to their capability to dissolve a range of solutes, especially catalytic complexes.13 More recently, they have attracted significant interest from material scientists, as promising thermal fluids,14 lubricants15 and components of novel composite materials: luminescent ionogels,16 highly conducting elastic polymers17 or so-called buckygels.18 Already known to be useful solvents for inorganic synthesis,19 ionic liquids have gained an interest as ionothermal solvents20 and templating agents for preparation of inorganic nanomaterials.21 Since nanoparticles in solutions are only kinetically stable, they have to be stabilised against aggregation, which can be achieved by electrostatic or steric protection.22 Ionic liquids, initially applied to take advantage of their high thermal stability and ability to dissolve inorganic materials,23 were soon found to act as capping agents24 and to stabilise nanoparticles against aggregation.25
The combination of the properties of ionic liquids and nanoparticles in the form of IL-nanofluids impart certain additional or optimised properties to ionic liquids. For example, the use of colloidal self-assembly of inorganic nanomaterials in ionic liquids for dye-sensitised solar cells (DSSCs) has been reported.26 Lunstroot et al. described luminescent dispersions of lanthanide-doped LaF3:Ln3+ (Ln = Eu, Nd) nanocrystals in ionic liquids.5 Watanabe et al. demonstrated the long-term colloidal stability of ionic liquid-based suspensions of PMMA-grafted silica particles.27 Photoluminescence of stable CdTe semiconductor nanoparticle dispersions in ionic liquids was accomplished by Nakashima et al.,28 while magnetorheological fluids with dispersions of magnetic Fe2O3 nanoparticles in ionic liquids have been reported by Schubert and co-workers.2 Recently, the synthesis of stable dispersions of Fe3C29 and NiO30nanoparticles in ionic liquids have been reported, with the ionic liquid acting as templating and stabilising agent. An enhanced electrical conductivity was achieved by introducing copper metal nanoparticles into an imidazolium ionic liquid,31 whereas an enhancement in thermal conductivity was gained by dispersing carbon nanotubes in the ionic liquids.32
Metal oxide nanoparticles have numerous applications33 and nanosized CuO in particular has applications as material in solar cells, in gas sensing, as magnetic storage media,34 in nanodevices,35 for heterogeneous catalysis,36 and as a superconductor.37 Synthetic routes to CuO nanostructures, with ionic liquids as templating agents, were explored by Taubert and co-workers38 and Liu et al.,39 who used ionic liquid-assisted aqueous methods. Furthermore, Mudring and co-workers40 prepared CuO nanorods in 1-butyl-3-methylimidazolium bistriflamide, with the aid of a strong base and sonication.
The purpose of this study was to produce stable IL-nanofluids in a simple, inexpensive and reproducible manner. We aimed at a one-pot synthesis in a neat ionic liquid, with no additives except for the source of copper. Thus, highly stable dispersions of CuO nanoparticles or Cu2O clusters in ionic liquids based on acetate anion were prepared. Furthermore, the mechanism of their formation has been studied, and their physical properties were investigated.
The synthesis was initially investigated by carrying out the reactions in sealed DSC pans. Stock solutions of copper(II) acetate hydrate in [P8 8 8 12][OAc] and in [C4mim][OAc] were heated slowly (2 °C min−1) to 150 °C, then cooled rapidly to room temperature and the cycle was repeated. As shown in Fig. 1, for both solutions, exothermic reactions (negative peaks) took place only in the first cycles. The reactions were not reversible, and completed within the time of the first cycle and, therefore, in second cycles the heat flow vs. temperature remained linear, i.e. no exothermic effect was observed.
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Fig. 1 Thermal profiles of the DSC-monitored formation of IL-nanofluids based on [P8 8 8 12][OAc] (top) and [C4mim][OAc] (bottom). In both cases, first heating cycles show exothermic (negative) processes that are not reproduced in the second and subsequent heating cycles, indicating complete and irreversible reactions. |
Based on the DSC tests, a temperature of 120 °C was chosen for further reactions. This temperature enabled fast reaction rates, and at the same time prevented thermal decomposition of the acetate-based ionic liquids (for thermal properties of the used ionic liquids see Table 2).
Ultrasound treatment of the reaction mixture is known to decrease the size of the formed nanoparticles and reduce aggregation. Therefore, for each ionic liquid system, two modes of agitation were tested: stirring using a conventional magnetic stirring bar, and ultrasound agitation with a sonic probe.
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Fig. 2 XPS spectra for Cu 2p3/2 spectral lines of nanoparticles synthesised using magnetic stirring in: (a) [C4mim][OAc] and (b) [P8 8 8 12][OAc]. Peak 1 represents copper(I) and peaks 2–4 represent copper(II). |
Fig. 2a shows the peak fit of Cu 2p3/2 core level and its corresponding shake-up satellites for the sample based on [C4mim][OAc]. The dotted blue line represents the original data; peak fits of the individual components are shown using coloured solid lines, while their sum is shown as the black solid line. Cu 2p3/2 peak 1, positioned at the binding energy value of 932.5 eV (red line) is assigned to the Cu(I) state and peak 2 positioned at 933.1 eV (green line) is assigned to Cu(II) state.45 The additional peaks (3 and 4) around 940 eV in the Cu 2p3/2 peak region result from a shake up process due to the open 3d9 shell of Cu(II). The relative amount of Cu(II) to Cu(I) state present on the surface was [Cu2+]/[Cu+] = 2.4, obtained from the ratio of the sum of the areas of peaks 2–4 to that of peak 1.46
For the sample based on [P8 8 8 12][OAc]— Fig. 2b —the Cu 2p3/2 peak is positioned at the binding energy value 932.6 eV (green line) along with shake-up satellites around 940 eV indicating Cu(II) state.
XPS analysis strongly indicate, that a mixed valent copper(I,II) oxide has formed in [C4mim][OAc], while copper(II) oxide has been synthesised in [P8 8 8 12][OAc]. Regarding other possible copper-containing species, it has already been demonstrated by the DSC experiment (Fig. 1), that no residual copper(II) acetate has been left after the synthesis. In the XPS experiments, no evidence for the presence of copper hydroxide, copper acetate or a copper carbene complex was found, as the binding energy for the Cu 2p3/2 peak from copper acetate species would be seen at ca. 934.0 eV; copper hydroxide 934.5–935.0 eV, while that of a copper carbene complex species would be seen at ca. 936.0 eV.47
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Fig. 3 TEM images of copper(I,II) oxide clusters and copper(II) oxide nanoparticles synthesised in [C4mim][OAc], using magnetic stirring (a) and ultrasound (b), and in [P8 8 8 12][OAc], using magnetic stirring (c) and ultrasound (d). |
Fig. 3a and b show small droplets of [C4mim][OAc] with barely-visible mixed-oxidation state clusters of copper(I,II) oxide. Nanoparticles of copper(II) oxide prepared in [P8 8 8 12][OAc] were much larger, with diameters of ca. 50 nm, as shown in Fig. 3c and d. Surprisingly, the use of ultrasound did not have observable influence on the decrease of the nanoparticles size. This may indicate a very rapid formation of the particles, with no time for the cavitation bubbles to affect the process.
At room temperature, a solution of copper(II) acetate hydrate in [P8 8 8 12][OAc] was blue, with the typical absorption band of a hydrated copper(II) acetate species at 370 nm and 690 nm.48 Upon heating to about 60 °C, the colour changed to green (absorption bands at 370 nm, 420 nm and 700 nm), indicating a transition to a partly dehydrated, or possibly hydroxygenated, copper(II) species. Further heating of the solution to about 120 °C resulted in the formation of a dark brown liquid, with an absorption maximum at 850 nm, which suggests the formation of copper(II) oxide.
The solution of the copper(II) acetate hydrate in [C4mim][OAc] was also blue at room temperature (broad absorption band at 630 nm, see Fig. 4, curve 1).
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Fig. 4 Colour changes observed during the synthesis of Cu2O clusters in [C4mim][OAc], illustrated by photographs and corresponding UV-VIS spectra. |
Upon careful heating to about 70 °C, the solution turned green, with a broad band at about 700 nm (Fig. 4, curve 2). Further heating to 120 °C resulted in a deep red liquid, very different in appearance from the product obtained in [P8 8 8 12][OAc]. The UV-VIS spectrum exhibited an absorption maximum at 530 nm (Fig. 4, curve 3), indicating the formation of a colloid.
The UV-VIS studies indicate, that both processes go through a similar first step, involving a partial dehydration of the solution copper(II) species. For [P8 8 8 12][OAc], the second step appears to involve thermal decomposition of the coordination of the solution species (in accordance with the shoulder on the DSC scan, Fig. 1). At the same time, the second step in [C4mim][OAc] must include decomposition of the solution species, along with a partial reduction of the copper(II) to copper(I). The partially dehydrated intermediate species (green solution) appeared to be unstable, and the green colour was only observed for a few minutes, after very slow and careful heating, with quick subsequent transformation to the final product. Therefore, the formation of the partially dehydrated species is most likely the rate-limiting step, followed by a rapid decomposition. This is in agreement with a single peak found in the DSC scan (Fig. 1).
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Fig. 5 13C NMR spectra (100.6 MHz, neat liquid, DSMO as reference) of [C4mim][OAc] at 27 °C (a), as well as a copper(II) acetate hydrate solution in [C4mim][OAc], at: 27 °C (b), 60 °C (c), 120 °C (d), and subsequently cooled down to 27 °C (e). The insert shows the magnified part of the spectrum (e), with the carbene signals clearly visible. |
13C NMR spectrum of neat [C4mim][OAc] shows the pure ionic liquid (Fig. 5a), with the 13C NMR signal from the (CO) carbon at δ = 174 ppm. This signal is not observed in the spectra of the copper(II) acetate hydrate solution in [C4mim][OAc] at 27 °C (blue) and 60 °C (green), because all acetate anions in solution are in a dynamic equilibrium with those complexed to the paramagnetic copper(II). Upon further heating, the solution species of copper(II) decompose to yield copper(I,II) oxide (red colloid), and the acetate anions are no longer coordinating to copper. Therefore, at 120 °C the acetate signal is visible again (Fig. 5d); it is also visible in the ambient temperature spectrum of the prepared IL-nanofluid (Fig. 5e), showing that an irreversible reaction took place. The analogous observations regarding the signal from the (C
O) carbon were made for the acetate signals in the 13C NMR spectra of the [P8 8 8 12][OAc]-based mixtures.
Regarding the 13C NMR signals from the cations of the respective ionic liquids, it was found that the [P8 8 8 12]+ cation remained unaffected throughout the reaction, whereas for the IL-nanofluid based on [C4mim][OAc], a new set of low-intensity signals appeared after the reaction (see Fig. 5d, e, and the insert). This set of peaks, corresponding to a new species based on the 1-butyl-3-methylimidazolium cation, was firstly observed at 120 °C (i.e. after the final step of the reaction), and subsequently also in the spectrum of the IL-nanofluid at 27 °C. The peaks of the imidazolium ring are shifted upfield, which indicates increased electron density on the ring, and thereby a possible carbene formation, which has been reported earlier to easily occur for imidazolium salts in the presence of copper(II) under basic conditions.49
Upon heating the highly concentrated (1.8 M) solution of copper(II)acetate hydrate in [C2mim][OAc] to about 70 °C, the solution colour changed gradually from blue to green. Subsequent slow cooling to room temperature resulted in the crystallisation of small, transparent, green crystals. Single crystal X-ray structure determination of this crystalline precipitate revealed the structure of the compound [C2mim][Cu3(OAc)5(H2O)(OH)2]·H2O (see Table 1). This structure consists of [C2mim]+ cations and an anionic copper oxy-hydroxy strand (Fig. 6a), consisting of triangles of copper atoms bridged by μ3-OH. Each of those Cu triangles is connected to two adjacent μ3-OH bridged Cu triangles, via an edge on the one side and via the corner on the other (Fig. 6b). The three crystallographically independent copper atoms are also connected by acetate anions in bridging and chelating-bridging mode, as illustrated in Fig. 6c—leading to an approximately hexagonal Cu3O3 motif. The coordination spheres of the three copper atoms can roughly be described as CuO6 octahedra, however with a Jahn–Teller distortion with Cu–O distances ranging for Cu1 from 1.948 to 2.522 Å; for Cu2 from 1.944 to 2.788 Å, and for Cu3 from 1.946 to 2.557 Å. The copper–copper distances are rather short, with Cu1–Cu2 = 3.172(10) Å, Cu2–Cu3 = 3.0022(9) Å and Cu3–Cu1 = 3.166(10) Å.
Empirical formula | C32H60Cu6N4O28 |
Formula weight | 1330.08 |
T/K | 100(2) |
Wavelength | Mo-Kα0.71073 |
Crystal system | Monoclinic |
Space group | Cc |
a/Å | 10.5009(3) |
b/Å | 21.7757(6) |
c/Å | 10.8067(3) |
α (°) | 90.00 |
β (°) | 93.286(3) |
γ (°) | 90.00 |
volume/Å−3 | 2467.05(12) |
Z | 2 |
D c (mg m−3) | 1.791 |
Absorption coefficient μ/mm−1 | 2.633 |
F(000); no. of parameters | 1356; 349 |
Crystal size/mm | 0.20 × 0.20 × 0.20 |
θ range (°) | 5.88, 52.74 |
Flack parameter | 0.085(17) |
Index ranges | −12≤h≤13, −27≤k≤27, −13≤l≤12 |
Independent reflections, Rint | 4284, Rint = 0.0487 |
Goodness-of-fit (F2) | 1.054 |
Final R indices I > 2σ(I) | R = 0.0373, wR = 0.0900 |
R indices (all data) | R = 0.0454, wR = 0.0947 |
R int | R int = 0.0487 |
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Fig. 6 Crystal structure of [C2mim][Cu3(OAc)5(H2O)(OH)2]·H2O: (a) cationic and anionic strand; (b) connectivity and distances along the copper strands; (c) schematic drawing of the structure. |
The coordinating water molecules were hydrogen bonding to the acetate anions, with O–H⋯O distances ranging from 1.735 Å to 1.815 Å. Further hydrogen bonding was found for the acidic hydrogen of the imidazolium cation to one of the oxygen atoms of a coordinating acetate anion, with an O–H⋯O distance of 2.245(4) Å. In the packing of the structure the distance of the copper strands (measured for the shortest distance of the centroids of the copper triangles of adjacent strands) is about 10.5 Å.
Only a few copper(II) hydroxy salts with the hexagonal Cu3O3 motif have been reported to date, most of them related to the solid state structure of compounds such as the mineral botalallackite, Cu2(OH)3Cl, or the compound Cu2(OH)3(NO3), which are all based on Cu2(OH)3X layers of hexagonally packed copper atoms. Furthermore, the crystal structure of a basic copper(II) acetate, Cu2(OH)3(CH3COO)·H2O, reported by Masciocchi et al.,50 had a similar sheet structure. Apparently, in the presence of the [C2mim]+ cation, the formation of an anionic strand in [C2mim][Cu3(OAc)5(H2O)(OH)2]·H2O is favoured. The ionic nature of this compound is probably facilitating its crystallisation, in contrast to the mentioned basic copper(II) acetate that precipitates as a microcrystalline powder.50
The postulated reaction mechanism follows eqn (1), starting from the binuclear copper(II) acetate.
Cu2(OAc)4·2H2O + 2 [P8 8 8 12][OAc] → [P8 8 8 12]2[Cu2(OAc)4(OH)2] + 2 HOAc → 2 CuO + 2 [P8 8 8 12][OAc] + 4 HOAc | (1) |
In the course of the reaction, the acetate anions in the ionic liquid remain in a dynamic equilibrium with those coordinating to copper atoms, and are thus invisible on the 13C NMR spectra (see Fig. 5b and c). After the reaction, the solubilised acetic acid is dissociated in ionic liquid, and all the acetate groups are equal by 13C NMR spectroscopy (Fig. 5d and e). The ionic liquid here has multiple roles of the base (acetate) donor, stabiliser of the proposed ionic intermediate, [Cu2(OAc)4(OH)2]2−, and stabiliser of the nanoparticles in the formed IL-nanofluid.
The first stage of the formation of copper(I,II) oxide clusters in [C4mim][OAc] resembles the formation of copper(II) oxide. As shown in Fig. 4, upon heating, the solution of hydrated copper species (blue) is partially dehydrated (green), and forms copper oxo-hydroxy species. This was supported by UV-VIS spectroscopy, and by the crystallisation of the compound [C2mim][Cu3(OAc)5(H2O)(OH)2]·H2O from its analogue, green [C2mim][OAc]-based precursor solution (Fig. 6). Of course, it is worth noting, that the solution species cannot be identical with this polymeric structure, and the solid-state structures may be merely indications for the structure of the solution species.52 This rate-limiting step was followed by decomposition of the complex, combined with a reduction of copper(II) to copper(I), to yield the red IL-nanofluid containing Cu2O clusters.
Since this second step only took place in the imidazolium-based ionic liquid, and not in the phosphonium-based one, it can be concluded that the [C4mim]+ cation is actively involved in the reduction process. Considering the basicity of the acetate solutions and the acidity of the proton on the imidazolium ring, this step may involve the removal of acetic acid from the complex presented in Fig. 6c, and formation of a 1,3-dialkylimidazolium carbene. A similar mechanism has been found for several other transition metals in the presence of imidazolium and a base.53 It may be postulated, that decomposition of such a carbene complex could result in the partial reduction of copper(II) to copper(I), which is supported by the presence of carbene signals in the 13C NMR spectra after the reaction (Fig. 5d and e).
glass transition/°C | ||||
---|---|---|---|---|
system | glass → liquid | liquid → glass | T d/°C | H2O content/wt % |
[C4mim][OAc] | 60.18 | 60.11 | 208.71 | 0.0978 |
[C4mim][OAc]-CuxO (st) | 63.01 | 62.18 | 200.03 | 0.0102 |
[C4mim][OAc]-CuxO (us) | 63.90 | 66.28 | 199.86 | 0.0119 |
[P8 8 8 12][OAc] | 75.74 | 74.32 | 274.74 | 0.0248 |
[P8 8 8 12][OAc] - CuO (st) | 80.09 | 79.14 | 272.68 | 0.2536 |
[P8 8 8 12][OAc] - CuO (us) | 79.64 | 81.31 | 268.16 | 0.2249 |
Presence of the copper oxide nanostructures affected slightly both glass transition temperatures and decomposition temperatures. However, this effect is too small to have any practical significance. The materials incorporating the imidazolium-based anions have lower decomposition temperatures, as the acetate anion can easily remove the acidic proton from the imidazolium ring.
Density. Densities of neat ionic liquids and corresponding IL-nanofluids were measured in a range of temperatures (see Table 3 and Fig. 7).
ρ /g cm−3 | |||
---|---|---|---|
T/°C | [C4mim][OAc] | + CuxO (st) | + CuxO (us) |
25 | 1.0651 | 1.0670 | 1.0667 |
35 | 1.0588 | 1.0603 | 1.0603 |
45 | 1.0526 | 1.0538 | 1.0539 |
55 | 1.0465 | 1.0475 | 1.0476 |
65 | 1.0406 | 1.0414 | 1.0414 |
75 | 1.0346 | 1.0353 | 1.0352 |
85 | 1.0288 | 1.0292 | 1.0291 |
[P8 8 8 12][OAc] | + CuO (st) | + CuO (us) | |
25 | 0.8897 | 0.9118 | 0.9076 |
35 | 0.8836 | 0.9054 | 0.9014 |
45 | 0.8775 | 0.8991 | 0.8952 |
55 | 0.8714 | 0.8927 | 0.8889 |
65 | 0.8653 | 0.8862 | 0.8827 |
75 | 0.8593 | 0.8797 | 0.8764 |
85 | 0.8533 | 0.8733 | 0.8703 |
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Fig. 7 Experimental densities as a function of temperature, plotted for [P8 8 8 12][OAc], ×, two IL-nanofluids prepared based on this ionic liquid using magnetic stirring, ●, and ultrasound, ◇, as well as for pure [C4mim][OAc], ■, and two IL-nanofluids prepared based on this ionic liquid using magnetic stirring, ○, and ultrasound, ▲. |
Interestingly, densities of the IL-nanofluids based on [C4mim][OAc] were almost identical with the neat ionic liquid, despite the addition of copper(II) acetate–probably due to the small size of the clusters.
Densities of the IL-nanofluids based on [P8 8 8 12][OAc] were, as expected, higher than the density of the neat ionic liquid. This may be explained by the presence of relatively large (ca. 50 nm) nanoparticles of copper(II) oxide, of a density obviously higher than the ionic liquid itself. Furthermore, density of the IL-nanofluid produced using ultrasound was found to be slightly higher than the density of the IL-nanofluid synthesised using magnetic stirring. This may suggest that the ultrasound treatment had some effect on the morphology and/or size of the nanoparticles, albeit it was too subtle to be observable by TEM.
The experimental densities, ρ/g cm−3, were correlated with temperature, T/°C using a linear regression, according to eqn (2).
ρ = aT + b | (2) |
The correlation parameters for are shown in Table 4.
system | a | b | R 2 |
---|---|---|---|
[C4mim][OAc] | −0.00060 | 1.07997 | 0.99982 |
[C4mim][OAc]-CuO st | −0.00063 | 1.08232 | 0.99966 |
[C4mim][OAc]-CuO us | −0.00063 | 1.08222 | 0.99992 |
[P88812][OAc] | −0.00061 | 0.90485 | 0.99999 |
[P88812][OAc]-CuO st | −0.00064 | 0.92792 | 0.99999 |
[P88812][OAc]-CuO us | −0.00062 | 0.92321 | 0.99999 |
IL-nanofluids based on [P8 8 8 12][OAc] seemed to be unaffected by atmospheric conditions, while those incorporating [C4mim][OAc] changed colour to green after prolonged (overnight) exposure to the open air, which indicates the reoxidation of Cu(I) to Cu(II).
Footnote |
† CCDC reference number 839595. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c1dt11578b |
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