Polyol Synthesis of Magnetite Nanocrystals in a Thermostable Ionic

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Polyol Synthesis of Magnetite Nanocrystals in a Thermostable Ionic Liquid Durgesh Vinod Wagle, Adam J. Rondinone, Jonathan David Woodward, and Gary A. Baker Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01511 • Publication Date (Web): 14 Mar 2017 Downloaded from http://pubs.acs.org on March 19, 2017

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Crystal Growth & Design

Polyol Synthesis of Magnetite Nanocrystals in a Thermostable Ionic Liquid Durgesh V. Wagle,† Adam J. Rondinone,‡ Jonathan D. Woodward,*,§ and Gary A. Baker*,† †

Department of Chemistry, University of Missouri, Columbia, MO 65211, USA.

‡

Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN

37831-6493, USA. §

Nuclear Security & Isotope Technology Division, Oak Ridge National Laboratory, Oak Ridge,

TN 37831-6229, USA. * Authors to whom correspondence should be addressed.

ABSTRACT: We report on the development of a facile, one-pot synthesis of single-crystalline magnetite (Fe3O4) nanoparticles (NPs) based on the thermal decomposition of the non-toxic iron precursor iron(III)acetylacetonate within the ionic liquid trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide ([P6,6,6,14][Tf2N]) using 1,2-hexadecanediol as a polyol reducing agent in an “iono-polyol process”. In this expedient approach, the [P6,6,6,14][Tf2N] acts both as a low-volatility, thermostable solvent and as the colloid-stabilizing agent, eliminating the requirement for additional surface-capping agents. Performing the synthesis at 300 or 350 °C yielded quasi-spherical, monodispersed Fe3O4 NPs with a mean size of 14 nm. Evidence from thermogravimetry, X-ray fluorescence, and infrared analysis is consistent with nanocrystal coverage by a partial bilayer of [P6,6,6,14][Tf2N], accounting for the excellent dispersibility of the Fe3O4 NPs in solvents such as hexane, toluene, and methylene chloride. Time-dependent thermogravimetric analysis reveals that [P6,6,6,14][Tf2N] is transiently stable at 300 °C for 30 min (sufficient for nanocrystal formation), but rapidly degrades at 350 °C or higher. By employing a reaction temperature of 300 °C, the [P6,6,6,14][Tf2N] can be recycled and re-used multiple times for the subsequent preparation of Fe3O4 NPs with no ill effects in terms of particle size, uniformity, or agglomeration. Finally, we demonstrate that the Fe3O4 NPs can be dispersed into [P6,6,6,14][Tf2N] as a solventless carrier fluid to produce an “iono-ferrofluid” responsive to an external magnetic field.

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INTRODUCTION Iron oxide nanoparticles (NPs) have been the focus of significant scientific interest due to their characteristic physicochemical properties leading to numerous electronic and biomedical applications including magnetic switches,1 inks,2 data storage,3 targeted drug delivery,4,

5

magnetic resonance imaging (MRI) contrast agents,6 immunoassays,4 and tissue repair.4 Traditionally, the synthesis of iron oxide NPs has been achieved via co-precipitation of ferrous and ferric salts in basic aqueous solution.4,

7-10

Other approaches include synthesis within

constrained environments or cages (e.g., microemulsions, micelles, dendrimers, cyclodextrins),4, 11-17

hydrothermal and sol–gel techniques,4, 18-22 acoustic cavitation from sonolysis,23, 24 and the

thermal decomposition of organometallic precursors such as Fe(CO)5 and Fe(acac)3 (acac = acetylacetonate) in high-boiling organic solvents4,

20, 25, 26

to produce variously-shaped

nanostructures (e.g., spheres, cubes, rods) of uniform size. Additional surface capping and templating agents are generally required to achieve precise control over the resulting NP size and morphology, as well as for providing stabilization against aggregation.4,

7

However, the low

boiling temperatures of common organic solvents (for example, 80, 115, and 139 °C for benzene, pyridine, and m-xylene, respectively) limits the accessible temperatures at which the thermal decomposition of many organometallic precursors can be carried out.7 Additionally, it is difficult to recycle and reuse conventional organic solvents, leading to sustainability, exposure, and disposal concerns. These limitations have led to the search for thermally-stable and recyclable solvents capable of producing uniformly sized and shaped NPs. Ever since landmark reports detailing the first air- and water-stable room temperature ionic liquids (ILs) over two decades ago,27 these tailorable fluids have been considered as versatile and potentially more sustainable alternatives to conventional organic solvents.28,

29

ILs frequently

feature many attractive physicochemical properties, including high thermal stability, low vapor pressure, electrical conductivity, broad liquid range, and the flexibility of tailoring to specific tasks by changing their composition, making them ideal solvents in many respects.28, 30 Recently, ILs have earned recognition as media for the facile and rapid synthesis of monodisperse nanomaterials with minimal size-selection steps compared to conventional organic and aqueous approaches.31-35 Unlike organic solvents, ILs possess favorable solvation properties that arise from their high ionic strength,36 extended networks of hydrogen bonds,30 ability to form nanobiphasic hydrophilic and hydrophobic domains,37,

38

and self-organizing nature.39,


40

Together,

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these solvent features provide unique conditions for the formation of cage-like domains potentially suited to the soft confinement required for fabricating a wide variety of nanomaterials.2,

41

Accordingly, ILs have been employed to synthesize various nanoporous

42

materials, as well as nanoparticles,43 nanosheets,44 and nanorods.35, 45, 46 A number of recent reports have demonstrated size- and shape-controlled production of iron oxide nanomaterials in imidazolium ILs using additional surfactants, capping agents, and polymers,

such

as

oleic

acid,

oleylamine,

polyvinylpyrrolidinone

(PVP),

and

poly(ethylene)glycol.46-51 For example, Wang and Yang have reported the synthesis of monodisperse maghemite nanocubes, nanorods, and nanospheres in neat 1-butyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf2N]) using Fe(CO)5 in the presence of structure-directing and capping agents such as oleic acid and oleylamine48 whereas Lee et al. synthesized Fe2O3 nanorods and nanowires from Fe(CO)5 in an [omim][Tf2N]/DMF mixture (omim = 1-octyl-3-methylimidazolium, DMF = N,N′-dimethylformamide) in the absence of stabilizing surfactants.49 In another example, Hu et al. reported the microwave-assisted polyol synthesis of Fe3O4 NPs in a [bmim][BF4]/dibenzyl ether mixture using Fe(acac)3 as a precursor with stabilizing effects and size and shape control mediated by both the IL and oleic acid additives.52 Cao and Zhu reported that the IL employed in the microwave-assisted hydrothermal synthesis of hollow iron oxide nanospheres in aqueous [bmim][BF4] mixtures using Fe(NO3)3·9H2O and urea as precursors exerted a dramatic effect on the resulting crystalline phase and morphology of the product.53 Lian et al. reported the synthesis of mesoporous hollow microspheres, microcubes, and porous nanorods of α-Fe2O3 in neat [bmim][Cl].54 Finally, by using [bmim][Tf2N] in solvent-recyclable processes, Wang et al.55, Hu et al.52 and Oliveira et al.50 each demonstrated the production of iron oxide NPs, however, these researchers employed toxic precursors such as Fe(CO)5 or additional capping agents (e.g., oleic acid), or both. Currently, researchers are becoming increasingly keen on streamlining nanoscale synthesis using ILs in a more sustainable fashion, a recent example being work by Riche et al. involving the parallel synthesis of uniform Pt NPs using a network continuous flow platform.56 Here we describe the one-pot synthesis of single-crystalline Fe3O4 NPs based on an ionopolyol process involving the thermal decomposition of the low-toxicity iron source Fe(acac)3 in the IL trihexyltetradecylphosphonium bis(trifluoromethanesulfonyl)imide, [P6,6,6,14][Tf2N]. In this process, the [P6,6,6,14][Tf2N] acts as both the solvent proper as well as the templating agent, 3 ACS Paragon Plus Environment


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thus circumventing the need for supplemental surface capping agents or surfactants. The resulting Fe3O4 NPs are easily dispersed into aprotic, nonpolar solvents (e.g., hexane, toluene, methylene chloride, chloroform, diethyl ether) due to near bi-layer coverage of the nanoparticles by [P6,6,6,14][Tf2N] species. Furthermore, we demonstrate that [P6,6,6,14][Tf2N] is thermally stable for 30 min at 300 °C and can be recycled and reused for subsequent cycles to produce fairly monodisperse Fe3O4 NPs. Finally, the magnetite NPs were dispersed into [P6,6,6,14][Tf2N] as a carrier fluid to yield an “iono-ferrofluid” responsive to an external magnetic field, suggesting interesting magnetic switching, extraction, or microfluidic applications, among others.

EXPERIMENTAL SECTION Materials. Fe(acac)3 (97%, acac = acetylacetonate), 1,2-hexadecanediol (90%), and anhydrous hexane were purchased from Sigma-Aldrich and used as received. Isopropanol (HPLC grade) was purchased from Fluka. [P6,6,6,14][Tf2N] (>97%, CYPHOS IL 109; CAS No. 460092-03-9), was obtained from Cytec Canada Inc. Alternatively, [P6,6,6,14][Cl] (CYPHOS IL 101; CAS No. 258864-54-9) was converted to the [P6,6,6,14][Tf2N] by well-established ion-exchange methods.57 All reagents were used without further purification. Physical characterization techniques are detailed in the Supporting Information (SI). Methods. Initially 0.071 g (0.2 mmol) of Fe(acac)3, 0.26 g (1.0 mmol) of 1,2-hexadecanediol and 5 mL of [P6,6,6,14][Tf2N] were loaded into a 3-neck, round-bottom flask equipped with a gas adapter, stirring bar, and stainless-steel thermocouple immersed into the reaction medium. The mixture was constantly stirred throughout the reaction and heated to 100 °C under vacuum for 30 min followed by heating at 200 °C for 30 min under argon; this step-wise process is critical to producing monodisperse NPs.58 The reaction mixture was then heated to 300 °C (or 350 °C), for a 30 min period under an Ar atmosphere. The resulting viscous, dark-brown tar like mixture was gradually cooled to room temperature (with no specific control over the cooling rate) and then dispersed into 10 ml of hexane. The NPs were isolated and purified by performing three cycles of precipitation using isopropanol, followed by redispersion into hexane. Specifically, 30 mL of isopropanol was added to the NP suspension in hexane followed by centrifugation at 12,000 rpm for 5 min. The supernatant was carefully decanted and the NPs redispersed in hexane via bath sonication for 5 min. The supernatant was retained in order to recover and reuse the [P6,6,6,14][Tf2N]. 4 ACS Paragon Plus Environment

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In order to recycle the [P6,6,6,14][Tf2N] for reuse, the supernatant collected was recovered by rotary evaporation (60 °C, 150 rpm, 1 mm Hg) to remove the hexane and isopropanol phases, leaving behind yellowed [P6,6,6,14][Tf2N]. It is important to note that the recycled [P6,6,6,14][Tf2N] gave characteristic NMR and FTIR spectra which matched the original (unused and unheated) [P6,6,6,14][Tf2N]. This recovered [P6,6,6,14][Tf2N] was subsequently tested for Fe3O4 NP synthesis, with similar results obtained to that for fresh [P6,6,6,14][Tf2N].

RESULTS AND DISCUSSION The Fe3O4 NPs were synthesized based on a procedure adapted from Sun et al.,25,

58-60

in

which we replace the high-boiling organic solvent normally used with a thermostable ionic liquid. Specifically, our method entails a one-pot, in situ process involving the thermal decomposition of Fe(acac)3 using 1,2-hexadecanediol as a polyol reductant within the IL [P6,6,6,14][Tf2N] at 300 °C (or 350 °C) for 30 min. In this modified polyol method, the IL acts as both the solvent as well as the templating and capping agent, allowing the NPs to be synthesized and stabilized in the absence of additional surfactants such as oleic acid and oleylamine.46, 47 The obtained NPs are readily dispersed in hexane (as well as solvents such as toluene) with no appreciable precipitation over a period of several weeks, suggesting that the surfaces are capped and stabilized by hydrophobic species. A number of studies have reported the effect of varying experimental parameters (e.g., metal salt, concentration of reactants, templating stabilizers, solvent, reaction temperature, time), all of which critically affect the resulting NP diameter, shape, and size distribution.61-65 To this end, various reaction times (15–60 min) and temperatures (250–350 °C) were investigated, however, the highest quality Fe3O4 NPs were produced by reaction at 300 °C for 30 min. Reactions conducted at 250 °C produced ill-defined NPs and copious amounts of poorly crystalline, unidentified particulates that were difficult to image. At 350 °C, NPs similar to those produced at 300 °C were formed, but in significantly smaller quantities. In contrast to temperature, reaction time apparently exerted little influence over NP size and shape. Although the exact mechanism is poorly understood, magnetite NP formation can be rationalized from the polyol process reported by Fievet et al.66 and modified by Sun et al.25, 60 more recently. It is generally accepted that the 1,2-hexadecanediol partially reduces the Fe(III) cations to an Fe(II)-intermediate precursor that decomposes at high temperatures, generating 5 ACS Paragon Plus Environment


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“FeO” monomers or the minimum subunits of NPs.25,

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58, 60, 61

When the concentration is

saturated, these FeO monomers precipitate and homogeneous nucleation of seeds occurs, followed by growth, to eventually produce magnetite NPs. Well-separated nucleation and growth steps are assumed to give rise to uniformly sized NPs.61,

66-70

The 1,2-hexadecanediol, which

undergoes dehydration and oxidation in the polyol process, plays a multifunctional role as a solubilizer of organometallic reagents in the solvent, reducing agent, and templating ligand– especially in the absence of other stabilizing molecules such as oleic acid and oleylamine.71 In particular, increasing the relative concentration of 1,2-hexadecanediol has been shown to cause a decrease in average NP diameter, increasing size distribution but having no significant effect on morphology.63 Transmission electron microscopy (TEM) images of the Fe3O4 NPs synthesized at 300 °C (Figure 1) reveal a quasi-spherical morphology with distinct lattice fringes (Figure 1 inset). The NPs are relatively monodisperse with an average diameter of 13.82±2.62 nm. No individual NPs smaller than 10 nm or larger than 20 nm were observed. Aggregation is minimal and the NPs typically form clusters containing a few to several dozen NPs on the TEM grid due to their ferrimagnetic nature. In addition to the well-defined population of Fe3O4 NPs, a population of small seeds is also evident in Figure 1. As we will show, the agreement between TEM- and XRD-determined NP sizes suggests that these “seed particles” are amorphous in nature and thus do not contribute to XRD peak broadening. Consistent with this, this phase does not represent magnetite and is thus not considered in the TEM size analysis. In fact, at this point, we cannot fully rule out the fact that they may even be partially organic (or organometallic) in nature. In any case, of more practical importance, it is noteworthy that this population of seed particles is not observed for TEM images of Fe3O4 NPs made from once- or multiply-recycled [P6,6,6,14][Tf2N], nor are they seen in samples purified additional times. A final observation is that these small particles do not form chains or aggregates but remain isolated on the TEM grid, revealing that they are not ferrimagnetic in nature. At this point, the presence of these ultrasmall seed particles remains an unresolved issue requiring additional study. The rather narrow size distribution of the NPs can be explained by a homogenous nucleation model with effective separation between the nucleation and growth steps.61, 66, 69 Well-defined spherical or cubic NP morphologies are often produced using oleic acid and oleylamine 58, 61, 72

25, 48, 55,

as surfactants and thus the lack of additional templating ligands likely accounts for the 6 ACS Paragon Plus Environment

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more irregular and dispersed morphology of the NPs reported in this work. The Fe3O4 NPs synthesized at 350 °C are similar in shape and size (quasi-spherical and ~15 nm in diameter) compared to those formed at 300 °C but are produced in significantly lower quantity (Figure S1 of the Supporting Information). Although increasing reaction temperature in the polyol process is expected to produce greater quantities of NPs (by increasing the rate of nucleation),66 in this case, compromised stability of the [P6,6,6,14][Tf2N] at this higher temperature may account for the lower yield.

Figure 1. A) Wide-view TEM image of Fe3O4 NPs synthesized at 300 °C for 30 min, showing clusters of a few dozen NPs. B) A slightly higher magnification TEM image reveals a maximum NP diameter of approximately 20 nm. The inset shows a high-resolution TEM image of a single, crystalline Fe3O4 NP. The room temperature X-ray diffraction (XRD) pattern of the Fe3O4 NPs synthesized at 300 °C, along with a Rietveld refinement (GSAS)73 and indexed peaks, is provided in Figure 2. The upper plot depicts the Rietveld fit (red line) overlaying the experimental data (black line). The lower plot is the residuals curve (blue line) indicating a reasonably good fit (RF2 = 0.0962, χ2 = 2.737 Rwp = 0.1345, Rexp = 0.0822) to the measured pattern. The sharp peaks indicate good crystallinity and Scherrer broadening is evident, consistent with the size of the NPs.74 All of the observed peaks are assigned on the basis of a cubic spinel structure (peaks at 18.4, 30.1, 35.5, 43.2, 53.5, 57.2 and 62.7° 2θ correspond to the [111], [220], [311], [400], [422], [511], and [440] crystal faces respectively) except for the first peak at 16.3° 2θ which is attributed to an unidentified impurity phase. Common phosphide, sulfide, phosphate and sulfate phases are not 7 ACS Paragon Plus Environment


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evident and do not match the unidentified peak. The Rietveld refinement confirmed the identity of the magnetite phase in the cubic space group Fd¯3 m with lattice constants a,b,c = 8.385(3) Å. The NP diameter calculated using a Williamson-Hall approach75 (GSAS, pseudo-voigt profile function 2, strain corrected) is 14.5 nm which is in excellent agreement with the diameter measured from TEM images, suggesting that the NPs are single-crystalline. In the XRD pattern of the NPs synthesized at 350 °C (Figure S2), the intensity of the peaks is lower and the background is noisier compared to the data in Figure 2. The Rietveld refinement confirmed the spinel structure (a,b,c = 8.399(2) Å) but converged with slightly worse statistics (RF2 = 0.4302, χ2 = 6.510, Rwp = 0.3624, Rexp = 0.1423). The calculated NP diameter was 15.8 nm, agreeing with the measured TEM diameter, again indicating that the NPs are single crystals. A second distinguishing feature of the XRD pattern from the NPs formed at 350 °C sample is the lack of the unidentified peak at 16° 2θ that was observed in the 300 °C sample. Tentatively, this suggests that the potential impurity phase may anneal away or be decomposed at higher temperature.

Figure 2. XRD data (black) plus Rietveld refinement (red) and residuals (blue) for Fe3O4 NPs synthesized at 300 °C for 30 min. Miller indices for magnetite are labeled above each peak. Note that the peak at 16.3° 2θ, indicated by the green asterisk and arrow, is attributed to an impurity or additional phase. To evaluate the suitability of [P6,6,6,14][Tf2N] as an alternative medium (to high-boiling organic solvents) for synthesizing NPs, the stability of neat [P6,6,6,14][Tf2N] as a function of 8 ACS Paragon Plus Environment

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temperature and heating time was investigated by thermogravimetric analysis (TGA) and proton nuclear magnetic resonance (1H NMR) spectroscopy. TGA plots depicting the weight loss of [P6,6,6,14][Tf2N] from 200–400 °C under inert gas (nitrogen) are shown in Figure 3. Images of unheated [P6,6,6,14][Tf2N] alongside [P6,6,6,14][Tf2N] heated for 30 min at 200 °C followed by 30 min heating periods at 300, 350, and 400 °C (Figure S3A) are given in the Supporting Information. The corresponding results for 1H NMR analysis of these same samples are provided in Figure S4. The TGA measurements clearly demonstrate that [P6,6,6,14][Tf2N] is stable at lower temperatures. Only a miniscule weight loss is observed after 2 h of heating at 250 °C. When heated at 300 °C, although somewhat discolored within 30 min, [P6,6,6,14][Tf2N] loses less than 5% of its initial mass within 30 min and suffers just a 10% mass loss after 60 min, indicating that the majority of the IL remains intact during the timeframe of a typical NP synthesis. In fact, the linear nature of the TGA trace at 300 °C is consistent with some (or the majority) of this mass loss originating from evaporation of the IL rather than decomposition.76 Consistent with this supposition, the 1H NMR spectrum of [P6,6,6,14][Tf2N] heated at 300 °C is essentially identical to the spectrum of the unheated sample; the aliphatic methyl and methylene peaks from 0.75–1.75 ppm are unchanged. This slight discrepancy between TGA and 1H NMR results is likely due to slight volatility of the [P6,6,6,14][Tf2N] at 300 °C or minor loss of volatile decomposition products which evolve during the course of heating and are thus absent during NMR analysis. At higher temperatures, however, [P6,6,6,14][Tf2N] decomposes rapidly, as shown by both TGA and NMR analysis. At 350 °C, [P6,6,6,14][Tf2N] becomes only slightly darker compared to the IL heated at 300 °C but loses 25% of its initial weight within 30 min and 70% after 60 min. In line with previous reports,77 at 400 °C, [P6,6,6,14][Tf2N] loses 30% of its initial mass within 20 min before reaching the destination temperature, with essentially complete decomposition by 30 min to render a blackened char. The nonlinear TGA traces at these higher temperatures is consistent with IL decomposition.76 In the NMR spectra, heating the [P6,6,6,14][Tf2N] at 350 and 400 °C causes the fine structure of the aliphatic methyl and methylene peaks to become less distinct with visible broadening; in particular, the resonance at 1.28 ppm shifts downfield slightly by 0.006 ppm. The rapid decomposition of the [P6,6,6,14][Tf2N] at 350 °C may account for the significantly fewer Fe3O4 NPs produced at that temperature (vide supra) compared to 300 °C where the solvent remains largely intact. We note that kinetic TGA scanning typically leads to severe overestimation in IL thermal stability, in some cases in excess of 150 °C.76 Based on these 9 ACS Paragon Plus Environment


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results, the [P6,6,6,14][Tf2N] considered here should be considered to possess only transient stability for exposure to temperatures exceeding a practical limit of 250–300 °C.

Figure 3. Isothermal TGA traces showing the mass loss associated with heating [P6,6,6,14][Tf2N] at a rate of 20 °C min–1 under nitrogen to the designated peak temperature and holding. The dashed vertical lines denote the respective times required for the sample to reach 200 °C (black) and 400 °C (orange) during the experiment. The dispersibility of the Fe3O4 NPs in hexane and toluene suggests the surfaces are capped with hydrophobic molecules, most likely [P6,6,6,14][Tf2N] species alongside oxidation products of the 1,2-hexadecanediol. Other more polar by-products, such as water and acac degradation species, will likely evaporate during the reaction or be removed during purification of the product.71 Elemental analyses of dried Fe3O4 NP samples were measured using energy-dispersive X-ray fluorescence (EDXRF) spectroscopy to investigate this assumption. EDXRF plots of Fe3O4 NPs synthesized at 300 and 350 °C are presented in Figures 4 and S5, respectively. In each of the plots, the Fe-Κα and Fe-Κβ lines between 6–7 keV dominate the spectra, while low intensity P-Κα, S-Κα, S-Κβ and Fe--Κα-escape peaks are observed between 2–5 keV. The relative elemental composition for the Fe3O4 NPs synthesized at 300 °C is 2.72 (±0.38) %P, 5.34 (±0.29) %S and 90.96 (±0.22) %Fe. For Fe3O4 NPs prepared at 350 °C the composition is 8.53 (±0.24) %P, 14.30 (±0.18) %S and 76.77 (±0.08) %Fe. These measurements, consistent with essentially a 2:1 sulfur-to-phosphorus (S:P) ratio, suggest that [P6,6,6,14][Tf2N] (or coupled decomposition products) are likely present on the NP surfaces, accounting for their lipophilicity. Since EDXRF 10 ACS Paragon Plus Environment

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cannot detect the presence of C and O, it is not possible to determine whether 1,2hexadecanediol oxidation products might also be present using this technique. For Fe3O4 NPs synthesized at 300 °C, the S:P ratio (1.96) is indeed very close to 2, consistent with intact ion pairs residing on the Fe3O4 NP surfaces. This speculation of [P6,6,6,14][Tf2N] stability under these conditions is supported by our TGA results discussed earlier. Although P and S are present in higher quantities for the NPs prepared at 350 °C, the S:P ratio is slightly lower (~1.67) suggesting that a combination of [P6,6,6,14][Tf2N] ion pairs, as well as decomposition products might be present on the Fe3O4 NP surfaces for this reaction temperature, in agreement with TGA and 1H NMR experiments suggesting partial stability of [P6,6,6,14][Tf2N] beyond 300 °C. It should also be noted that elemental ratios calculated from EDXRF spectra are not rigorously quantitative but only provide rough estimates regarding the nature of the chemistry present at the NP surface. The species capping the Fe3O4 NP surfaces were also examined using Fourier transform infrared (FTIR) spectroscopy. Figure S6 compares the FTIR spectra for fresh (unused and unheated) [P6,6,6,14][Tf2N] with those for Fe3O4 NPs made in fresh and recycled [P6,6,6,14][Tf2N] at 300 °C. For completeness, the FTIR spectrum measured for twice-recycled [P6,6,6,14][Tf2N] is also provided. Consistent with NMR results, the integrity of the recycled [P6,6,6,14][Tf2N] is completely maintained after two reaction cycles at 300 °C, an encouraging outcome for applying [P6,6,6,14][Tf2N] as a recyclable medium for other nanosynthetic routes. A number of lowintensity, broad peaks appear between 1000 and 1700 cm–1 for the Fe3O4 NPs, suggesting a mixture of organic components may be present on the particle surface. Peaks for the Fe3O4 NP samples principally arise from Fe–O stretches (3430, 1627, 1460, 1371, 1100 and 585 cm–1) and aliphatic C–H stretches (2959, 2924, 2875, 2855 cm–1) and deformation/scissoring (1467 cm–1) vibrations from the [P6,6,6,14]+ cation.78-81 In the fresh and recycled [P6,6,6,14][Tf2N] samples, the appearance of νaSO2 (1352 cm–1), νaCF3 (peak at 1196 cm–1, shoulder at 1227 cm–1), νsSO2 (1138 cm–1) and νaSNS (1059 cm–1) bands arise from the [Tf2N]– anion. Overall, the FTIR spectra for the Fe3O4 NPs made in fresh and recycled [P6,6,6,14][Tf2N] are consistent with capping by [P6,6,6,14]+ and [Tf2N]– species, however, minor contribution from 1,2-hexadecanediol degradation cannot be definitively ruled out. Given the low thermal stability of 1,2hexadecanediol, contribution of such by-product species is expected to be minor under the conditions of the Fe3O4 NP synthesis. Indeed, for heat-and-hold TGA analysis under nitrogen, 11 ACS Paragon Plus Environment


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the mass loss for 1,2-hexadecanediol is ~95% after 10 min at 200 °C (data not shown). Unfortunately, a more rigorous molecular elucidation based on NMR analysis eludes us due to the paramagnetic nature of the Fe3O4 NPs. Major decomposition products from the [P6,6,6,14][Tf2N] appear unlikely based on the S:P ratio determined by EDXRF. On the metal oxide surface, certain modes originating from the [Tf2N]– fingerprint region (1000 to 1400 cm–1) are shifted and broadened into a single band around 1100 cm–1.82 We note that the most prominent peak in the FTIR spectrum for the Fe3O4 samples at 585 cm–1 is assigned to the Fe–O vibrational band associated with the magnetite phase.83, 84 There is a notable difference between the FTIR spectra for [P6,6,6,14][Tf2N] and for the Fe3O4 NPs (Figure S7, inset of panel A) in the C–H alkane region from 2800–3000 cm–1. The peaks at 2959 and 2870 cm–1 correspond to the asymmetric and symmetric methyl stretches and the 2930 and 2858 cm–1 peaks are assigned to asymmetric and symmetric methylene stretches for the aliphatic chains.83 These peaks were slightly shifted to lower frequencies (2957, 2867, 2924, and 2854 cm–1, respectively) indicating a slightly more “ordered” or crystalline state of the aliphatic chains on the Fe3O4 NP surface.85 Based on our EDXRF and FTIR assignments and the demonstrated stability of [P6,6,6,14][Tf2N] under our reaction conditions, we propose that these aliphatic modes primarily originate from the [P6,6,6,14]+ cation. An estimate of the number of organic capping layers present on the Fe3O4 NP surface (ranging, for example, from sub-monolayer coverage to bi- or multi-layer configuration) provides some insight into the dispersibility of the [P6,6,6,14][Tf2N]-capped Fe3O4 NPs in nonpolar solvents. To determine the relative amount of organic capping material, TGA was performed on a dried sample of purified Fe3O4 NPs synthesized at 300 °C (Figure S8). This experiment reveals that ~20% of the total mass arises from organic species present on the NP surface (i.e., 80% residual weight at 600 °C). Using this result and the TEM-determined particle size (13.8 nm), combined with knowledge of the densities and molecular weights for [P6,6,6,14][Tf2N] and Fe3O4, allows for estimating the thickness (T) of the organic capping layer at 2.09 nm. Full details on these calculations are provided in the Supporting Information. We note that, for the purposes of simplicity and for reasons justified earlier, the capping layer is presumed to comprise solely [P6,6,6,14][Tf2N]. A van der Waals radius of 0.748 nm was determined from an optimized geometry for the [P6,6,6,14][Tf2N] ion pair calculated at the M06-2X/6-31++G(d,p) level in Gaussian09. From this value and the capping layer thickness T already determined, we are able 12 ACS Paragon Plus Environment

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to estimate the number of [P6,6,6,14][Tf2N] layers on the Fe3O4 NP surface to be ~1.4 layers, consistent with full but sub-bilayer coverage of the Fe3O4NP surface by [P6,6,6,14][Tf2N], accounting for their dispersibility in nonpolar solvents. This sub-bilayer arrangement might, for example, result from a partially inter-digitated packing arrangement of the long alkyl chains of the [P6,6,6,14]+ cation in a fashion shown schematically in Figure S14. For Fe3O4 NPs synthesized at 350 °C, the larger quantities of P and S detected by EDXRF may possibly suggest bilayer coverage, however, we did not perform a similar calculation due to the lower thermal stability of [P6,6,6,14][Tf2N] displayed at 350 °C in our experiments. The potential to recover and reuse the solvent provides a compelling reason for employing ILs for sustainable nanosynthesis. To demonstrate the feasibility for solvent recycle, [P6,6,6,14][Tf2N] used to make Fe3O4 NPs was recovered and then reused for two successive rounds of Fe3O4 NP synthesis. In this case, [P6,6,6,14][Tf2N] recovery simply involves the removal of the volatile purification solvents (i.e., hexane and isopropanol) by rotary evaporation, leaving behind the slightly-discolored [P6,6,6,14][Tf2N] immediately ready for use in subsequent Fe3O4 NPs synthesis. Figure 5 shows representative TEM images and corresponding size distribution histograms of Fe3O4 NPs synthesized at 300 °C for 30 min using fresh [P6,6,6,14][Tf2N], alongside results for once- and twice-recycled [P6,6,6,14][Tf2N]. Using recycled [P6,6,6,14][Tf2N], a quasispherical Fe3O4 NP morphology was observed similar to that seen using fresh [P6,6,6,14][Tf2N]. The mean NP diameter, calculated by fitting each histogram to a standard Gaussian distribution, increased from 13.8 to 14.8 nm, with a slight broadening of the size distribution when using recycled [P6,6,6,14][Tf2N] instead of fresh [P6,6,6,14][Tf2N]. Interestingly, however, for twicerecycled [P6,6,6,14][Tf2N], the mean diameter was 15.2 nm with a noticeably narrower size distribution. Whether this size-focusing effect is real remains an open question, however, these results clearly demonstrate that [P6,6,6,14][Tf2N] is a suitable solvent for recycling and reuse in nanoscale thermal reactions of this type, an outcome which bodes well for applying [P6,6,6,14][Tf2N] and related ILs in sustainable materials synthesis. The Fe3O4 NPs produced using recycled [P6,6,6,14][Tf2N] were readily dispersed in hexane and toluene (among other solvents) in the same manner as NPs produced using fresh [P6,6,6,14][Tf2N]. This is not surprising given that the TGA trace measured for Fe3O4 NPs produced from recycled [P6,6,6,14][Tf2N] also reveals an identical organic capping content of ~20% for these NPs (Figure S8). As shown in Figure S6, FTIR spectra for Fe3O4 NPs made from fresh versus once-recycled 13 ACS Paragon Plus Environment


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[P6,6,6,14][Tf2N] are essentially identical. The spectral profile for the asymmetric and symmetric aliphatic C–H stretching from 2800–3000 cm–1 remains unchanged, signifying that the [P6,6,6,14][Tf2N] packing onto the Fe3O4 NP surface is similar whether fresh or recycled [P6,6,6,14][Tf2N] is used. As an eventual replacement for high-boiling organic solvents, it is also significant that the more expensive IL be amenable to facile reuse multiple times. In this regard, the demonstration that [P6,6,6,14][Tf2N] can be recycled multiple times with no adverse effects in terms of the nanoscale product emphasizes the potential of highly thermostable ILs such as [P6,6,6,14][Tf2N] for cost-effective and sustainable materials synthesis. The small but monotonic increase in NP diameter with successive [P6,6,6,14][Tf2N] reuse might be explained by a seed-mediated growth mechanism25,

58, 86-88

wherein seeds generated

from the decomposition of the Fe(II)-intermediate precursor to an “FeO” monomer remain within the [P6,6,6,14][Tf2N] after synthesis to act as sites for nucleation and growth of Fe3O4 NPs in subsequent reactions. Two broad, poorly-defined peaks (or groupings of peaks) over the 30–80° 2θ region in the background-corrected XRD pattern for recycled [P6,6,6,14][Tf2N] presented in Figure 6 are tentatively assigned to such “FeO” seeds. These broad peaks occur in the proximity of several known ferrite phases, however, the experimental diffraction peaks were not sufficiently resolved to allow for phase assignment, likely because the crystallite size of the “FeO” seeds is below the useful range of powder diffraction using Cu radiation. Nevertheless, this assignment is reasonable given that [P6,6,6,14][Tf2N] heated at 300 °C for 30 min (in the absence of reagents) showed no defined peaks over the 30–80° 2θ range. Residual Fe(acac)3 can also be excluded because all prominent diffraction peaks from Fe(acac)3 (2θ/° = 10.60, 13.04, 16.86, 20.22, 21.29, 22.19, 22.69, 23.08, 23.65, and 25.16) were noticeably absent in an XRD pattern of [P6,6,6,14][Tf2N] containing Fe(acac)3 and 1,2-hexadecanediol heated to ≥150 °C for several minutes. Iron sulfates and phosphates can also be excluded by consideration of S:P ratios determined from the EDXRF analysis discussed earlier. Additionally, the presence of the residual “FeO” seeds is associated with significant visual darkening of the [P6,6,6,14][Tf2N] compared to once- and twice-thermally cycled [P6,6,6,14][Tf2N], as shown in panel B of Figure S3. In addition, as displayed in Figure S9, TGA plots of fresh and once-recycled [P6,6,6,14][Tf2N] plotted together exhibit a difference in residual weight remaining after complete [P6,6,6,14][Tf2N] decomposition (>450 °C) which we assign to the presence of ~1 wt% of residual “FeO” seeds in the recycled [P6,6,6,14][Tf2N]. In order to validate the baseline reliability during TGA scanning, 14 ACS Paragon Plus Environment

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we measured the thermal stability of fresh [P6,6,6,14][Tf2N] and compared it to once- and twicethermally cycled [P6,6,6,14][Tf2N] (that is, the first three samples shown in Figure S3B), where a thermal cycle consists of a 30 min heating period at 200 °C, followed by a 30 min dwell time at 300 °C. As shown in Figure 7, even when subjected to two successive thermal cycles, the TGA weight loss trace for [P6,6,6,14][Tf2N] closely resembles that of fresh (untreated) [P6,6,6,14][Tf2N], with the onset of decomposition occurring beyond 300 °C and complete degradation resulting before 450 °C. Once more, this further corroborates the outstanding thermostability of [P6,6,6,14][Tf2N] and suggests it will find use in the synthesis of other nanoscale materials.

Figure 4. EDXRF spectrum of Fe3O4 NPs synthesized in [P6,6,6,14][Tf2N] at 300 °C. The inset shows the P-Κα, S-Κα, and S-Κβ peaks from the IL present at the NP surfaces.

The FTIR spectra of fresh and twice-recycled [P6,6,6,14][Tf2N] (Figure S6) are indistinguishable within the error of the measurement. Thus, any potential bands due to residual “FeO” seeds within twice-recycled [P6,6,6,14][Tf2N] are not apparent. Any possible contribution from a Fe–O vibrational mode near 585 cm–1, if present, is likely overshadowed by more conspicuous vibrations occurring from [P6,6,6,14][Tf2N] in this region. Although [P6,6,6,14][Tf2N] becomes progressively more discolored through successive heating cycles, TGA and FTIR results indicate that the integrity of the fluid is completely maintained after two thermal cycles



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involving heating at 300 °C for 30 min, an excellent benchmark for a thermostable IL suitable for recycling and reuse. Dried samples of Fe3O4 NPs synthesized at 300 °C were easily redispersed in a number of organic solvents, including methylene chloride, chloroform, diethyl ether, toluene and hexane. The nanoscale Fe3O4 powders were partially soluble in ethyl acetate but were categorically insoluble in acetonitrile, acetone, isopropanol, ethanol, and methanol. The Fe3O4 NPs could also be

redispersed

within

[P6,6,6,14][Tf2N]

as

well

as

its

ammonium

analog

trihexyltetradecylammonium bis(trifluoromethylsulfonyl)imide ([N6,6,6,14][Tf2N]). This was accomplished by employing hexane as a co-solvent followed by subsequent removal of hexane under vacuum. Surprisingly, attempts to disperse Fe3O4 NPs within shorter-chained IL analogs such as tributylmethylammonium bis(trifluoromethylsulfonyl)imide ([N4,4,4,1][Tf2N]) failed, even when using the same hexane co-solvent approach. For a number of biotechnology applications, it is necessary to render Fe3O4 NPs colloidally stable in water. Attempts to transfer the Fe3O4 NPs from an organic phase to water using αcyclodextrin (α-CD) were completely unsuccessful (Figure S10). One possible explanation is that the packing of the [P6,6,6,14]+ on the NP surface does not allow for threading of the long aliphatic chains with α-CD molecules. In contrast, Fe3O4 NPs were rendered water-soluble by reaction with dopamine. It is possible that the presence of the benzene ring in dopamine promotes C–H···π interaction with the [P6,6,6,14][Tf2N].89 The more likely explanation, however, is ligand exchange, resulting in displacement of [P6,6,6,14][Tf2N] species from the NP surface by dopamine, which is capable of binding to the metal oxide surface via a catechol anchor. This ligand exchange process leads to efficient transfer of the Fe3O4 NPs from hexane to water. The phase transfer of the Fe3O4 NPs is easily observed by the transfer of brown color from hexane to the lower aqueous phase. After dispersing Fe3O4 NPs into [P6,6,6,14][Tf2N], we discovered that an “iono-ferrofluid” was created that was responsive to a permanent magnet. A traditional ferrofluid is a magneticallyresponsive dispersion of nanoscale iron oxide (hematite or magnetite) within an organic solvent (e.g., kerocene, motor oil) that exhibits super-paramagnetic behavior (i.e., magnetization is not retained upon removal of an external field).90 In the case of an iono-ferrofluid, the carrier solvent is replaced by an IL, in this case [P6,6,6,14][Tf2N]. Based on TGA analysis, the iono-ferrofluid we produced contained ~1.67 wt% Fe3O4 NPs (Figure S11). The “solventless” iono-ferrofluid 16 ACS Paragon Plus Environment

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exhibited actuation and moved in response to an externally-applied magnetic field, as illustrated in Figure 8. We note that the Fe3O4 NPs, initially homogenously distributed throughout the [P6,6,6,14][Tf2N], “drag” the solvent toward the permanent magnet. Although uncommon, ionoferrofluids have been reported previously using [bmim][Tf2N] as the carrier solvent.91 The magnetically-responsive properties of iono-ferrofluids hold potential as smart actuation materials for a number of applications, including theranostics92 and molecular separation or extraction.93 In a final set of experiments, to demonstrate its generality, we extended our iono-polyol process to the creation of the bimetallic cobalt ferrite (CoFe2O4). Using [P6,6,6,14][Tf2N] as solvent for dual acac metal precursors and a final heating stage of 300 °C for 30 min, we showed the successful preparation of CoFe2O4 NPs which displayed a mean diameter of ~8 nm (Figure S12). Similar to the case for Fe3O4 NPs, the CoFe2O4 NPs were readily dispersed in toluene and showed no signs of precipitation after 2 weeks. Overall, this experiment shows in particular the utility of [P6,6,6,14][Tf2N] as a robust solvent for performing thermal reactions (nanoscale and otherwise) as well as the universality of the iono-polyol process for the creation of nanoscale materials in general.



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Figure 5. Measured diameter histograms for Fe3O4 NP diameters synthesized in (A) fresh [P6,6,6,14][Tf2N], (B) once-recycled [P6,6,6,14][Tf2N], and (C) twice-recycled [P6,6,6,14][Tf2N]. Representative TEM images (all scale bars denote 50 nm) are inset in each panel with the corresponding average NP diameter shown. For size analysis, 300 NPs were counted for each sample.


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Figure 6. XRD patterns of fresh (unused, unheated) [P6,6,6,14][Tf2N] (black) and recycled [P6,6,6,14][Tf2N] recovered following Fe3O4 synthesis in the solvent (red). The latter sample tentatively contains residual “FeO” seeds.

Figure 7. Thermogravimetric analysis of [P6,6,6,14][Tf2N] for fresh (black), once thermally-cycled (red), and twice thermally-cycled (blue) samples. A thermal cycle consists of a 30 min heating period at 200 °C, followed by 30 min at 300 °C. These thermal conditions match those used for the Fe3O4 NP synthesis, minus the addition of Fe(acac)3 and 1,2-hexadecanediol. 19 ACS Paragon Plus Environment


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Figure 8. A time sequence of images showing the movement of an iono-ferrofluid droplet comprising Fe3O4 NPs dispersed in [P6,6,6,14][Tf2N] in response to an external magnetic field (cylindrical NdFeB magnet).

CONCLUSIONS A facile, single-pot synthesis of Fe3O4 NPs by the thermal decomposition of a non-toxic organometallic iron precursor (Fe(acac)3) within the phosphonium ionic liquid [P6,6,6,14][Tf2N], which acts as both solvent and templating agent, has been developed which avoids the use of additional stabilizing surfactants. In this process, a reaction temperature of 300 °C produced monodisperse Fe3O4 NPs with an average diameter of ~14 nm. The [P6,6,6,14][Tf2N] was shown to be abundantly thermally stable at 300 °C for 30 min while beginning to suffer more rapid decomposition for temperatures in excess of 350 °C. The purified NPs could be readily dispersed in hydrophobic solvents (e.g., hexane, toluene, chloroform), consistent with NP capping by [P6,6,6,14][Tf2N]. The Fe3O4 NPs can also be dispersed within [P6,6,6,14][Tf2N] to yield a magnetically-responsive “iono-ferrofluid”. Recovery and reuse of the [P6,6,6,14][Tf2N] from previous synthetic cycles gave NPs with similar sizes and uniformity, although there is some evidence for a size-focusing advantage with [P6,6,6,14][Tf2N] reuse. In particular, an improvement in uniformity was observed when using twice-recycled [P6,6,6,14][Tf2N], a phenomenon we tentatively attribute to the presence of residual “FeO” seeds within the [P6,6,6,14][Tf2N] which act as nucleation sites in subsequent reactions, a topic that merits further exploration and exploitation. The iono-polyol method described here is of general applicability, as illustrated by its successful extension to the preparation of ~8 nm CoFe2O4 NPs.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:


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Additional experimental details, supplemental XRD, HR-TEM, EDXRF, TGA, FTIR, and 1

H-NMR figures, photographs of [P6,6,6,14][Tf2N] after various thermal treatments,

calculations estimating the surface coverage of [P6,6,6,14][Tf2N] on Fe3O4 NPs surfaces (PDF)

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected].

ORCID Durgesh Wagle: 0000-0002-2522-0670 Adam Rondinone: 0000-0003-0020-4612 Jonathan Woodward: 0000-0001-6159-7574 Gary Baker: 0000-0002-3052-7730

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Wigner program at Oak Ridge National Laboratory and the Interdisciplinary Intercampus Funding Program (IDIC) program at the University of Missouri. X-ray diffraction and X-ray fluorescence measurements were conducted at the Center for Nanophase Materials Sciences, (CNMS) which is a DOE Office of Science user facility. The CYPHOS IL 109 phosphonium salt was a kind gift from Dr. Al Robertson of Cytec Canada Inc.

REFERENCES (1) Zhang, Y.; Liu, D.; Wang, X.; Song, S.; Zhang, H. Chem. Euro. J. 2011, 17, 920–924. (2) Sambucetti, C. J. IEEE Trans. Magn. 1980, 16, 364–367. (3) Sun, S. Adv. Mater. 2006, 18, 393–403. (4) Laurent, S.; Forge, D.; Port, M.; Roch, A.; Robic, C.; Vander Elst, L.; Muller, R. N. Chem. Rev. 2008, 108, 2064–2110. (5) Veiseh, O.; Gunn, J. W.; Zhang, M. Adv. Drug Deliv. 2010, 62, 284–304.



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For Table of Contents Use Only Polyol Synthesis of Magnetite Nanocrystals in a Thermostable Ionic Liquid Durgesh V. Wagle,† Adam J. Rondinone,‡ Jonathan D. Woodward,*,§ and Gary A. Baker*,†

We report on a template-free, single-pot route to single-crystalline magnetite nanocrystals using a phosphonium ionic liquid as solvent using iron(III) acetylacetonate as a non-toxic iron source coupled with a polyol reductant.


Polyol Synthesis of Magnetite Nanocrystals in a Thermostable Ionic

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