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Ionic Liquid Anion Controlled Nanoscale Gold Morphology Grown at a Liquid Interface Nakara Bhawawet, Jeremy B Essner, Durgesh Vinod Wagle, and Gary A. Baker Langmuir, Just Accepted Manuscript • Publication Date (Web): 23 May 2017 Downloaded from http://pubs.acs.org on May 31, 2017

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Ionic Liquid Anion Controlled Nanoscale Gold Morphology Grown at a Liquid Interface Nakara Bhawawet, Jeremy B. Essner, Durgesh V. Wagle, and Gary A. Baker*

Department of Chemistry, University of Missouri-Columbia, Columbia, MO 65211 *Email: [email protected]

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ABSTRACT: Two different ionic liquids comprising the tetrabutylphosphonium cation ([P4444]) paired with the strongly-coordinating anions 6-aminocaproate ([6-AC]) or taurinate ([tau]) were prepared and employed in an aqueous/organic liquid bilayer system to generate nanoscale gold by Au(OH)4– photoreduction. Generally, as the concentration of ionic liquid in the organic phase was increased, the resulting quasi-spherical gold nanoparticles reduced in size and presented less aggregation, leading to marked increases in the catalytic efficiency for 4-nitrophenol reduction using borohydride. The diffusion of the ionic liquids across the liquid/liquid interface was also investigated, revealing partition coefficients of 6.0 and 7.6 for [P4444][6-AC] and [P4444][tau], respectively. Control studies elucidated that biphasic interfacial reduction was necessary to achieve stable nanoparticles possessing high catalytic activity. When the ionic liquid anion was instead replaced by the weakly-coordinating bis(trifluoromethylsulfonyl)imide ([Tf2N]), photoreduction of Au(OH)4– led to holey, wavy gold nanowires instead of spherical nanoparticles, indicating the dramatic morphological control exerted by the coordination strength of the ionic liquid anion. This strategy is straightforward and simple and opens up a number of intriguing avenues for controllably preparing plasmonic colloids for a range of applications from catalysis to optical sensing.

KEYWORDS: Ionic liquids, gold nanoparticles, photoreduction, 4-nitrophenol reduction TOC graphic:

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INTRODUCTION Gold nanoparticles (AuNPs) have attracted broad interest in recent decades due to their fascinating properties, including unique optoelectronic behavior and high surface-tovolume ratios which lead to efficient targeting and detection systems.1-3 These properties, namely their intense optical absorption (derived from surface plasmon resonance) and scattering cross sections, can be exploited in a wide range of applications including catalysis, chemical sensors, biological detection, medicinal treatments, nanoelectronics, and optics.1, 3-7 Their optical and catalytic properties are highly dependent on and can be tailored by varying the AuNP size, morphology, and chemical surroundings (e.g., surface ligands, local dielectric).2, 3, 8 Therefore, precise control over the physical properties of AuNPs and an in-depth understanding of their formation and growth mechanisms remains important. The most popular approach for AuNP synthesis remains the classical Turkevich method which employs sodium citrate as a reducing and stabilizing agent to produce fairly monodisperse nanoparticles roughly 20 nm in diameter from aqueous HAuCl4.9 The Turkevich method for preparing citrate-stabilized AuNPs introduced many decades ago has more recently undergone a renaissance and been systematically refined to better control particle size for a wealth of potential applications.10-13 Despite these efforts, while this facile approach provides some control over AuNP size, it has limitations in terms of both size regime accessible and overall morphological control. With the goal of exerting greater control over particle morphology, numerous synthetic methods have been developed, such as those involving biphasic chemical, photochemical, electrochemical, or microwave-assisted reduction.14-22 In particular, the interface between two immiscible liquids has been shown to be a strong potential platform for the assembly

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of differently sized and shaped Au nanostructures23-28 due to the liquid/liquid interface possessing a “soft interface”, meaning that it lacks fixed nucleation sites.29 This desirable property might potentially allow capping agents to freely target certain nucleation sites, which can lead to different morphologies of nanoscale growth. For example, Soejima et al. recently reported the interfacial growth of holey Au nanowires through a biphasic aqueous–organic

photoreduction

that

employed

the

semi-organic

salt

tetrabutylammonium hexafluorophosphate (TBAPF6) for the directional growth and stabilization of the resultant nanowires.28 Inspired by work with TBAPF6, we propose that ionic liquids (ILs), which are low-melting semi-organic salts or salt mixtures composed entirely of ions,30 represent an extremely promising vector for the morphological control over Au nanostructures formed at liquid interfaces, due to the ability to broadly tailor these task-specific solvents.31 In addition, ILs possess remarkable solvent properties (e.g., low vapor pressure, broad liquidus range, intrinsic ionic conductivity, high thermal stability), opening up other opportunities.30,

32-35

Although some organic solvents share certain of these properties,

ILs have enormous flexibility in terms of the number of possible cation-anion combinations (including facility for incorporating functional ions), potentially allowing for unprecedented control over nanoscale growth.31 Regrettably, ILs have only been minimally explored for the preparation and stabilization of nanoparticles so,31,

36-41

in

spite of enormous potential, a vast knowledge gap exists in the mechanistic understanding of nanoscale growth and colloidal stabilization when utilizing these designer solvents. In this contribution, two tailor-made ILs, tetrabutylphosphonium 6-aminocaproate ([P4444][6-AC]) and tetrabutylphosphonium taurinate ([P4444][tau]) were explored within a

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biphasic aqueous–organic photoreduction system to develop a better understanding of the nanoscale growth processes of Au nanostructures that occur at the liquid/liquid interface when employing coordinating anions. Notably, 6-aminocaproate and taurinate anions each contain a free amine end plus a dissociated acid group capable of coordinating with metal ions and metal/metal oxide surfaces,42-45 making their selection interesting for elucidating anion-controlled nanoscale growth. As a benchmark for comparison, similar photoreduction was conducted using ILs comprising the weakly-coordinating anion bis(trifluoromethylsulfonyl)imide (Tf2N–) in anticipation that dissipative nanostructures (wavy Au nanowires) similar to those observed by Soejima et al.28 would be observed. A conceptual overview of this strategy is provided by the cartoon schematic shown in Scheme 1. The chemical structures of the various ILs tested in this approach are given in Figure S1 of the Supporting Information.

EXPERIMENTAL SECTION Materials and reagents Experiments were carried out using Ultrapure Millipore water (18.2 MΩ·cm). Acetonitrile (HPLC grade, ≥99.9%), 6-aminocaproic acid (≥99%), chloroform (ACS, ≥99.8%), deuterium oxide (99.9 atom % D), gold(III) chloride trihydrate (≥99.9% trace metals basis), methanol (HPLC grade, ≥99.9%), 4-nitrophenol (ReagentPlus®, ≥99%), sodium borohydride (99.99%, trace metals basis), sodium hydroxide (99.99%, trace metals basis), taurine (≥99%), and tetrabutylphosphonium hydroxide (40 wt % in water) were all purchased from Sigma-Aldrich (St. Louis, MO). Deuterochloroform (CDCl3, 99.8% atom % D) was acquired from Cambridge Isotope Laboratories, Inc. (Andover, MA). All chemicals were used as received.

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Characterization techniques A Bruker AVIII 500 MHz NMR spectrometer was used to characterize synthesized ILs. All UV-Vis spectra were recorded on a Cary 50 UV-Vis spectrophotometer using 1-cm disposable PMMA cuvettes. Transmission electron microscopy (TEM) studies were conducted on carbon coated copper grids (Ted Pella, Inc. 01814-F, support films, carbon type-B, 400 mesh copper grid) using a FEI Tecnai (F30 G2, Twin) microscope operated at a 300 keV accelerating electron voltage. Ionic liquid synthesis The ILs used in this investigation, tetrabutylphosphonium 6-aminocaproate ([P4444][6AC]) and tetrabutylphosphonium taurinate ([P4444][tau]), were synthesized via a one-pot neutralization. Briefly, an equimolar mixture of tetrabutylphosphonium hydroxide and 6aminocaproic acid (or taurine) were co-dissolved in water and stirred overnight at room temperature, followed by water removal under rotary evaporation. The final products were further dried under vacuum overnight on a Schlenk line at 60 °C to remove traces of moisture. All other ILs used were synthesized as reported earlier.34, 46, 47 Interfacial photoreduction A stock solution of 20 mM Au(OH)4– was first prepared by adding 4 mL of 1.0 M NaOH to 16 mL of an aqueous 25 mM HAuCl4 stock. Next, using a pipette, 1 mL of freshlyprepared Au(OH)4– was carefully layered atop 1 mL of neat IL or 1 mL of IL in chloroform (5, 10, 100, 500, or 1000 mM). The biphasic solutions were immediately irradiated vertically under a 450 W xenon arc lamp (SVX 1450, Müller) for 1 h. Upon completion of the photoreduction, the colorless chloroform layer was carefully withdrawn and discarded using a Pasteur pipet. The remaining aqueous layer was combined with 1 mL of acetonitrile and then centrifuged at 12,000 rpm for 5 min. The

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supernatant was discarded and the pellet was re-dispersed in an additional 1 mL of acetonitrile via manual shaking and mild sonication for ~30 s. The washing process was repeated twice. After the third and final washing step, the final pelletized AuNP sample was re-dispersed in 1.5 mL of water. For comparison with ILs containing weaklycoordinating anions, in keeping with studies by Soejima et al.,48 only two IL concentrations of 300 and 600 mM were evaluated. Photoreduction control studies All control studies were conducted similarly to the above-described photoreduction, with slight modifications: (i) For evaluating “dark” reactions, samples were generated in an identical fashion except that instead of irradiating the samples for 1 h under Xe arc lamp irradiation, they were simply stored shielded from ambient light in a lab drawer, monitoring the visual appearance and UV-Vis spectra at set time intervals. (ii) For noninterfacial, homogeneous reactions, 0.50 mL of aqueous Au(OH)4– was combined with 0.50 mL of aqueous IL, both at twice the desired final concentration, and the samples vortexed for 60 s prior to performing photoreduction to ensure fully homogenized solutions. (iii) Finally, for equilibrated control studies, the biphasic solutions were vortexed for 60 s and the phases then allowed to fully disengage (phase separate) prior to performing the photoreduction. Ionic liquid interfacial partitioning Liquid/liquid partitioning experiments designed to probe the partition-coefficient (in this case, the ratio of concentrations of the IL in the two immiscible phases at equilibrium) for the coordinating IL were initiated by carefully layering 1 mL of D2O onto 1 mL of 1000 mM IL in deuterochloroform (CDCl3). For parity with our interfacial photoreduction experiments, the biphasic solution was then irradiated with a Xe arc lamp during which

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50 µL of the D2O phase was carefully extracted at set time intervals (typically, after 0, 5, 10, 30, 60, 120, 240, and 360 min). Each D2O fraction was then transferred into an NMR tube, followed by the addition of 750 µL of methanol-in-D2O. The methanol served as an internal standard and had previously been prepared by combining 6 µL of methanol with 9 mL of D2O. Specifically, the amount of MeOH in the NMR tube was calculated to be 0.0123 mmol, as shown by eq 1: 6 µL mg 1 mmol × 750 µL × 0.792 × = 0.0123 mmol (1) 9006 µL µL 32.04 mg

The integration at δ = 0.84 ppm, corresponding to the four terminal methyl groups of the [N4444+] phosphonium cation, was compared with that at δ = 3.2 ppm, corresponding to the methyl group of methanol. The concentration of IL in the D2O phase was then calculated from eq 2. [IL]D

2O

=

IL peak area 0.0123 mmol MeOH 1 × × 4 MeOH peak area 50 × 10–3 mL

(2)

Since the volume of D2O is equivalent to the volume of CDCl3 in the liquid/liquid partition experiment, the partition coefficient (K) of the IL between the aqueous and chloroform phases can be readily calculated according to:

K =

[IL]D

2O

[IL]0,CDCl – [IL]D2 O 3

Note that in these experiments [IL]0,CDCl = 0.100 M. 3

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(3)

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Catalytic rates for 4-nitrophenol reduction The catalytic activities of the synthesized AuNPs were evaluated for the kinetics of borohydride-assisted conversion of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). Upon the addition of NaBH4, the solution immediately turns dark yellow due to the formation of the 4-nitrophenolate ion (4-NPO), which shows an absorbance maximum around 400 nm. NaBH4 was used in large excess (>200-fold with respect to 4-NP) to ensure that the reduction followed pseudo-first-order kinetics. For the AuNPs made from [P4444][6-AC] and [P4444][tau], 10.50 µL of 10-fold diluted solutions of the as-synthesized AuNPs was added to a mixture of 0.14 mM 4-NP (1.40 mL of a 0.2 mM aq. stock) and 30 mM NaBH4 (0.60 mL of 0.1 M aq. stock) to give 5 mol% Au with respect to the 4-NP. The Au nanowire samples made using weakly-coordinating ILs were tested under identical conditions. For the control study, AuNPs were omitted, with all other experimental conditions remaining identical to those just stated. The UV-Vis spectrometer was set up to collect spectra every 15 s over a 15 min period in the range of 250–550 nm at a scan speed of 24,000 nm min–1. RESULTS AND DISCUSSION The coordinating phosphonium ILs were synthesized via a one-pot neutralization route using tetrabutylphosphonium hydroxide and 6-aminocaproic acid (or taurine). The successful preparation and purity were confirmed by 1H NMR spectra (Figures S2 and S3). All other ILs were prepared as described elsewhere.34, 46, 47 The photoreduction of Au(III) was carried out by the photoirradiation of a biphasic solution of aqueous Au(OH)4– layered over either neat IL or IL dissolved at the desired concentration in chloroform. Upon completion of the photoreduction, AuNP samples were thoroughly

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washed with acetonitrile (three times; see Experimental Section) to remove excess IL and then re-dispersed into water. The UV-Vis spectra of the AuNPs generated from [P4444][6-AC] and [P4444][tau] are shown in Figure 1A and 1B, respectively. As the initial concentration of IL in the organic phase was increased, the resulting localized surface plasmon resonance (LSPR) hypsochromically shifted (Figure 1C) and the peak width concurrently decreased, both being typical indicators of the presence of smaller, more uniform AuNPs. The striking visual color transition observed for AuNPs formed across a range of [P4444][6-AC] concentrations (Figure 1C, inset) dramatically illustrates this distinct LSPR shift. Concentrations of [P4444][6-AC] or [P4444][tau] below 5 mM did not result in any notable AuNP formation, likely due to insufficient colloidal stabilization. Indeed, as shown in Figure S4, a photoreduction experiment conducted using 1 mM [P4444][6-AC] in the organic phase resulted in electroless plating of a thin gold film onto the vessel wall rather than formation of discrete and stable AuNPs. Careful TEM analysis reveals that, rather than significant changes in colloid size and uniformity taking place, the changes in LSPR are instead largely the result of changes in the state of colloid interaction and agglomeration. The general trend followed is that, as the IL concentration decreases, the AuNPs show increasing aggregation and particle irregularity. Although not definitive, there are also features in the TEM images, particularly for low IL concentrations, that suggest the possibility of cold welding having taken place.49 Overall, we find that the changes in LSPR peak shape and position are a manifestation of the increased aggregation of similarly-sized, quasi-spherical AuNP building blocks. Figure 2 provides TEM images for AuNP samples made from 1000 mM (Figure 2A,B) compared with 5 mM (Figure 2C,D) [P4444][6-AC]. The AuNPs made from

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1000 mM [P4444][6-AC] were fairly uniform, quasi-spherical particles showing very little evidence of cold welding (i.e., “fused” nanostructures representing a bridging between discrete colloids). On the other hand, the 5 mM [P4444][6-AC] AuNP samples were highly aggregated, comprising mixtures of irregular and quasi-spherical particles, including those reminiscent of cold-welded structures. These results confirm that as the [P4444][6AC] concentration decreases, the nanoparticles become less stabilized and more prone to aggregation, resulting in assembled nanostructures accounting for the red-shifted LSPR. TEM images and the accompanying histograms for the remaining [P4444][6-AC] concentrations investigated can be found in Figure S5. Overall, they support the same trend of a relatively constant size for individual AuNPs but with a decidedly higher degree of aggregation for lower [P4444][6-AC] concentrations. Interestingly, when using neat [P4444][6-AC], not only did free, quasi-spherical AuNPs form but large non-crystalline spheres enveloping AuNPs were also generated, as shown in Figure S5A. The exact origin of these “raisin bun” spheres remains an open question, however, the generation of these large (few hundred nanometer) spheres is reproducible. Control experiments indicate that the material is not residual IL, as originally suspected. Additional support for this stance derives from the observation that these spheres show no signs of degradation (beam damage) during TEM imaging even under prolonged electron beam exposure, suggesting they might be inorganic in nature. Due to the increased particle irregularity and aggregation of the 10 mM and 5 mM [P4444][6-AC] AuNPs (Figure S6), a uniform size analysis was too difficult to conduct but the free, non-aggregated particles were sized at 7.1 ± 2.3 and 5.4 ± 1.2 nm, respectively. The AuNPs made in the presence of [P4444][tau] also exhibited the same trend; as the ionic liquid concentration decreased the quasi-spherical particle size only slightly

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increased but the uniformity of the individual particles and the frequency of large aggregates drastically increased, which accounts for the observed bathochromic shift of the LSPR (Figure 1B). TEM images and the accompanying histograms of the [P4444][tau] AuNPs samples are provided in Figure S7 and S8. Akin to the lower concentrations of [P4444][6-AC], uniform size analyses on the 10 and 5 mM [P4444][tau] AuNP samples was too difficult to conduct due to the large amount of aggregation but the free, nonaggregated particles were 9.6 ± 4.3 and 12.2 ± 5.2 nm in diameter, respectively. A comparison of the nanoparticle size vs. the LSPR of the AuNPs (Figure S9A and C) revealed that the individual quasi-spherical particle size did not substantially change as the ionic liquid concentration decreased, therefore the bathochromic shift of the LSPR could be mostly attributed to the observed aggregation. While the photoreduction system employed here is similar to that reported by Soejima et al.28, due to the resulting morphology of the nanoparticles within this work, the reported reduction mechanism of the formation of dissipative nanostructures via ionpairing of the cation (TBA+) with Au(OH)4– is likely not a plausible mechanism in this case. Therefore, to elucidate the plausible growth mechanism the diffusion of the ILs across the interface was monitored via NMR using methanol as an internal standard. The diffusion study, details of which are provided in the experimental section, was conducted in the same manner as the previously discussed photoreductions, however, the water and chloroform were replaced by deuterium oxide (D2O) and deuterated chloroform (CDCl3), respectively. The D2O phase was sampled at set time intervals (0, 5, 10, 30, 60, 120, 240, and 360 min) to examine the amount of IL (both cations and anions) diffusing from the CDCl3 phase. The NMR spectra of [P4444][tau] in the D2O phase sampled at the above times are shown in Figure 3A, with the data offset for clarity. All peaks were

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standardized to the methanol peak intensity prior to calculating peak area. Each peak area was then normalized to its longest respective time interval and the results were plotted vs. time (Figure 3B) to show the system’s temporal equilibration. Figure 3B indicates that the partitioning of the ionic liquids between the two phases took around one hour to reach near-equilibrium, which was further supported by a timed reduction study that showed no significant changes in the LSPR and catalytic activity after 60 min (Figure S10). The NMR spectra of the [P4444][6-AC] fractions and the time-lapsed diffusion plot are provided in Figure S11. The diffusion studies also afforded the calculation of the partition coefficients of the two ILs within the biphasic system, which were found to be 6.0 and 7.6 (at 27 °C) for [P4444][6-AC] and [P4444][tau], respectively. Since both ions comprising the ILs diffuse across the interface at similar rates, it’s reasonable to conclude that the ILs cross the interface as their own ion pair and do not form [IL+][Au(OH)4–] pairs. In Soejima’s work,28 these pairs form at the water–chloroform interface and continuously diffuse into the aqueous phase, leaving a new interface available for further ion pairing. The spontaneous formation of these assemblies at the interface is promoted by vectorial transport of cations across the interface, which is driven by osmotic pressure. However, the NMR diffusion studies indicate that a different reduction mechanism is at play which will be addressed in a latter section but may actually be related to the ionic liquid structures themselves. Additionally, the catalytic efficacy of the AuNPs was investigated through the model reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). To test the reaction kinetics, an aliquot of AuNPs that would result in 5 mol% Au with respect to 4-NP was added to a solution containing 4-NP and NaBH4 (>200-fold excess with respect to 4-NP). The absorbance at 400 nm was collected over a 15-min period (Figure 4A) and the

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apparent rate constants (kapp) for each AuNP catalyst were estimated from the slopes of the linear correlation of ln(At/A0) vs time, where A0 is the initial absorbance and At is the absorbance at time (t). The kinetic activities of the AuNPs made from both ionic liquids resulted in a similar trend of decreasing kapp values as the IL concentration decreased, eventually resulting in negligible catalytic activity for the AuNPs made in the presence of 10 mM and 5 mM IL, which was due to poorly stabilized and highly aggregated particles (Figure 4B and S12). An interesting and intriguing difference between the catalytic activities of the AuNPs synthesized in the presence of neat [P4444][6-AC] vs. [P4444][tau] is that the [P4444][tau] AuNPs produced one of the highest catalytic activity out of all the samples tested, while the neat [P4444][6-AC] AuNPs produced negligible 4-NP reduction. The non-existence catalytic activity of the [P4444][6-AC] AuNPs is attributable to the unexpected formation of the large amorphous spheres that have entrapped an appreciable amount of the AuNPs, thereby decreasing the quantity of catalytically active sites. As shown in Figure 4B, the kapp values for AuNPs made from 1000 mM [P4444][6-AC], neat [P4444][tau], and 1000 mM [P4444][tau] were 0.24 ± 0.06 min–1, 0.29 ± 0.09 min–1, and 0.27 ± 0.07 min–1, respectively (Figure 4C). Correlations between the size of the nanoparticles and the observed catalytic rates are shown in Figure S9B and S9D for [P4444][6-AC] AuNPs and [P4444][tau] AuNPs, respectively. The effects of the slight increase in particle size and the aggregation phenomena are reflected in the catalytic properties of the AuNPs, in which the kapp values decreased with decreasing ionic liquid concentration. The changes in LSPR, particle size, and kapp are attributed to the decreasing quantity of IL available for adequate stabilization of the particles.

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Furthermore, since the photoreduction system came close to equilibrium in one hour, which was the same duration that the sample was photoirradiated, there was concern for continued particle growth and/or stabilization after synthesis. Therefore to confirm that the nanoparticle stabilization was fully saturated, samples were generated in which the purification of the AuNPs was stayed for 24 h. The experiment was carried out by employing 1000 mM of both [P4444][6-AC] and [P4444][tau] under identical photoreduction conditions as those previously described, however, the AuNPs were washed the following day to compare any morphological and catalytic changes with respect to samples that were washed immediately after synthesis. TEM images of the 1000 mM [P4444][6-AC] AuNPs (Figure S13) washed on different days show no significant size deviations, although the AuNPs washed 24 h after synthesis appear to be more uniformly dispersed and less aggregated than those immediately washed. On the other hand, the 1000 mM [P4444][tau] AuNPs that were washed 24 h after synthesis displayed a slightly larger average particle size (Figure S14) so a delayed purification step to provide a ripening period for the AuNPs may actually have an adverse effect, contrary to what was expected. Oddly enough, the average kapp values remained nominally unchanged regardless of when the purification was conducted (Figure S15). These results help fortify that the AuNPs were adequately stabilized within the equilibration period and did not require any additional ripening period. To confirm that the AuNPs were formed through an interfacial photoreduction (i.e. the interface and photoirradiation are necessary), a series of control experiments were conducted, details of which can be found in the experimental section. Interfacial reductions were conducted under identical conditions to the aforementioned photoreductions except the samples were not illuminated but were stored in a lab drawer.

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The photos in Figure S16 highlight the results of this study using neat, 1000, 100, and 5 mM concentrations of the ILs. After 1 h in dark conditions, only slight color changes in the aqueous layer were visible indicating that photoirradiation is necessary to arrive at adequate AuNP formation. Additional time under dark conditions did result in some visible evidence of AuNP formation (mostly in the case of 5 mM IL) while other solutions developed a pale to vibrant yellow, the origin of which is still currently under investigation. While the dark conditions produced intriguing results, the take home message is that, after 1 h, the drastic differences between the dark vs. illuminated samples clearly indicates that the irradiation step is essential. Next, to assess the vitality of the organic-aqueous interface, photoreductions were conducted in which the ILs were directly added to the aq. Au(OH)4–, with no organic layer present (i.e. no interface and no IL diffusion across said interface). The samples were homogenized and then irradiated for 1 h. Figure S17A, B show that under non-interfacial conditions, some AuNP formation occurs, especially when employing [P4444][6-AC], but not to the extent of the interfacial conditions, highlighting the key role that the slow diffusion of IL across an interface plays in the formation of these AuNPs. Even further, comparable samples that were generated under constant magnetic stirring, actually resulted in even less AuNP formation (Figure S17C), indicating that continued equilibration of the samples is actually detrimental within these systems. As further evidence that the interface and IL diffusion were central in the formation of stable, catalytically active AuNPs, photoreductions were conducted in which the samples were vortexed pre-irradiation to allow for equilibration (Figure S18). Figure S18A, B, D, and E show that the visual appearance of the equilibrated (eq.) samples (except in the case of neat IL) and their absorbance profiles (and therefore LSPR values)

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were all similar to the results obtained for the non-equilibrated (non-eq.) indicating that diffusion across an interface may not actually be necessary. Interestingly though, the catalytic rates tell a different story (Figure S18C, F). While the kapp values for the eq. sample made in the presence of neat [P4444][tau] were similar or slightly higher than those observed for the comparable non-eq. sample, all other eq. samples had drastically lower kapp values with some showing negligible activity. These results indicate that, although in most cases equilibration will produce spectroscopically similar AuNPs, the diffusion of IL across an interface throughout the photoreduction is crucial to afford highly active AuNPs. Lastly, thermal effects were ruled out as a major contributor in the photoreduction by monitoring the aqueous layer temperature with a thermocouple (Figure S19), which shows that the solutions only increased by about 6 °C throughout the photoreduction. As previously mentioned, the data indicates that the reduction mechanism within this work doesn’t follow the ion pairing mechanism proposed by Soejima et al.28 but likely originates from the actual IL structures. Since the cation of the ILs remained the same and the resulting physical and spectroscopic properties of the AuNPs were similar regardless of the IL employed and in conjunction with the control studies, it is reasonable to conclude that the current choices of counter anions in the ILs play a minute role (if any) in the reduction process but may contribute to the growth mechanisms of the AuNPs. Contrary to the weakly-coordinating PF6 anion employed in Soejima’s work28, the two free amine-containing anions studied here are strongly coordinating anions. Therefore, the coordinating nature of these ILs may actually be responsible for the formation of the observed quasi-spherical nanoparticles instead of dissipative nanostructures that result in holey Au nanowires. To test this proposed mechanism,

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photoreductions were conducted using TBAPF6, as well as ILs based on the weaklycoordinating anion bis(trifluoromethylsulfonyl)imide ([Tf2N]), namely [emim][Tf2N], [N4441][Tf2N], and [C3(C1Im)2][Tf2N]. The UV-Vis spectra and TEM images of the resulting particles are provided in Figures 5 and S20-22. Employing TBAPF6 resulted in holey Au nanowire structures (Figure S20) similar to those reported by Soejima,28 although the samples were more aggregated especially for higher concentrations of TBAPF6. All of the poorly coordinating ILs produced wire like structures akin to those observed when using TBAPF6, however, most of them were poorly defined and/or highly aggregated (Figures S21-22). Interestingly, when [N4441][Tf2N] was employed, fairly well-defined holey Au nanowires, possibly more pristine than the TBAPF6 samples, were generated (Figure 5). These results reveal that the proposed growth mechanism is indeed correct and elucidate the possibility of finely tuning the Au nanostructure morphology by carefully tailoring ILs that have a wide range of coordination strength. Additional, weakly-coordinating ILs that were tested ([(N111)2N][Tf2N], [C4mpip][Tf2N], and [N8881][Tf2N]) also produced similarly coloured solutions and spectroscopic results, indicating that they may have also produced nanowires and further supporting the coordinating strength growth mechanism. Select samples were also tested for catalytic activity (Figure S23) and not surprisingly, the aggregated holey Au nanowire samples displayed marginal kapp values, although the activity for [N4441][Tf2N] was unexpectedly high which likely was due to the small, less-aggregated nanoparticles that were observed in TEM (Figure 5C and E).

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CONCLUSIONS In conclusion, two tetrabutylphosphonium-based ILs, [P4444][6-AC] and [P4444][tau], demonstrated IL-assisted photoreduction of Au(III) at the water–chloroform interface resulting in quasi-spherical AuNPs. Regardless of which IL was employed, by increasing the IL concentration in the organic phase, the LSPR of the AuNPs hypsochromically shifted and the peak broadening decreased. These spectroscopic results are typically indicative of smaller and more uniform AuNPs, although imaging analysis confirmed that the LSPR shifts are due to decreased aggregation from a more adequate stabilization of the AuNPs. The decrease in particle stabilization correlates well with the observed catalytic rates in that as the quantity of stabilizer (IL) decreased and aggregation increased, the kapp values decreased. Diffusion experiments elucidated that both the cations and anions migrate across the interface at similar rates, revealing partition coefficients of 6.0 and 7.6 for [P4444][6-AC] and [P4444][tau], respectively. Control studies confirmed that both the photoillumination and interfacial diffusion were necessary for the formation of stable, catalytically active AuNPs. The resulting AuNP morphologies were attributed to the strongly coordinating nature of the anions within the ILs, evidenced by the drastically different morphologies (i.e., holey nanowires) obtained when employing weakly-coordinating ILs. These results highlight the potential for finely tuning Au nanostructures by tailor-making ILs with incrementally increasing coordination strength. Future studies will focus on a complete understanding of how the coordination strength affects the reduction and growth mechanisms of the diverse Au nanostructures that form at the interface by screening a library of ILs with varying coordination strengths.

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ASSOCIATED CONTENT SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website at DOI: ■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Gary Baker: 0000-0002-3052-7730 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the Research Cooporation for Science Advancement.

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Scheme 1. Overview of the interfacial photoreduction and the resulting products, quasi-spherical AuNPs (right) vs. holey gold nanowires (left) when employing coordinating and non-coordinating ionic liquids, respectively.

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Figure 1. Normalized UV-Vis spectra of AuNPs made by the assistance of (A) [P4444][6-AC] and (B) [P4444][tau] for incremental IL concentrations. Overall, these spectra reveal that as the IL concentration decreases the LSPR red-shifts accompanied by substantial peak broadening. (C) Extracted LSPR values from (A) and (B) further highlight this trend. The inset image in (C) shows the striking color transition displayed for AuNPs prepared using different [P4444][6-AC] concentrations.

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Figure 2. Representative TEM images of AuNP samples made in the presence of (A and B) 1000 mM and (C and D) 5 mM [P4444][6-AC]. At 5 mM [P4444][6-AC], samples show significant aggregation due to inadequate IL available for colloid stabilization, accounting for the observed bathochromic shift in the LSPR. Surprisingly, however, upon close inspection of these aggregates under higher magnification, they appear to be composed of nominally the same-sized AuNPs as for the free, non-aggregated colloids rather than comprising larger Au nanostructures.

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Figure 3. (A) 1H NMR spectra of [P4444][tau] fractions drawn from the D2O phase at various time intervals under liquid/liquid photoreduction conditions. The asterisk denotes the methanol internal standard and the remaining peaks are color-coded to match the corresponding legend found in panel B. (B) Integrated intensities (standardized to the methanol peak and then normalized to the longest time measured, 6 h) of the time-dependent [P4444][tau] 1H NMR spectra given in panel (A) showing that, after approximately 1 h, the system is already near equilibrium.

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Figure 4. (A) Time-dependent UV-Vis absorption spectra of the NaBH4-assisted reduction of 4-NP catalyzed by AuNPs made by the assistance of 1000 mM [P4444][6-AC]. For clarity, spectra are plotted at 1 min intervals. (B) Plots of ln(At/A0) for 4-NP absorbance at 400 nm against time for various AuNP catalysts. (C) Calculated kapp values (from the linear portion of data in panel B) of IL-stabilized AuNPs, showing that as IL concentration increases the apparent catalytic rate also increases owing to decreased colloid aggregation.

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Figure 5. (A) UV-Vis spectra and (B) photograph of samples made in the presence of 304 and 616 mM [N4441][Tf2N]. The broad LSPR near 600 nm and the gun-barrel blue color of the solutions are indicative of possible nanowire formation. TEM imaging analysis of both samples (D–F) confirm the formation of holey Au nanowires with select holes highlighted within blue boxes in the insets of panels D and F. Both concentrations of [N4441][Tf2N] generated similar results, which were among the most pristine nanowires generated from the non-coordinating ILs investigated.

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