UVâvis Spectroscopy, Microfocus X-ray Absorption - American

Article pubs.acs.org/JPCC

In Situ Spectroelectrochemistry at Free-Standing Liquid−Liquid Interfaces: UV−vis Spectroscopy, Microfocus X‑ray Absorption Spectroscopy, and Fluorescence Imaging Yvonne Gründer,† J. Frederick W. Mosselmans,‡ Sven L. M. Schroeder,*,†,§ and Robert A. W. Dryfe*,† †

School of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom Diamond Light Source, Didcot, Oxfordshire OX11 0DE, United Kingdom § School of Chemical Engineering and Analytical Science, University of Manchester, The Mill, Sackville Street, Manchester M13 9PL, United Kingdom ‡

ABSTRACT: A windowless electrochemical cell for the spectroscopic investigation of the liquid−liquid interface, using a dual droplet configuration, has been designed. The setup permits in situ probing of the bulk solutions and the interfacial region by fiber-optic UV−vis spectroscopy, microfocus X-ray fluorescence (XRF) elemental mapping, and microfocus X-ray absorption near-edge structure (μXANES) spectroscopy. The electrodeposition of Au, induced by ion transfer of the tetrachloroaurate complex from a halogenated solvent (containing a weak reducing agent) to the aqueous phase, has been monitored by a combination of the three techniques. The reaction can be followed in situ by UV−vis spectroscopy by detecting the oxidized form of the reducing agent. Voltammetric evidence suggests the formation of interfacial Au(I) species, whereas μXANES detect the presence of metallic Au(0).

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INTRODUCTION Nanoparticle formation continues to be widely investigated due to the potential for technological innovation in areas such as catalysis, electronics, and coating applications.1−3 Current molecular level insight into nanoparticle formation processes, however, is insufficient to permit physical and chemical properties of nanoparticles to be tailored through control of molecular self-assembly, nucleation, and growth.4 The structural properties of supersaturated solutions and their evolution during particle nucleation and growth are essentially unknown.5−9 Available quantitative models for nucleation make minimal assumptions about the molecular structure of the nucleating species and their surrounding medium.5−9 Consequently, there is a need for developing experimental methodologies that provide more incisive insight into the molecular basis of nucleation and growth processes in solutions.6,10 The liquid−liquid interface provides an ideal system for studying the underlying atomic and molecular processes of nucleation and growth. Many preparative routes to nanoparticles are based on spontaneous reaction at liquid−liquid interfaces.11,12 The interface is free from high-energy defects that serve as heterogeneous nucleation sites in other interfacial systems. Electrochemical polarization can be used to generate metastable interfacial supersaturation,13−15 providing a means for experimentally confining the locus of nucleation to a thin layer with a small volume. The nucleation and growth of metallic particles has been investigated previously at the liquid− liquid interface under electrochemical control: additional © 2013 American Chemical Society

information was limited to ex situ structural characterization of deposits (via electron microscopy or X-ray diffraction).16−19 Here we show how imaging X-ray absorption fine-structure (XAFS) and UV−vis spectroscopies can be applied in a windowless liquid−liquid electrochemical cell to achieve the in situ characterization of an interfacial reaction and of the resultant growth of interfacial metal nanoparticles using electrochemical control. As the demonstration system, we have chosen the formation of gold nanoparticles at the water/ organic solvent interface, induced by the heterogeneous electron transfer between Au(III) and an aromatic amine, which functions as the organic phase electron donor. Liquid− liquid interfaces have previously been studied in situ using UV− vis and X-ray absorption or fluorescence spectroscopies20−24 as well as scattering techniques using neutrons, X-rays, or visible light.25−30 A preliminary report on neutron reflectivity at the aqueous electrolyte/organic interface suggested that the rootmean-square interfacial roughness was on the order of 1 nm, although the experiment was complicated by the need for a very thin (10 μm) layer of aqueous phase. X-ray reflectivity at the polarized interface has been shown to be a particularly powerful approach, when combined with molecular dynamics (MD) simulations of the electrical double-layer formed at the polarized interface. These combined MD and X-ray reflectivity studies have highlighted the inadequacy of methods based on Received: December 7, 2012 Revised: February 25, 2013 Published: February 27, 2013 5765

dx.doi.org/10.1021/jp312060f | J. Phys. Chem. C 2013, 117, 5765−5773

The Journal of Physical Chemistry C

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a liquid/liquid junction to be formed with an Ag/AgCl reference electrode. Through standard polytetrafluoroethylene (PTFE) fittings (Bohlender, Grünsfeld, Germany), both glass compartments are connected to glass tubes with an outer diameter of 6 mm and inner diameter of 4 mm. A hemispherical droplet of the electrolyte solution can be pushed out from the open end of each tube. Exact control of the volume of these droplets is achieved using syringe pumps (Aladdin-1000, World Precision Instruments) that are connected to the other end of each glass half-cell, permitting electrolyte dosing/withdrawal with an accuracy of ∼1 μL. The mouths of the two glass tubes are mounted at a vertical distance of about 2 mm from each other in a purpose-designed environmental chamber shown in Figure 1b. By expanding the electrolyte volumes with the syringe pumps, the two liquid phases can be brought in contact with each other to form a liquid−liquid junction. In this way a stable, free-standing liquid−liquid interface with a diameter of ∼3 mm is established (Figure 1c), and its current/potential response can be studied by using a potentiostat to control the potential difference applied across the liquid−liquid interface. A gas inlet permits control of the gaseous environment around the liquid−liquid interface, thereby giving some control over solvent evaporation rates as well as minimizing the influence of reactive gases such as O2. Alternatively, this facility also permits dosing of additional gaseous reaction partners into the cell. For the Xray absorption spectroscopy measurements a cylindrical cell enclosure was used (covering an angle of ∼300°; Figure 1b) which is made from polyimide foil (Kapton) that acts as a corrosion- and radiation-resistant X-ray transparent window, allowing the interface to be aligned in the synchrotron X-ray beam as well as detection of the transmitted beam and X-ray fluorescence with a fluorescence detector aligned at an angle of ∼90° relative to the incident X-ray beam. For the UV−vis absorption, a different cubic-shaped cell holder was employed. Similar to the setup for the X-ray absorption measurement, a gas inlet permits the control of gaseous environment and the cell is mounted in the same way. The walls consist of two glass windows opposite one another, so that the shape of the droplet can be adjusted, and two glass walls into which the optical fibers of the UV−vis spectrometer can be screwed. These walls can slide up and down and can be fixed relative to the bottom and top by screws that allow the accurate alignment of the transmitted beam relative to the interface. UV−vis Spectroscopy. For the characterization of the process at the liquid−liquid interface, a commercial QE65000 UV−vis spectrometer (Ocean Optics, The Netherlands) was employed. The light beam, supplied by a DH-2000-BAL deuterium−halogen light source (Ocean Optics, The Netherlands), was both conducted to, and collected from, the spectroelectrochemical cell by 230 μm diameter optical fibers (Ocean Optics, The Netherlands). The beam was focused with optical lenses to about 0.5 mm at the sample position. X-ray Absorption Spectroscopy (XAS) and X-ray Fluorescence (XRF). XRF mapping and XAS experiments were carried out at the microfocus spectroscopy beamline I18 of the Diamond Light Source (Harwell Science and Innovation Campus, UK).34 Data were acquired in fluorescence-yield mode, monitoring the intensity of the Au L-alpha emission line using an Ortec multielement solid-state Ge detector. The beam size during the experiments was approximately 50 μm × 50 μm. Au L3 edge spectra were normalized in the program Athena.35

mean-field theories at describing the double layer structure at the liquid−liquid interface28 and the need to include specific ion−ion correlations on the organic side of the interface.29 The novelty of the approach reported here lies in two developments: First, the absence of any liquid/window contact removes experimental uncertainty related to the presence of heterogeneous nucleation sites at the window surface, which can arise from adherence of irreversibly formed reactive species and/or particles formed in previous voltammetric cycles. Second, the use of a microfocus photon beam (here, both synchrotron X-rays and UV−vis fiber optics) permits spatially resolved imaging of the chemical and structural state of the system, taking advantage of spectroscopic contrast associated with the high local concentrations of active species near the interface.

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EXPERIMENTAL SECTION Spectroelectrochemical Cell. The principle of the windowless experiment we have developed is based on hanging meniscus electrochemical cells commonly employed for in situ X-ray diffraction characterization of the single crystal electrolyte interface.31,32 The modified version employed here consists of two immiscible phases, each containing a reference and a counter electrode to control the potential and current flow across the liquid−liquid interface (Figure 1a). By convention,

Figure 1. (a) Schematic of the windowless developed for deposition studies at the liquid−liquid interface. RE and CE denote reference and counter electrodes, respectively; the upper (aqueous) phase is colored blue, and the lower (organic) phase is orange. (b) Photograph of the cell used in situ at the Diamond light source. (c) A higher magnification photograph of the interfacial contact area highlighted in (a).

the potential scale refers to the potential of the aqueous phase relative to the organic phase, and a positive current refers to the transfer of positive charge from the aqueous to organic (or, equivalently, a negative current flow in the opposite direction).33 The glass compartment of the aqueous electrolyte contains an Ag/AgCl reference electrode, while the organic electrolyte glass cell has an additional compartment that allows 5766



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Figure 2. Electrochemical liquid−liquid cell investigated by UV−vis and XAS. The double bar denotes the interface, and org denotes either DCE or DCB.

Materials and Chemical Procedures. Glassware and Teflon parts were cleaned first in a fresh mixture of concentrated nitric acid (HNO3) and concentrated hydrochloric acid (HCl) (1:3) to remove metal contaminants. After rinsing with Milli-Q water, the glassware was immersed in a 1:4 mixture (by volume) of piranha solution (30% hydrogen peroxide, H2O2, and concentrated sulfuric acid, H2SO4 (CARE is required on handling and disposal of this solution), Fisher Scientific), boiled in ultrapure water, and dried. Solutions for the aqueous phase were prepared using ultrapure water (18.2 MΩ cm). LiCl (99.99%, Aldrich) was used as the aqueous base electrolyte. The organic phase electrolyte bis(triphenylphosphoranylidene)ammonium tetrakis(pentafluorophenyl)borate (BTPPATPBF) was prepared as described elsewhere.33 The organic solvents were 1,2-dichloroethane (DCE, CHROMASOLV, ≥99.8%, supplied by Sigma-Aldrich) or 1,2-dichlorobenzene (DCB, ≥99% supplied by Fluka). DCB was chosen as the organic phase for the XRF and absorption measurements as it is less volatile than DCE. Minimising evaporative losses during the experiment results in a more stable position of the liquid−liquid interface, and measurements are possible over a longer time period. The metal precursor, tetraoctylammonium tetrachloroaurate (TOAAuCl4), was prepared as an organic solution via a phasetransfer process.12 Initially, a 10 mM aqueous solution of hydrogen tetrachloroaurate trihydrate, HAuCl4·3H2O (≥99.9%, Aldrich), was prepared. The aqueous solution was brought in contact with a solution of 10 mM tetraoctylammonium chloride (TOACl) in either DCE or DCB. Tetrachloroaurate and chloride undergo spontaneous ion exchange, leading to the formation of an organic solution of TOAAuCl4.12 This process was finished when the aqueous phase became colorless and the color of the organic phase changed to yellow. The organic and aqueous phases were separated, and the resulting 10 mM TOAAuCl4 solution in DCE or DCB was used to prepare the organic phases for the electrochemical cell. The reducing agent, tri-(p-tolyl)amine (TPTA, 97%, Aldrich), was purified by recrystallization from isopropanol. Microelectrode voltammery was used to determine standard potentials: these experiments were performed with a 5 μm radius Au microelectrode (supplied by IJ Cambria Scientific, Burry Port, UK) in a three-electrode configuration. For nonspectroelectrochemical measurements, a conventional glass cell (inner diameter of ∼1 cm) was employed for polarization of the liquid−liquid interface. Cyclic voltammetry experiments were performed using a four-electrode configuration33 with an Autolab potentiostat (PGSTAT100) for the measurements based in our laboratory. An IVIUM Compactstat potentiostat was used for the X-ray absorption experiment. Homemade Ag/AgCl reference electrodes (RE) were directly immersed in the chloride containing aqueous phase. An aqueous solution of 0.1 mM LiCl and 1 mM

BTPPACl (bis(triphenylphosphoranylidene)ammonium chloride) was brought in contact with the organic solution and formed a liquid junction for the organic reference electrode. The resulting electrochemical cell is summarized in Figure 2.

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RESULTS Electrochemistry. To achieve the deposition of gold nanoparticles at the liquid−liquid interface, we exploit the fact that the standard reduction potential of the anionic [AuCl4]− complex differs by about 2 V between the organic solvent and the aqueous solution.36 It is therefore stable in the organic solvent in the presence of a weak reducing agent such as the TPTA employed here, but it is expected to undergo reduction by TPTA on transfer to the aqueous phase. The oxidation of TPTA was measured voltammetrically using a Au microelectrode in DCE and DCB with the ferrocene couple (which has a standard potential of 0.64 and 0.73 V vs SHE in DCE and DCB, respectively37) as an internal reference: standard potentials of 0.97 and 1.01 V, respectively (both vs SHE), were determined. TPTA is assumed to undergo an initial one-electron oxidation in these solvents to form the corresponding radical cation.38 From the thermodynamic properties of the individual solutions one may therefore expect the TPTA to reduce aqueous [AuCl4]− to either Au(I) or metallic gold via an interfacial electron transfer reaction, since the reduction potentials of [AuCl4]− to these species are on the order of 1.0 V vs SHE.36,39 For our system (Figure 2) the interfacial distribution of [AuCl4]− can be varied by potential control (eq 1). Gold reduction to Au(I), eq 2a, and/or metallic gold formation should take place when the transferred [AuCl4]− complex has been resolvated by the aqueous phase, and the electron transfer rate should be variable under voltammetric control (eq 2b): [AuCl4]−(org) → [AuCl4]−(aq)

(1)

followed by [AuCl4 ]−(aq) + 2TPTA (org) → [AuCl 2]−(aq) + 2Cl−(aq) + 2TPTA+(org)

(2a)

and/or [AuCl4 ]−(aq) + 3TPTA (org) → Au(s) + 4Cl−(aq) + 3TPTA+(org)

(2b) −

A cyclic voltammogram for the transfer of [AuCl4] from DCE to the aqueous phase in the presence of organic phase TPTA is shown in Figure 3a. Upon transferring the complex a small prewave is observed, which was previously identified as the corresponding Au(I) complex, [AuCl2]−.36 Figure 3b shows the cyclic voltammogram of the same system after holding the 5767



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Figure 3. (a) Cyclic voltammogram of the gold transfer in the presence of the reducing agent (TPTA) at the water|DCE interface. The cell defined in Figure 2 was used with x = 0.2 mM and y = 4 mM. (b) Cyclic voltammogram obtained after holding the potential at 0.175 V for 15 min. The evolution of [AuCl2]− through Au(III) reduction is evident.

potential at 0.175 V for 15 min. The prewave associated with [AuCl2]− is much more prominent, although the main feature associated with [AuCl4]− ion transfer is still present. This suggests that some of the potentiostatically transferred Au(III) ions are reduced to Au(I) in the aqueous phase. None of the voltammetric features provide evidence for the existence of Au(0) species. Based solely on electrochemical data, no conclusion can therefore be drawn about any possible reduction of the Au complexes to Au(0). UV−vis Absorption. To obtain further information on the Au(III) reduction process, interfacial UV−vis spectroscopic data were recorded. All spectra were normalized to the absorbance signal at 800 nm, where no signal change is observed as a result of the electrochemical processes, to account for possible background changes due to evaporation of the droplet. In a first step, the setup was tested examining the [AuCl4]− ion transfer without the reducing agent present, i.e., for the cell in Figure 2 with x = 0.2 and y = 0. The UV−vis absorption spectra of the aqueous and the organic phases during a cyclic voltammogram recorded at 2 mV s−1 are shown in Figure 4, which gives the applied interfacial potential waveform and the associated currents (Figure 4a) and displays the time-resolved spectra in the aqueous (Figure 4b) and in the organic phase (Figure 4c), each recorded ca. 0.5 mm away from the interface. The [AuCl4]− complex is associated with two absorbance maxima at approximately 237 and 326 nm in water and at 242 and 311 nm in DCE. These values compare well with previously reported data for aqueous solution.40 The transfer of the Au(III) ion into the aqueous phase is clearly evident through the increase of [AuCl4]− absorbance when the potential is positive. During the reversal of the potential scan,

Figure 4. Time-resolved spectroscopic UV−vis imaging of [AuCl4]− at the water/DCE interface during voltammetric potential sweeps using the cell defined in Figure 2. (a) Applied voltammetric waveform (black) with current data (red); (b) aqueous phase in the absence of TPTA (x = 0.2 mM, y = 0); (c) organic phase in the absence of TPTA, i.e., as (b); (d) aqueous phase in the presence of TPTA (x = 0.2 mM, y = 4 mM); (e) organic phase in the presence of TPTA, i.e. as (e); (f) the interfacial spectrum in the presence of TPTA (as (d)).

the [AuCl4]− absorbance decreases due to reversible backtransfer into the organic phase and diffusion of the complex into the bulk solution. The second voltammetric sweep is associated with the same spectral features, although the overall intensity in the absorbance signal from the Au(III) complex is higher due to a buildup of Au(III) transferred from the first scan. The absorbance spectra recorded in the DCE phase while the potential scan proceeds in a positive direction show a decrease in the absorbance signal due to transfer of the gold complex into the aqueous phase. During the negative potential scan an increase in the absorbance peak of the [AuCl4]− complex can be observed arising from back-transfer from the aqueous phase. As was the case for the aqueous phase, the second scan shows similar features but with higher overall intensity of the Au(III) 5768



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absorbance. This indicates accumulation of the gold complex at the interface during the potential cycles. In a second experiment (Figure 4d−f), the [AuCl4]− ion transfer was investigated with TPTA present in the organic phase, i.e., for the cell in Figure 2 with x = 0.2 mM and y = 4 mM. Upon the transfer of the [AuCl4]− ion to the aqueous phase, interfacial electron transfer takes place from [AuCl4]− to TPTA. The time-dependent UV−vis spectra of the aqueous phase (Figure 4d) show an increase in the intensity of the absorbance from [AuCl4]− when the potential is scanned positively. However, with the reducing agent present, there is no increase in intensity during the second potential sweep, indicating a fast (i.e., on the time scale of seconds) reaction of transferred [AuCl4]− within the aqueous phase. Consistent with this, the spectra of the organic phase (Figure 4e) reveal a decrease of the [AuCl4]− concentration during the first positive sweep, and only a small increase in [AuCl4]− absorbance due to back-transfer during the negative potential sweep. The magnitude of the changes in the organic phase is lower overall than in the case without reducing agent (Figure 4c), which again indicates that [AuCl4]− reacts with TPTA at the interface. This observation is consistent with the cyclic voltammograms in Figure 3a, where a decrease in the [AuCl4]− ion-transfer current was detected upon extending the positive potential range. It was difficult to measure the absorbance at the interface because the curvature of the organic phase close to the interface resulted in most of the incident light beam being reflected away from the curved interface, and it was therefore not detected by the fiber-optic spectrometer. In addition, the curvature of the droplet changed during the measurement due to evaporation and as a function of the potential, giving rise to background changes. To overcome this problem, a more confined interface was created in the upper glass tube (the cell part containing the aqueous electrolyte). In this way the interface remained flat enough to obtain stable absorbance spectra over a long time. The resulting interfacial absorbance spectra during a cyclic voltammogram recorded with a sweep rate of 2 mV s−1 are shown in Figure 4f. An increase in the absorbance at about 680 nm can be observed when the potential is swept negatively. This absorbance corresponds to the previously reported spectrum of the radical cation formed from the reducing agent (TPTA+).41 However, no characteristic features due the plasmon absorbance of metallic gold at the interface could be observed, indicating that Au nanoparticles are not present in detectable quantities under these conditions or that the particles initially formed are very small (few nanometers in diameter), preventing the formation of a surface plasmon.42 XRF Imaging of the Interface and Microfocus X-ray Absorption Near-Edge Structure (μXANES). XRF imaging was carried out to investigate the ion transfer of the gold complex and the associated reactions of the ion once the gold was present in the aqueous phase. The cell in Figure 2 was set up with x = 1 mM and y = 0. The concentration of [AuCl4]− was chosen to be higher than in the UV−vis absorbance experiments to give a more intense fluorescence signal. XRF maps of the resulting interface are shown in Figure 5. The schematic in Figure 5a provides a key to interpreting the color changes in the three phases sampled (organic, aqueous, and the adjacent vapor phase) in the maps. The red color depth indicates Au fluorescence intensity, while the intense blue color indicates high X-ray transmission. The entirely blue circle

Figure 5. (a) Schematic key to areas visible in XRF maps of the dualdroplet liquid−liquid interface. The blue circle segment on the left of the images stems from the gas phase. To the right of the scheme in (a) are actual XRF maps of the interface between the aqueous and DCB phases as described in the text at (b) open circuit potential (OCP), (c) immediately after applying an external potential of 0.1 V, and (d) 13 min later. The accumulation of gold in the aqueous phase is evident through the stronger red color. Each map has dimensions of 2.9 mm (horizontal) by 3.5 mm (vertical) and was measured with a pixel size of 50 μm. The cell defined in Figure 2 was used, with x = 1 mM and y = 0 mM.

segment on the left of the maps stems from high transmission through the vapor surrounding the dual-droplet interface. The high X-ray absorbance of the chlorinated organic solvent results in no blue coloration in the area of the organic phase. This high absorbance of the organic phase also strongly attenuates Au fluorescence signal from this phase, even though the concentration of Au in the organic phase is relatively high. The Au fluorescence therefore escapes only from a shallow region near the surface of the strongly absorbing solvent. Au fluorescence is therefore mainly evident as emission from the droplet edge (purple color from a superposition of blue and red) visible at the left of the organic phase. The most significant changes in the maps are evident in the aqueous phase in the upper right region of the images. The map acquired at open circuit potential (OCP), i.e., before any external potential was applied, is shown in Figure 5b. The aqueous phase appears light blue due to the initial absence of Au and the low X-ray absorbance of the water. Figure 5c is an Au XRF image recorded immediately after applying an interfacial potential of 0.1 V, while Figure 5d was collected 13 min later. The expected potential-induced accumulation of Au in the aqueous phase is evident through the enhanced red color in the top right region of the images. This observation of Au transfer into the aqueous phase is in line with the results obtained by UV−vis spectroscopy. XRF maps upon transfer of [AuCl4]− into the aqueous phase in the presence of the reducing agent, TPTA, are shown in Figure 6. The cell in Figure 2 was set up with x = 1 mM and y = 4 mM. The Au fluorescence emission intensities are scaled in the same way as in Figure 5, and the same color scheme has been applied. Note that the right-hand edge of the droplet is shown in this case so, in contrast to Figure 5, the vapor adjoining the contacted droplets is visible on the right-hand side of each plot in Figure 6. A map of the initial system held at OCP is shown in Figure 6a. The map recorded immediately after applying a potential difference of 0.05 V is shown in Figure 6b: this potential, with reference to Figures 3 and 4, should be sufficient to induce [AuCl4]− transfer to the aqueous phase; however, no significant spectral changes are evident. After a further 15 min the potential difference was adjusted to 0 V, and the map displayed in Figure 6c was recorded; again no significant change is evident from the map. However, after holding the potential at 0 V for 200 min, accumulation of Au close to the interface becomes visible (Figure 6d). A second potential pulse was applied, now increased to 0.15 V to transfer 5769



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Figure 6. XRF maps of the interface between the aqueous (upper half of each image) and the DCB (lower half) phases containing [AuCl4]− at open circuit potential (OCP) and gentle polarization (top row of maps) and after applying an external potential of 0.15 V. The potential-induced accumulation of gold in the aqueous phase is evident through the stronger red color in the aqueous phase. (a) Initial state of the system at open circuit potential. (b)−(h) show the potentials subsequently applied and, where the time was significant, the times these potentials were applied for. Each map has dimensions of 3.9 mm (horizontal) by 4.65 mm (vertical) and was measured with a pixel size of 50 μm. The cell defined in Figure 2 was used, with x = 1 mM and y = 4 mM.

Figure 7. Au L3-edge XANES spectra of (from bottom to top) the reference Au foil, Au metal deposits visible in the maps in Figure 6e−g, the aqueous phase at a depth of 100 μm from the interface after 150 min of [AuCl4]− transfer, and the [AuCl4]− in DCB before potentialinduced ion transfer. The cell composition was identical to that of Figure 6.

more [AuCl4]− into the aqueous phase. XRF maps recorded immediately after applying this potential, as well as 105 and 150 min later, are shown in Figure 6e−g. A progressive increase of the intensity of the Au fluorescence signal near the interface with time is evident. A strong accumulation of Au is taking place that was not seen in the fluorescence maps acquired in the absence of TPTA (Figure 5). This observation suggests that the intensity increase stems from the formation of metallic gold by interfacial reaction with TPTA. After the potential had been held at 0.15 V for 160 min, it was adjusted to −0.05 V, and the fluorescence map shown in Figure 6h was acquired. This map shows an increase in intensity due to gold, suggesting that more deposition has occurred on application of a negative potential. With reference to Figure 3, this (final) potential is associated with the return of the [AuCl4]− from the aqueous phase to the organic phase. To ascertain that metallic Au had been formed from the reaction with TPTA, a μXANES spectrum of the interfacial Au deposits was recorded. It should be noted that the time required for accumulation of the series of high-resolution maps in Figure 6 did not permit us to measure this μXANES spectrum before further potential changes were applied to the system. Instead, the μXANES was collected from a freshly prepared interface, having used smaller maps to locate the interfacial species. The spectra in Figure 7 are (from bottom to top) a reference transmission XANES spectrum from metallic Au foil, a μXANES scan of the putative Au metal deposits visible in the maps in Figure 6e−g, a μXANES spectrum of the aqueous phase about 100 μm away from the interface after 150 min of [AuCl4]− transfer, and the XANES spectrum of [AuCl4]− in DCB before potential-induced ion transfer. The L3-edge XANES shows that Au(III) signal dominates toward the interior of the aqueous phase while the signal recorded at the interface is dominated by the contribution from metallic Au.

interface are presented, illustrated here for Au deposition using TPTA as the reducing agent. The system relies on the difference in reduction potential of the gold complex between the aqueous and organic phases, which is mostly due to the difference in the Gibbs energy of solvation of the halide ion liberated during the reduction of the gold complex. Upon potential induced transfer of the gold complex into the aqueous phase, the tetrachloroaurate ion should be reduced by the TPTA remaining in the organic phase. The Nernst potentials for the reduction of Au(III) to Au(I) and Au(0) in aqueous solution are 0.93 and 1.02 V (vs NHE), respectively.39 The potential for reducing Au(I) to Au(0) in aqueous solution is 1.15 V (vs NHE).39 As the tetrachloroaurate ion transfers at a Galvani potential of ΦΔDCE = 0.075 V, and the standard w potential of the TPTA reducing agent in DCE is 0.97 V (vs NHE), Au reduction should occur spontaneously upon the transfer of the tetrachloroaurate ion into the aqueous phase. The investigation of the gold reduction by cyclic voltammetry shows that not all of the gold(III) complex is reduced upon the ion transfer, as the ion transfer current in the cyclic voltammogram remains observable (Figure 3). Upon transferring gold into the aqueous phase, a second ion transfer previously identified as the [AuCl2]− complex becomes visible,36 with some ion transfer due to the [AuCl4]− remaining observable. These results suggest that the tetrachloroaurate complex is reduced partly to Au(I) during the ion transfer, but no conclusion about the formation of metallic gold can be drawn from this data alone. Our previous investigation of a related chemical system involved Au deposition driven by an aqueous reducing agent (ferrocyanide): no reverse ion transfer of the Au(III) complex back to the organic phase was observed. This was explained by a total reduction of the Au(III) to Au(I) due to a homogeneous reaction in the aqueous phase. The difference between the two cases is that the [AuCl4]−/ ferrocyanide electron transfer is homogeneous, whereas the [AuCl4]−/TPTA case is heterogeneous, since the gold

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DISCUSSION In this article, a new experimental cell and spectroscopic approach to studying metal deposition at the liquid−liquid 5770



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voltammetric cycles lasting up to 20 min, given that metallic Au is detected over this time scale in XANES experiment. We note that X-ray-induced reduction of Au(III) to metallic gold via Au(I) has been reported recently in aqueous solution; however, our data suggest that the Au(III) species are stable under X-ray irradiation under the conditions employed here.45 Taken together, these data suggest that Au(I) is formed due to the lack of nucleation sites at the bare liquid−liquid interface or is an intermediate to Au(0) formation, but the latter probably does not exist in nanoparticulate form, or at least not in sufficient concentration to give rise to a strong surface plasmon resonance. This is plausible, given that no specific capping agent has been added to the cell. A further notable feature of the spectroscopic data is the potential dependence of the Au reduction process, most clearly visible through the growth (and loss) of the TPTA+ signal at wavelengths centered on 680 nm in Figure 4f. As noted above, the growth of the TPTA+ signal is most clearly seen on the reverse portions of the potential waveform, i.e., when [AuCl4]− is returning to the organic phase. The only difference between the two parts of the voltammetric cycle is that, in the former, the [AuCl4]− ion is leaving the organic phase and could still be partially solvated by the organic solvent; in the latter case the returning [AuCl4]− should be fully hydrated. A partial desolvation of the [AuCl4]− as it leaves the organic phase will reduce the reduction rate of the latter, as the change in reduction potential is due to the change in halide solvation on reduction. If the reduction process is genuinely interfacial, it must occur in a small reaction layer which is governed by [AuCl4]− becoming fully solvated by water, but limited by the [AuCl4]− diffusing over a distance that is too large to permit bimolecular electron transfer (normally assumed to occur on a length scale of ∼1 nm46). An alternative explanation for the delay in Au formation would be that the reaction is autocatalytic and therefore is enhanced by the presence of initially formed Au. This, however, would be expected to give an increase in reaction rate regardless of the direction of the potential scan, whereas the reaction is seen as the interfacial potential is decreased. We therefore conclude that the competition between resolvation dynamics of [AuCl4]− and electron transfer rate is governed by the former, which is too slow to permit reduction of the ion as it enters the aqueous phase.

precursor must be transferred out of the organic phase to be reduced and the reaction is therefore confined to the interface. A further difference is that the net driving force for Au reduction is greater in the [AuCl4]−/ferrocyanide case than in the [AuCl4]−/TPTA system. Finally, the former reaction involves electron transfer between two negatively charged reagents, whereas the reducing agent in the latter case is a neutral species. The UV−vis experiment recorded the absorption spectra in the aqueous and organic phases simultaneously with the voltammetric gold ion transfer, with and without the TPTA reducing agent. Without reducing agent present, the absorbance signal from the tetrachloroaurate follows the ion transfer of the gold complex between aqueous and organic phase. With the reducing agent present in the organic phase, the absorbance signals in the organic and aqueous phases initially look similar to those without the reducing agent up to the potential of reverse transfer of the gold complex to the organic phase. In the spectra recorded at the interface, the signal from the TPTA oxidation can be observed (see the longer wavelength features in Figure 4f). From this point (300 s in Figure 4) a decrease in both absorbance signals in the organic and aqueous phase can be seen. Recording the absorbance spectra directly at the water−DCE interface allowed us to monitor the formation of the TPTA+ ion, which is formed upon reduction of the Au(III) complex. In principle, the cation should form on the transfer of the [AuCl4]− ion to the aqueous phase, i.e., the positive-going potential scans, which correspond to times of 0−200 and 400− 600 s (see Figures 4a and 4f). However, the strongest growth of the TPTA cation absorbance occurs on the reverse scans (200− 400 and 600−800 s), i.e., when the [AuCl4]− ion is dragged back to the interface to return to the organic. We suggest that this result may be explained in terms of the slow rate of the deposition (see above) coupled with the dynamics of the [AuCl4]− solvation change. As discussed above, [AuCl4]− only becomes reactive on transfer from the aqueous phase; however, the ion will diffuse toward the bulk of the aqueous phase, and once the ion is more than ca. 1 nm from the interface, the probability of the multiple electron transfer from the TPTA is negligible. If the process of desolvation by the organic solvent is incomplete within the time taken for [AuCl4]− to transfer over this nanometer, then metal deposition will not occur. On the reverse transfer, however, the [AuCl4]− returning to the organic phase has been fully hydrated and so is in the more reactive state. These interpretations are supported by the XAS. The gold transfer could be monitored in situ by XRF imaging. In the presence of the reducing agent increased gold fluorescence was observed close to the interfacial region upon the transfer of the gold. XANES measurements in the organic phase before transfer and in the aqueous phase after transfer of the gold complex confirm the existence of Au(III) in the organic and the aqueous phase but also indicate the formation of metallic gold at the interface. No increased XRF intensity was observed at the interface at open circuit potential in the presence of the reducing agent, which strongly indicates that the metallic gold is formed by the potential-induced [AuCl4]− transfer, i.e., via an interfacial electron transfer, rather than X-ray induced formation of metallic gold as reported previously for different metals.43,44 One unexplained feature at present is the lack of plasmon absorption seen in the UV−vis spectrum, even over multiple

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CONCLUSIONS A new windowless cell, based on contact between droplets, is described as a method to investigate the process of deposition at the liquid−liquid interface. Because the cell is windowless, it permits the in situ spectroscopic study of chemical processes at the liquid−liquid interface. Further, the lack of windows means that the apparatus is particularly suitable for the study of deposition reactions, where cell walls and/or residue from previous reactions can influence nucleation. The reaction between [AuCl4]− and the organic electron donor TPTA is investigated using this cell. The reaction is performed under electrochemical control by using the potential-induced transfer of the [AuCl4]− ion and exploiting the large difference in reduction potential of Au(III) between organic and aqueous solution. The advantages of the droplet cell with regard to spectroscopic studies are illustrated by in situ X-ray absorption and UV−vis absorption spectroscopy of the gold reduction process. It is shown that the deposition of metallic gold at the liquid−liquid interface is hindered due to the lack of nucleation sites; instead, the Au(III) complex is reduced to Au(I). In 5771



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(17) Johans, C.; Liljeroth, P.; Kontturi, K. Electrodeposition at polarisable liquid|liquid interfaces: The role of interfacial tension on nucleation kinetics. Phys. Chem. Chem. Phys. 2002, 4, 1067−1071. (18) Platt, M.; Dryfe, R. A. W.; Roberts, E. P. L. Electrodeposition of palladium nanoparticles at the liquid-liquid interface using porous alumina templates. Electrochim. Acta 2003, 48, 3037−3046. (19) Platt, M.; Dryfe, R. A. W. Structural and electrochemical characterisation of Pt and Pd nanoparticles electrodeposited at the liquid/liquid interface: Part 2. Phys. Chem. Chem. Phys. 2005, 7, 1807− 1814. (20) Tanida, H.; Nagatani, H.; Harada, M. X-ray absorption fine structure of bromide ions attracted by cationic surfactants at the heptane-water interface. J. Phys.: Conf. Ser. 2009, 190, 012061. (21) Izquierdo, D.; Martinez, A.; Heras, A.; Lopez-Palacios, J.; Ruiz, V.; Dryfe, R. A. W.; Colina, A. Spatial scanning spectroelectrochemistry. Study of the electrodeposition of Pd nanoparticles at the liquid/ liquid interface. Anal. Chem. 2012, 84, 5723−5730. (22) Martinez, A.; Colina, A.; Dryfe, R. A. W.; Ruiz, V. Spectroelectrochemistry at the liquid|liquid interface: Parallel beam UV-vis absorption. Electrochim. Acta 2009, 54, 5071−5076. (23) Kakiuchi, T.; Takasu, Y. Potential-step chronofluorometric response of fluorescent-ion transfer across a liquid-liquid interface. J. Electroanal. Chem. 1995, 381, 5−9. (24) Ding, Z. F.; Wellington, R. G.; Brevet, P. F.; Girault, H. H. Differential cyclic voltabsorptometry and chronoabsorptometry studies of ion transfer reactions at the water|1,2-dichloroethane interface. J. Electroanal. Chem. 1997, 429, 35−41. (25) Benjamins, J.-W.; Jonsson, B.; Thuresson, K.; Nylander, T. New experimental setup to use ellipsometry to study liquid-liquid and liquid-solid interfaces. Langmuir 2002, 18, 6437−6444. (26) Schmidt, J. W. Structure of a fluid interface near the criticalpoint. Phys. Rev. A 1988, 38, 567−570. (27) Luo, G. M.; Malkova, S.; Yoon, J.; Schultz, D. G.; Lin, B. H.; Meron, M.; Benjamin, I.; Vanysek, P.; Schlossman, M. L. Ion distributions near a liquid-liquid interface. Science 2006, 311, 216−218. (28) Laanait, N.; Yoon, J.; Hou, B.; Vanysek, P.; Meron, M.; Lin, B.; Luo, G.; Benjamin, I.; Schlossman, M. L. Monovalent ion condensation at the electrified liquid/liquid interface. J. Chem. Phys. 2010, 132, 171101. (29) Laanait, N.; Mihaylov, M.; Hou, B. Y.; Yu, H.; Vanysek, P.; Meron, M.; Lin, B. H.; Benjamin, I.; Schlossman, M. L. Tuning ion correlations at an electrified soft interface. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 20326−20331. (30) Strutwolf, J.; Barker, A. L.; Gonsalves, M.; Caruana, D. J.; Unwin, P. R.; Williams, D. E.; Webster, J. P. R. Probing liquid vertical bar liquid interfaces using neutron reflection measurements and scanning electrochemical microscopy. J. Electroanal. Chem. 2000, 483, 163−173. (31) Renner, F. U.; Gründer, Y.; Zegenhagen, J. Portable chamber for the study of UHV prepared electrochemical interfaces by hard x-ray diffraction. Rev. Sci. Instrum. 2007, 78, 033903. (32) Magnussen, O. M.; Krug, K.; Ayyad, A. H.; Stettner, J. In situ diffraction studies of electrode surface structure during gold electrodeposition. Electrochim. Acta 2008, 53, 3449−3458. (33) Samec, Z. Electrochemistry at the interface between two immiscible electrolyte solutions. Pure Appl. Chem. 2004, 76, 2147− 2180. (34) Mosselmans, J. F. W.; Quinn, P. D.; Dent, A. J.; Cavill, S. A.; Moreno, S. D.; Peach, A.; Leicester, P. J.; Keylock, S. J.; Gregory, S. R.; Atkinson, K. D.; et al. I18 - the microfocus spectroscopy beamline at the Diamond Light Source. J. Synchrotron Radiat. 2009, 16, 818−824. (35) Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT. J. Synchrotron Radiat. 2005, 12, 537−541. (36) Gründer, Y.; Ho, H. L. T.; Mosselmans, J. F. W.; Schroeder, S. L. M.; Dryfe, R. A. W. Inhibited and enhanced nucleation of gold nanoparticles at the water/1,2-dichloroethane interface. Phys. Chem. Chem. Phys. 2011, 13, 15681−15689.

addition, the reduction of Au(III) is slowed down by the change in the solvation of the tetrachloroaurate ion upon transfer into the aqueous phase.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], tel +44 161-306-4502 (S.L.M.S.); e-mail [email protected], tel +44 161 306-4522 (R.A.W.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the EPSRC through an EPSRC-NSF “Materials World Network” grant (EP/H047786/1). We thank Diamond Light Source Ltd for the provision of synchrotron beamtime (award SP-7405).

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REFERENCES

(1) Hughes, M. D.; Xu, Y. J.; Jenkins, P.; McMorn, P.; Landon, P.; Enache, D. I.; Carley, A. F.; Attard, G. A.; Hutchings, G. J.; King, F.; et al. Tunable gold catalysts for selective hydrocarbon oxidation under mild conditions. Nature 2005, 437, 1132−1135. (2) Shipway, A. N.; Katz, E.; Willner, I. Nanoparticle arrays on surfaces for electronic, optical, and sensor applications. ChemPhysChem 2000, 1, 18−52. (3) Hakim, L. F.; King, D. M.; Zhou, Y.; Gump, C. J.; George, S. M.; Weimer, A. W. Nanoparticle coating for advanced optical, mechanical and rheological properties. Adv. Funct. Mater. 2007, 17, 3175−3181. (4) An, K.; Alayoglu, S.; Ewers, T.; Somorjai, G. A. Colloid chemistry of nanocatalysts: A molecular view. J. Colloid Interface Sci. 2012, 373, 1. (5) Davey, R. J.; Allen, K.; Blagden, N.; Cross, W. I.; Lieberman, H. F.; Quayle, M. J.; Righini, S.; Seton, L.; Tiddy, G. J. T. Crystal engineering - nucleation, the key step. CrystEngComm 2002, 257−264. (6) Davey, R. J.; Schroeder, S. L. M.; ter Horst, J. H. Nucleation of organic crystals - a molecular perspective. Angew. Chem., Int. Ed. 2013, 52, 2166−2179. (7) Weissbuch, I.; Lahav, M.; Leiserowitz, L. Toward stereochentical control, monitoring, and understanding of crystal nucleation. Cryst. Growth Des. 2003, 3, 125−150. (8) Gebauer, D.; Colfen, H. Prenucleation clusters and non-classical nucleation. Nano Today 2011, 6, 564−584. (9) Anwar, J.; Zahn, D. Uncovering molecular processes in crystal nucleation and growth by using molecular simulation. Angew. Chem., Int. Ed. 2011, 50, 1996−2013. (10) Pienack, N.; Bensch, W. In-situ monitoring of the formation of crystalline solids. Angew. Chem., Int. Ed. 2011, 50, 2014−2034. (11) Rao, C. N. R.; Kalyanikutti, K. P. The liquid-liquid interface as a medium to generate nanocrystalline films of inorganic materials. Acc. Chem. Res. 2008, 41, 489−499. (12) Brust, M.; Walker, M.; Bethhel, D.; Schiffrin, D. J.; Whyman, R. Synthesis of thiol-derivatized gold nanoparticles in a 2-phase liquidliquid system. J. Chem. Soc., Chem. Commun. 1994, 801−802. (13) Dryfe, R. A. W. The Electrified Liquid-Liquid Interface. Adv. Chem. Phys. 2009, 151, 153−215. (14) Cheng, Y.; Schiffrin, D. J. Electrodeposition of metallic gold clusters at the water/1,2-dichloroethane interface. J. Chem. Soc., Faraday Trans. 1996, 92, 3865−3871. (15) Trojanek, A.; Langmaier, J.; Samec, Z. Random nucleation and growth of Pt nanoparticles at the polarised interface between two immiscible electrolyte solutions. J. Electroanal. Chem. 2007, 599, 160− 166. (16) Johans, C.; Lahtinen, R.; Kontturi, K.; Schiffrin, D. J. Nucleation at liquid-liquid interfaces: electrodeposition without electrodes. J. Electroanal. Chem. 2000, 488, 99−109. 5772



Article

(37) Peljo, P.; Rauhala, T.; Murtomäki, L.; Kallio, T.; Kontturi, K. Oxygen reduction at a water-1,2-dichlorobenzene interface catalyzed by cobalt tetraphenyl porphyrine - A fuel cell approach. Int. J. Hydrogen Energy 2011, 36, 10033−10043. (38) Yurchenko, O.; Freytag, D.; Borg, L. Z.; Zentel, R.; Heinze, J.; Ludwigs, S. Electrochemically induced reversible and irreversible coupling of triarylamines. J. Phys. Chem. B 2012, 116, 30−39. (39) Bard, A. J.; Parsons, R.; Jordan, J. Standard Potentials in Aqueous Solutions; Marcel Dekker: New York, 1985. (40) Usher, A.; McPhail, D. C.; Brugger, J. A spectrophotometric study of aqueous Au(III) halide-hydroxide complexes at 25−80 degrees C. Geochim. Cosmochim. Acta 2009, 73, 3359−3380. (41) Hasegawa, H. Spectroscopic studies on the color reaction of acid clay with amines. II. The reaction with aromatic tertiary amines. J. Phys. Chem. 1962, 66, 834−836. (42) Alvarez, M. M.; Khoury, J. T.; Schaaf, T. G.; Shafigullin, M. T.; Vezmar, I.; Whetten, R. L. Optical absorption spectra of nanocrystal gold molecules. J. Phys. Chem. B 1997, 101, 3706−3712. (43) Muller, F.; Fontaine, P.; Remita, S.; Faure, M. C.; Lacaze, E.; Goldmann, M. Synthesis of nanostructured metal-organic films: Surface X-ray radiolysis of silver ions using a Langmuir monolayer as a template. Langmuir 2004, 20, 4791−4794. (44) Belloni, J.; Mostafavi, M.; Remita, H.; Marignier, J. L.; Delcourt, M. O. Radiation-induced synthesis of mono- and multi-metallic clusters and nanocolloids. New J. Chem. 1998, 22, 1239−1255. (45) Ma, Q.; Divan, R.; Mancini, D. C.; Keane, D. T. Elucidating chemical and morphological changes in tetrachloroauric solutions induced by X-ray photochemical reaction. J. Phys. Chem. A 2008, 112, 4568−4572. (46) Newton, M. D. Quantum chemical probes of electron-transfer kinetics - the nature of donor-acceptor interactions. Chem. Rev. 1991, 91, 767−792.

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