Article pubs.acs.org/ac
Spatial Scanning Spectroelectrochemistry. Study of the Electrodeposition of Pd Nanoparticles at the Liquid/Liquid Interface Daniel Izquierdo,† Alberto Martinez,† Aranzazu Heras,† Jesus Lopez-Palacios,† Virginia Ruiz,‡,§ Robert A. W. Dryfe,∥ and Alvaro Colina*,† †
Department of Chemistry, University of Burgos, Pza. Misael Bañuelos s/n, E-09001 Burgos, Spain New Materials Department, CIDETEC-IK4Centre for Electrochemical Technologies, Paseo Miramón 196, E-20009 Donostia-San Sebastián, Spain § Department of Applied Physics, Aalto University, P.O. Box 15100, FI-00076 Aalto, Finland ∥ School of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, U.K. ‡
S Supporting Information *
ABSTRACT: Spatial scanning spectroelectrochemistry is a new analytical technique that provides spectral information at different distances from an electrified liquid/liquid interface where an electrochemical process takes place. As a proof of concept, we have studied two different electrochemical processes at the electrified liquid/liquid interface: (1) Ru(bpy)32+ transfer through the water/1,2-dichloroethane interface and (2) electrodeposition of Pd nanoparticles at the water/1,2-dichloroethane interface. The instrumental setup developed consists of a movable slit for the light beam to sample at well-defined positions on both sides of the interface, providing important information about the chemical process occurring. If the slit is scanned at different distances from the interface during an electrochemical experiment, a complete picture of the reactions and equilibria in the diffusion layer can be obtained. For example, in the case of the Ru(bpy)32+, the experiments show clearly how the complex is transferred from one phase to the other. In the case of electrosynthesis of Pd nanoparticles, it is demonstrated that nanoparticles are not only deposited at the interface but diffuse to the aqueous bulk solution. These in situ observations were confirmed by ex situ experiments using transmission electron microscopy.
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sampling both the interface and the adjacent solution. Thus, depending on the incidence direction of the light beam with respect to the interface, UV/vis absorption spectroelectrochemical measurements can be performed in three main configurations: (1) normal beam configuration, in which the light beam passes across the L/L interface;14 (2) reflection configuration, in which the light beam is reflected from the interface;15−17 and (3) parallel beam configuration, in which the light beam grazes the interface.18 To date, the most widely used configuration in spectroelectrochemistry at the ITIES is the reflection one.15−17 Here we have used a new UV−visible absorption spectroelectrochemical cell for the study of the electrogeneration of Pd nanoparticles at the water/1,2-DCE
lthough chemical reduction is the method most commonly used to synthesize metal nanoparticles,1−3 a number of recent articles have described the use of electrodeposition as a controlled route to nanoparticle preparation.4,5 Electrodeposition of metals at the interface between two immiscible electrolyte solutions (ITIES)6−8 combines aspects of the chemical and electrochemical reduction processes. In this interesting approach, a metallic ion present in a liquid phase is reduced to the metal by a reducing agent dissolved in the other immiscible liquid phase. The metal nucleates and grows without any solid support, while the extent and rate of the reduction can be controlled electrochemically through the electrolysis time and the applied potential, respectively.6−8 Electrochemical deposition of metal nanoparticles (NPs) at liquid/liquid (L/L) interfaces can be studied using UV/vis absorption spectroelectrochemistry. This instrumental technique is useful for the study of complex electrochemical processes.9−13 Spectroelectrochemistry can be used for © 2012 American Chemical Society
Received: April 4, 2012 Accepted: June 2, 2012 Published: June 3, 2012 5723
dx.doi.org/10.1021/ac3009127 | Anal. Chem. 2012, 84, 5723−5730
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tetrachloropalladate(II) (Acros) were analytical grade and used as received without further purification. Bis(triphenylphosphoranylidene)ammonium tetrakis(pentafluoro)phenyl borate (BTPPA TPBF20) was prepared according to a reported metathesis procedure27 from equimolar amounts of bis(triphenylphosphoranylidene)ammonium chloride (BTPPACl, Lancaster, 97%) and lithium tetrakis(pentafluoro)phenyl borate (Boulder Scientific, 98%). The L/ L interface was formed using water with a resistivity of 18 MΩ cm (Milli-Q purification system, Millipore, UK), and 1,2dichloroethane (1,2-DCE, HPLC grade, obtained from SigmaAldrich). The cell was cleaned and rinsed after each measurement to ensure an apparently clean interface, it was cleaned by immersion in piranha solution [1:4 mixture (by volume) of 30% hydrogen peroxide, H2O2, and concentrated sulphuric acid, H2SO4], then rinsed with ultrapure water. For safety considerations, all handling and processing were performed carefully, particularly when 1,2-DCE and piranha solution were used. The new device, developed to measure in a parallel arrangement using a mobile slit (Figure S1, Supporting Information, consists of four main parts: (1) piezoelectric positioner, (2) support of the slit and lenses, (3) L/L spectroelectrochemical cell, and (4) cuvette fastening system. In all cases a 400 μm slit was placed after a collimating lens to ensure that a parallel beam passes through the slit. A TRA25CC miniature motorized actuator (Newport) controlled by a SMC100CC single axis motion driver (Newport) was used to control the light beam position. The piezoelectric positioner controls, with micrometer precision, the Z (interfacial normal) position of the light beam with respect to the interface, allowing light to pass at known distances from the interface. A block that supports the slit and lenses is attached to the positioner, so that the motorized actuator movement is transmitted to the whole block and the light beam passes with great accuracy across the cell. The slit is located between two collimating lenses with fitted optical fibers. The four-electrode spectroelectrochemical cell has been described previously.18 Briefly, it is based on a rectangular quartz spectrophotometric cuvette (45 × 12 × 2 mm) with a 2 mm light path (104.002F-QS, Hellma). Reference electrodes are accommodated in Luggin capillaries made in micropipet tips and placed close to the interface. The organic auxiliary electrode was coated with glass to avoid contact with the aqueous phase. Auxiliary and reference electrodes were 0.75 mm diameter platinum wires (Advent Technologies) and 0.37 mm diameter silver/silver chloride wires produced in-house, respectively. The silver/silver chloride reference electrodes were made by oxidation of a silver wire in a solution of lithium chloride. All potentials are reported with respect to these reference electrodes and, by convention, a positive potential difference implies that the aqueous phase is held positive with respect to the organic phase. The spectroelectrochemical cell is fastened in such a way that the support of the slit and lenses moves up and down without perturbing the electrochemical system and the position of the L/L interface does not change during the experiments. Spectroelectrochemical experiments were carried out using a PGSTAT 302N potentiostat (Eco Chemie B.V.) coupled to a QE65000 spectrometer (Ocean Optics). The light beam, supplied by a DH-2000 Deuterium-halogen light source (Ocean Optics), was both guided to and collected from the
interface, based on the parallel incidence of the light beam with respect to the interface. The spectroelectrochemical cell is based on a rectangular quartz spectrophotometric cuvette (45 × 12 × 2 mm) with a 2 mm light path-length.18 In this work, we performed spectroelectrochemical measurements in which the position of the light beam with respect to the ITIES was varied with a moving slit controlled by a piezoelectric micropositioner. In such a way we control, with micrometric precision, the exact distance of the light beam with respect to the L/L interface, allowing us to obtain information about the species in the adjacent solution during the reaction at the interface. The capability of varying and controlling the distance of the parallel light beam from the interface gives a noticeable improvement in the spatial resolution of spectroscopic techniques. A simple slit movement allows us to sample either the organic or the aqueous phase in a much easier way than with any other spectroelectrochemical setup. The idea of following the progress of the concentration profiles in a solid electrode/solution interface was developed by McCreery and co-workers,19,20 who used a laser beam to sample the diffusion layer. In this way they obtained the concentration profiles of species absorbing at the wavelength of the incident laser. Schindler and co-workers21,22 used a similar setup for studying polymerization processes, also on solid electrodes. In these works, a monochromatic light beam was positioned parallel to the electrode surface and absorbance changes with the distance to the electrode were recorded on a diode array, so that a concentration profile was obtained for each time. More recently, Amatore and co-workers23 also obtained concentration profiles by using a fiber-optic system and an epifluorescent microscopy detector based on a diode array. The results were limited to fluorescent species. In 2006, Garay and Barbero24,25 used a piezoelectric positioner to move a laser light in “probe beam deflection”, thereby also obtaining concentration profiles. Gyurcsanyi and Lindner26 suggested an approach based on microscopic measurements to follow changes of the free ionophore concentration across ionselective membranes. In our work, we have constructed a new device based on a piezoelectric positioner that is used to control and move a slit with respect to the L/L interface; thus, we do not obtain a concentration profile directly but by the evolution of the spectra with time and distance when the interface is sampled by moving the slit at a fixed scan rate. Computer-controlled piezoelectric positioners are commonly used in some analytical techniques such as scanning electrochemical microscopy (SECM). However, to the best of our knowledge, they have not been used in combination with UV/vis absorption spectroelectrochemical techniques. Combination of the spatial resolution, electrochemical data, and information obtained from molecular absorption spectroscopy can provide useful data on the L/L interfacial process. For these reasons we have called this technique spatial scanning spectroelectrochemistry. We have used the new device to study electrochemical processes at the ITIES in order to illustrate the straightforward alignment of the light beam in L/L spectroelectrochemistry and its suitability to analyze complex electrochemical processes in a single experiment.
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EXPERIMENTAL SECTION Tris(2,2′-bipyridine)ruthenium chloride, Ru(bpy)3Cl2·6H2O, lithium chloride (Sigma-Aldrich), lithium perchlorate (Merck), butylferrocene (BuFc, Sigma-Aldrich), and potassium 5724
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spectroelectrochemical cell by 230-μm diameter optical fibers (Ocean Optics). Figure 1 shows a schematic of the experimental setup.
absorption either within the aqueous or the organic phase. In this case, the slit is placed at a fixed position close to the aqueous solution (or the organic one) in such a way that the light beam passes parallel and adjacent to the interface. In both cases a potential of +0.38 V was applied for 40 s and UV/vis absorption spectra were measured during the potential step. Absorbance at a fixed position (adjacent to the interface) and recorded at different time in the two independent experiments is plotted in Figure 2A. As can be observed, the evolution of the
Figure 1. Schematic of the experimental setup.
Transmission electron microscopy (TEM) was performed with a Tecnai F30 FEG-TEM system operating at 300 kV. The samples were extracted directly from the electrochemical cell to the TEM grid (holey carbon, 400 mesh, supplied by Agar Scientific). The validation of the new device was performed using the following cell configuration (cell 1):
where the double bar denotes the interface to be polarized. Electrodeposition of Pd nanoparticles at the interface was studied using the following cell configuration (cell 2): Figure 2. Chronoabsorptometric experiments by applying +0.38 V to cell 1 for 40 s. (A) Spectra recorded during two separate chronoamperometric experiments, sampling the aqueous phase (green) and the organic phase (red). (B) Spectra recorded in a single experiment while the slit is being moved from the aqueous solution to the organic phase at 50 μm s−1. The positive distances denote the distance along the interface normal in the organic phase, the negative values correspond to the aqueous phase.
where the double bar denotes the interface to be polarized.
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RESULTS AND DISCUSSION Technique Validation. As one of the goals of this work was to develop a new device for UV−visible absorption spectroelectrochemical measurements at the ITIES, the performance of the technique was evaluated by following the transfer of Ru(bpy)32+ across the polarized L/L interface. This electrochemical process has been taken as a model reaction using different spectroelectrochemical configurations,16−18 because Ru(bpy)32+ exhibits a well-defined ion transfer at the water|1,2-DCE interface16 and has a large molar absorption coefficient in water, εwater = 12 794 M−1 cm−1 at 453 nm (experimental value estimated in our laboratory, in accordance with the reported value17). The validation of the new device was performed using cell 1, described in the Experimental Section. In order to compare the response at the two phases, twin chronoamperometric experiments were performed sampling
spectra in the two phases is almost a mirror image. There is no significant shift of the Ru(bpy)32+ absorption band due to the solvent, an observation consistent with the ease of transfer of this ion. The absorbance band around 453 nm corresponding to Ru(bpy)32+ decreases when the light beam passes, sampling the aqueous phase, while, on the contrary, it increases when it is sampling the organic one. This shows that we can precisely place the light beam very close to the interface with our device. The precise position of the slit is easily obtained, when the light beam passes through the two interfaces a marked change in absorbance is observed because of the different optical properties of the two solvents. When the light beam is placed adjacent to the interface and it is moved toward the bulk solution, if no electrochemical reaction takes place there is no change of absorbance. However, all the information related to the ion transfer can be extracted in a single experiment if the light beam is spatially 5725
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electron transfer between the aqueous phase tetrachloropalladate(II) anion and the organic phase electron donor, butylferrocene (BuFc), following the reaction7,8,31
scanned from the aqueous phase to the organic one during the experiment. The slit was placed 1 mm away from the interface and was moved from the aqueous phase to the organic one. A potential of +0.38 V was applied for 40 s, and the slit movement at 50 μm s−1 was initiated when the potential was applied. Figure 2B shows, as a contour plot, the spectra recorded with respect to distance from the interface during a chronoamperometric experiment. Only the distances close to the interface (
