Letter pubs.acs.org/Langmuir
Site Selective Generation of Sol−Gel Deposits in Layered Bimetallic Macroporous Electrode Architectures Hélène Lalo,† Yémima Bon-Saint-Côme,† Bernard Plano,‡ Mathieu Etienne,§ Alain Walcarius,§ and Alexander Kuhn*,† †
Université de Bordeaux, ISM, UMR 5255, 16 av. Pey Berland 33607 Pessac, France Université de Bordeaux, IMS, 351 cours de la libération 33405 Talence, France § LCPME, CNRS-Nancy-University, 405 rue de Vandœuvre, 54600 Villers-lès-Nancy, France ‡
ABSTRACT: The elaboration of an original composite bimetallic macroporous electrode containing a site-selective sol−gel deposit is reported. Regular colloidal crystals, obtained by a modified Langmuir−Blodgett approach, are used as templates for the electrogeneration of the desired metals in the form of a well-defined layered bimetallic porous electrode. This porous matrix shows a spatially modulated electroactivity which is subsequently used as a strategy for targeted electrogeneration of a sol−gel deposit, exclusively in one predefined part of the porous electrode.
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electrodeposition of metals into the colloidal crystal template.9 A precise control of the layer thickness is possible due to regular current oscillations, observed during the metal growth through the colloidal crystal, which are due to periodic variations in the active electrode surface area. It is therefore possible to deposit different metals successively in the same crystal while controlling perfectly the thickness of each metal film at the nanometer scale.10 If the different deposited metals have different electrocatalytic activities, it is possible to trigger subsequently specific reactions on one of the metals by choosing an adequate potential and reaction medium. We illustrate this in the present contribution in a proof-of-concept experiment by using a porous and layered bimetallic gold− platinum structure. These two metals have a very different reactivity with respect to the reduction of protons into hydrogen at a given potential. This leads to a localized change in pH, which allows to selectively trigger the formation of a silica deposit in the platinum pores via the polycondensation of appropriate precursor molecules. This strategy is complementary to other approaches that have been reported in the literature for the formation of porous sol−gel films on electrodes11,12 and opens up new perspectives for the generation of designer materials with spatially distributed properties, that are interesting for applications like prescreening and filtering of molecules via size exclusion, electrochemical gating, and tailoring of diffusion constants. If the silica fills completely the upper part of the pores, the obtained structures can also be used as micro- or nanocontainers for electroactive species with catalytic or mediating properties.
INTRODUCTION Controlling the spatial distribution of chemical reactions is of general interest for a broad variety of applications ranging from material science to catalysis and biomimetic systems.1 One possibility to achieve such selective reactivity is to use electrochemistry along with an inhomogeneous distribution of the electric field or patterned catalyst structures.2−6 In these cases, the selectivity has been mostly used to decorate surfaces, because those concepts are essentially restricted to twodimensions. An interesting challenge, that has been much less addressed so far, is to obtain spatial reaction selectivity in a three-dimensional system. Here we study this question by using a porous metal structure and selecting on purpose only parts of the system for a reaction by playing with the intrinsic electrochemical kinetics of the different porous layers. The rationale behind this approach is that different electrode materials show different reactivity for one and the same redox couple. This means that by carefully selecting the potential that is applied to an electrode composed of different materials, one can favor a certain oxidation or reduction reaction on one specific part of the electrode, whereas the same reaction is completely or at least partially suppressed on other parts of the electrode. In the case of an electrode that has three-dimensional features, like it is the case for porous systems, this gives access to the interesting possibility of focusing the electrochemical reaction to a well-defined position in the volume of the electrode. The porous structures used in this study are obtained by using the Langmuir−Blodgett (LB) technique. This technique that is typically employed for generating films of amphiphilic molecules is used here to generate a regular colloidal crystal composed of amphiphilic silica beads.7,8 These structures will act as a template for the subsequent electrode fabrication. A well-organized macroporous structure can be obtained by © 2012 American Chemical Society
Received: November 27, 2011 Revised: January 14, 2012 Published: January 19, 2012 2323
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Figure 1. (A) SEM side view of a colloidal template composed of five layers of 1200 nm silica particles and six layers of 500 nm particles on the top. (B) Chronoamperometric curve corresponding to the electrodeposition of gold in the first part of the template with the 1200 nm particles. Potential for deposition is E = −660 mV (vs EAg/AgCl). (C) SEM image of a section of the corresponding bimetallic material after template removal. The lower big pores are made out of gold, whereas the upper small pores are composed of platinum (deposition potential +140 mV vs EAg/AgCl).
Figure 2. (A) Comparison of water reduction signals for sol−gel polycondensation on gold and platinum (pH = 9). At the indicated potential (Eexp), almost no proton reduction occurs on gold, whereas hydrogen production is possible on platinum. This leads to a preferential pH increase in the platinum pores and thus to the polycondensation of the sol−gel in the platinum structure and not in the gold pores. (B) Principle of the silica sol− gel formation. The hydrolysis is carried out for 2.5 h under stirring. Then, by reducing electrochemically the protons, the pH can be locally increased and leads to the formation of a silica sol−gel at the electrode surface.15
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Scanning Electron Microscopy (SEM) Characterization. SEM experiments were carried out on a Hitachi TM-1000 tabletop microscope.
EXPERIMENTAL SECTION
Colloidal Templates. Silica particles of two diameters (500 and 1200 nm) were synthesized according to batch or semicontinuous procedures inspired from the Stöber sol−gel process and then assembled into a colloidal crystal on a gold coated glass slide (typical size in the cm2 range) as described previously.9,10,13,14 Potentiostatic Growth of the Metal Deposit. After the formation of the silica template, the gold-coated slides were used as working electrodes in a typical three-electrode cell with a platinum foil as counter electrode and an Ag/AgCl electrode as reference. A gold plating bath purchased from Metalor (ECF-63) or H2PtCl6 (Sigma) was used as received for the metal deposition. During the potentiostatic metal growth, the intensity of the faradaic current was measured with an Autolab PGSTAT 20 potentiostat (EcoChemie) system, monitored by a PC running the GPES 4.9 software. Electrogeneration of Silica. Sol−gel silica thin films were potentiostatically deposited (−0.8 V for 600 s) into the porous structure from precursor sols consisting of 20 mL ethanol (95−96%, Merck), 20 mL solution of 0.1 M NaNO3 (99%, Fluka), and 240 μL of 0.1 M HCl (37%, Riedel de Haen), to which was added tetraethoxysilane (TEOS, 99%, Sigma-Aldrich).
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RESULTS AND DISCUSSION
We have first elaborated a colloidal crystal that is composed of two sections with different bead sizes (Figure 1A). The deposition of a controlled number of layers of beads with a diameter of 1200 nm is followed by the deposition of several layers of beads with a diameter of 500 nm. In the illustrated example, gold with a thickness of five pore layers has been deposited into the interstices of the 1200 nm part of the template and then six pore layers of platinum in the 500 nm section of the same crystal. It is, in this case, absolutely crucial to be able to stop the electrodeposition precisely at a predefined thickness corresponding to the initially adsorbed number of bead layers of a given size. This is possible due to the occurring current oscillations during the potentiostatic metal deposition (Figure 1B). Indeed, as the metal grows into 2324
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the template, the active surface area decreases until a minimum is reached at the half-height of a bead layer. The current then increases again, due to the higher active area of the growth front, until the complete filling of a bead layer.9,10 After dissolution of the template beads with 5% hydrofluoric acid solution, a bimetallic macroporous electrode is obtained (Figure 1C). The most important aspect for the subsequent site selective generation of the sol−gel layer is the different reactivity of platinum and gold with respect to the reduction of protons into hydrogen. In fact the electrokinetics of this reaction is very different on both metals. Figure 2A illustrates that for achieving the same reaction rate for hydrogen production on the two metals, which is translated by the current density, a much higher potential is necessary at gold electrodes. Therefore, in the case of a platinum electrode, the reaction is much more efficient compared to gold, for a given potential. As the production of hydrogen involves at the same time an increase in pH at the surface of the electrode, this can be used to change locally the pH only in the porous platinum layer when a given overpotential is applied. This is the crucial point of the experiment, because it is known that such an electrochemically triggered pH change can be used to induce the formation of silica sol−gel layers at the electrode surface,15 via the mechanism illustrated in Figure 2B. This sol−gel process allows the generation of hybrid materials, which exhibit many interesting properties (mechanical, optical, ionic, etc.),16,17 that can be used for various applications ranging from biosensors to catalysis and fuel cells.18−22 The approach is simple, low cost and allows the fabrication of nanocomposite hybrid devices through hydrolysis of organically modified metal alkoxides. As in the present case, the sol−gel process is electrochemically triggered, it is possible to precisely fill the pores of the electrode with a controlled thickness as it has been observed for analogue systems.19,23 Most importantly, one can benefit from the different electroactivities of Pt and Au with respect to proton reduction in order to generate the sol−gel selectively in the part of the macroporous structure made out of platinum, if the right potential is chosen (Figure 2A). A similar selectivity has also been observed on surfaces decorated with Pt particles.24 In our example, another advantage of the configuration of the bimetallic electrode is the smaller pore size of the platinum layer. This allows confining even more the polycondensation process to the platinum pores and modify them quickly and selectively without generating a silica deposit in the gold pores. After having generated the layered bimetallic macroporous electrode, the sample is dipped into the silica sol, and a potential of −0.8 V is applied during 600 s. Thus, the reduction of H+ triggers the polycondensation of sol−gel and silica is generated, locally confined in the platinum pores. Depending on the duration of this silica deposition step, one can generate either very thin layers just along the pore walls (down to a few nanometers thickness) like it has been demonstrated recently19 or fill the pores completely as it has been shown for similar systems.23 The complete filling is possible because the generated silica has its own porosity and therefore does not prevent further proton reduction. The resulting electrode can be seen in Figure 3A. The SEM image shows that the porous electrode is composed of two sections with different pores sizes: a first one with big gold pores (1200 nm in diameter), and a second layer with smaller platinum pores (500 nm in diameter).
Figure 3. (A) SEM image of an electrode section. Irregularities of the section originate from the cutting with a diamond pen. The smaller pores at the top are made out of platinum, whereas the bigger pores at the bottom are composed of gold. The indicated black square illustrates the area where elemental cartography by EDX has been performed. Red squares indicate the parts of the sample that have been used for spectroscopic analysis. The blue line indicates the scan direction for which the chemical profile has been measured. (B) EDX cartography of a sample section (black frame in part A). The analysis shows platinum in upper layers and gold in lower ones. Silica is generated exclusively in the porous platinum layer. Small traces of silica are visible in the gold part due to a transfer of some silica by the diamond pen. (C) Element analysis from two distinct regions of the electrode (red frames in Figure 3A). In spectrum 1 (black), platinum, silicium, and oxygen bands are visible, while in spectrum 2 (blue) one can only detect the presence of gold
To get further insight, energy dispersive X-ray (EDX) analysis has been performed for identifying the elemental composition of the sample. The first analysis shown in Figure 3B illustrates the separation between gold and platinum in the electrode structure. One can see clearly that, as expected, a layered bimetallic macroporous electrode has been obtained. The middle image demonstrates the preferential presence of silica in the platinum layer. However, silica traces can also be observed in the gold layer. Most likely, this is a pollution coming from the upper part of the electrode. Indeed, during the cutting, small parts of the sol−gel are transferred from the top of the electrode to the bottom due to the use of a diamond cutter. Another possibility is that plumes of higher pH penetrate also slightly the gold layer at certain locations. The overall very good selectivity is confirmed by the element spectra in Figure 3C. One can clearly observe the simultaneous presence of platinum, silicium, and oxygen in the same part of the electrode (spectrum 1, black). In contrast, in spectrum 2 (blue), only the presence of gold is detected when averaging over the squares indicated in Figure 3A. This second analysis confirms the presence of polycondensed sol−gel only in the platinum part. This selectivity becomes even clearer when comparing the element profiles of the line scan presented in 2325
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Figure 4. EDX analysis of the presence of platinum, silica, and gold along the blue line in Figure 3. One can see that platinum is present on the left side of the scan whereas gold is detected in the right part. Silica is exclusively present in the part made out of platinum.
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ACKNOWLEDGMENTS We gratefully acknowledge the European Community’s Seventh Framework Programme (Grant Agreement No. 202167) for its financial support of the ERUDESP project.
Figure 4, obtained by scanning along the blue line indicated in Figure 3A. It is very obvious that the presence of the Si signal is intimately related to the presence of Pt.
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CONCLUSION
We have shown that hybrid macroporous materials can be prepared with a predefined distribution of the different components. In a first step, a layered bimetallic macroporous electrode is generated by using the Langmuir−Blodgett technique to build up a template structure with different bead sizes, combined with a subsequent electrodeposition of two different metals. Using the Langmuir−Blodgett approach has the advantage that samples of quite big dimensions (cm2 range and higher) can be produced with very good homogeneity, avoiding the usual crack formation that is typically observed for many other template formation processes.9 By choosing two metals with significantly different overpotentials for proton reduction (gold and platinum), it is possible to electrogenerate silica sol−gel deposits with a three-dimensional site selectivity. The smaller overpotential, required for water reduction on platinum, allows generating localized pH changes, that will trigger a targeted polycondensation of the silica sol−gel exclusively in the platinum pores, without affecting the gold layer. Additionally, the smaller size of the platinum pores, compared to gold, favors filling of the platinum layers without penetration of the sol−gel into the gold pores. This strategy is an easy way of electrochemically addressing and modifying well-defined areas of three-dimensional material structures, due to the spatial distribution of chemical reactivity. The resulting silica sol−gel deposit can be, for example, considered as a membrane, which might be used as a filter, a mechanical support, or as an immobilization matrix for molecules in the macroporous structure at specific locations. When the inside of the selected pores is only decorated with a thin silica layer, for example, enzymes can be fixed without hindering substrate diffusion.19 The targeted pores can also be completely filled, analogous to what has been obtained with electrophoretic paints,23 which allows in the present case delimitation of a confined porous reaction space in the layer below the silica modified pores, that can entrap specific molecules such as catalysts.
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[email protected]. Phone: +33 5 40 00 65 73. Fax: +33 5 40 00 27 17. Notes
The authors declare no competing financial interest. 2326
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