Article pubs.acs.org/Langmuir
Sol−Gel Route to Zirconia−Pt-Nanoelectrode Arrays 8 nm in Radius: Their Geometrical Impact in Mass Transport Olivier Fontaine, Christel Laberty-Robert,* and Clément Sanchez Laboratoire de Chimie de la Matière Condensée de Paris, UPMC-Paris 06, CNRS-UMR 7574, Collège de France, 11 Place Marcelin Berthelot, 75005 Paris, France S Supporting Information *
ABSTRACT: The fabrication of advanced nanoelectrode arrays and their electrochemical characterization are presented. These nanoelectrode arrays are constituted of nanoperforations of 8 nm in radius leading to platinum and protected by an inorganic matrix made of crystalline zirconia. These nanoelectrodes arrays provide a ceramic support with a high thermal and chemical stability. These devices present a well characterized structure with a control of size, shape, and spacing of the nanoelectrodes, allowing studying in depth both the mass transport and the charge transfer properties in the nanometer range. The radial diffusion occurs when the experimental scan rate is superior to a theoretical scan rate estimated from the model proposed by Amatore and colleagues. The coupling between electrochemical analysis and nanoscale structural characterizations successfully demonstrates that the theory defined for microelectrode arrays can be directly transposed for well-defined metal−ceramic nanocomposite nanoelectrodes.
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INTRODUCTION Over the past two decades, nanoelectrode and microelectrode arrays have gained importance in chemical and biochemical sensor technologies1−8 and in electronics/optoelectronics devices.9−11 Simultaneously, many works have been devoted to the theory and the simulation of electrochemistry data at the nanoscale.12−17 However, in all these studies, the electrode arrays have been described as the assembly of planar or recessed micro- or nanodisk electrode18 or micro- or nanoband electrode arrays,19−21 but no straightforward correlation between the materials structural characterization at the nanoscale and their electrochemical response has been performed. Two different strategies (top down and bottom up) have been mainly developed for the fabrication of microelectrode arrays. Both methods have pros and cons and exhibit their own specificity and potential. Physical methods (top down), including focused ion beam, nanosphere lithography,22 dip-pen nanolithography,23,24 and microcontact printing methodologies,25 allow the fabrication of microelectrode arrays exhibiting well-dispersed and organized microdisks onto the substrate surface. However, these processes require expensive commercial equipment that allow producing nanoelectrode arrays with individual diameters of about 100 nm. Chemical approaches (bottom up), such as dispersing block copolymers onto the electrode surface, have also been developed.26,27 Usually, these techniques are less expensive compared to physical ones and they allow to reach smaller nanoelectrode sizes (10−20 nm in diameter). However, it is often more difficult to control the geometry of the assembly of the microdisk onto the surface of the electrodes and diffusional interferences between adjacent electrodes are often critical with © 2012 American Chemical Society
nanoelectrodes built from soft templates. In general, achieving stable nanoelectrode arrays with a diameter much smaller than 100 nm appears to be difficult and constitute real challenges. The use of electrodes in the dimension of ∼10 nm is interesting because the size of the electrode and the solvated molecules are comparable. For electrodes having sizes approaching molecular dimensions, deviations between the theoretical and experimental electrochemical responses have been observed. For example, White and colleagues28,29 demonstrated that band nanoelectrodes having electrodes with 2 nm width exhibited current responses an order-of-magnitude smaller than the ones predicted by the standard microelectrode theory.21,22 Furthermore, Martin and colleagues30 showed that electrochemical responses of such nanoelectrodes agreed well with the theory for microelectrode arrays having overlapping diffusion regimes.31 In Figure 1, we have summarized the critical geometric parameters that need to be controlled to tune the performance of nanoelectrode arrays: (i) the interelectrode spacing, d, (ii) the radius of the nanoelectrode, (iii) the recessed (associated to the thickness of the inorganic layer) or the shape factor, and (iv) the nature of the layer (polymer, inorganic, hybrid). For optimum performance of the disk-shaped nanoelectrode arrays, the individual disk-shaped electrode should behave as isolated electrodes, appearing as sigmoids on cyclic voltammetry (CV) curves. Such a behavior is strongly dependent on the time scale of the experiment and on the interelectrode spacing. Received: July 13, 2011 Revised: January 17, 2012 Published: January 19, 2012 3650
dx.doi.org/10.1021/la202651b | Langmuir 2012, 28, 3650−3657
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Article
EXPERIMENTAL SECTION
Chemical and Materials. Nanoelectrodes array were synthesized from zirconium oxide nanoperforated membranes deposited on a conductive platinum layer on Si(001) substrates (Si-Tech, Inc.) by modification of our previously reported procedures.38−40 The process involved nanotexturation of a surface through solution deposition of zirconia precursors using block-copolymer micelles, followed by a short thermal treatment in air. Solution A consisted of polybutadieneb-polyethylene oxide [PB-b-PEO; MWPB = 5500 g·mol−1, MWPEO = 30 000 g·mol−1 (Polymer Source, Inc.)], EtOH (4.5 g; absolute ethyl alcohol, Normapur), and water (0.2 g). Solution A was placed in a drying oven at 70 °C until the copolymer PB-b-PEO was completely dissolved, and then it was left at room temperature to equilibrate. Solution B, made of ZrCl4 (100 mg, Cl > 98%; Fluka AG) and EtOH (1.5 g), was added to solution A. The final sol−gel solution C was stirred for 30 min at room temperature for homogenization. ZrO2based films were elaborated by dip-coating the Pt/Si(001) substrates into the sol−gel solution C at a constant withdrawal speed of 0.3 mm·s−1 under a constant temperature of 70 °C and controlled relative humidity (
