Fabrication of Zeolite‐Modified Electrodes via Electrophoretic Deposition

Abstract

Zeolite-modified electrodes (ZMEs) have been investigated for their novel properties of analyte preconcentration and ion-selectivity based on size, shape, and charge selectivity of zeolites. 1,2 A recent review of the literature on ZMEs by Walcarius, 3 which emphasizes the period of 1990-1995, includes references to previous reviews (such as Ref. 1, 2, 4) of preparation methods, the influence of supporting electrolyte, the molecular and electronic transfers, solid state chemistry, the use of ZMEs with various electrochemical probes, and numerous applications. Also discussed is the progress in understanding the importance of ion exchange and intrazeolite electron transfer and transport of ZMEs, particularly the ongoing work by Baker's group. 5,6 As reviewed by Walcarius, much effort has been made to prepare ZMEs in a reproducible way to display good sensitivity, selectivity, and durability. 3 Walcarius classified the wide range of different fabrication procedures to give seven basic types of ZMEs: zeolitic membrane, zeolite/polymer film, zeolite-containing carbon paste, zeolite-containing carbon composite, covalently linked zeolite, pressed zeolite, and zeolite suspensions. Early fabrication of ZMEs generally involved the immobilization of zeolite particles in a polymeric binder, traditionally nonconductive polystyrene. Such electrodes invariably have been plagued by poor reproducibility, lack of mechanical robustness in a stirred solution, and nonideal electrochemical behavior due to large regions of the metal electrode surface being in contact with the insulating binder. 2,7,8 Li, Calzaferri, 9,10 and with Pfanner, 11,12 and Calzaferri et al. 13 made monograin-layer ZMEs on glassy carbon electrodes by slow evaporation of an aqueous suspension, followed by deposition of a thin poly-strene film, to yield mechanically stable, dense ZMEs. However, polystrene is limited to mainly aqueous solutions due to its great sol-ubility in many organic solvents. By using polyacrylic acid as the polymer overcoat, this method was also shown viable for preparing ZMEs for aprotic media. 6,14 Also, the "monograin approach" has been modified recently by depositing carbon with the zeolite to increase the area of electrical contact. 14 Zeolite-containing carbon paste ZMEs are easy to prepare, mechanically stable for many hours in stirred solutions, and provide reproducible results if the particles are homogeneously dispersed into the paste. Carbon is incorporated to maintain electrical contact between the working electrode and the zeolite particle. The most robust zeolite-containing carbon composite is cited to be made of zeolite particles and carbon powder dispersed into a cross-linked copolymer of styrene and divinylbenzene (Shaw and Creasy 15). Since electroac-tive carbon is in contact with the electrolyte solution, zeolite-excluded ions may still be detected. However, carbon paste ZMEs recently have been made as sensor electrodes for the voltammetric determination of trace copper (0.3-1.0 mg/L) in aqueous solutions. 16 There has been discussion in the literature concerning the different electrochemical behaviors attributed to the different morphologies of the zeolite layers 12,17 and experimental protocol 6,18 for making ZMEs. The main problem of ZMEs, particularly for electro-analysis, was stated by Walcarius as the lack of reproducibility, 3 due to the heterogeneous nature of most ZMEs and the difficulty of surface renewal. The importance of preparing electrodes with a large direct contact area between the zeolite and the electrode for fundamental studies has been emphasized. 12 The influence of the type of carbon and the polymeric binder on the voltammetric response of carbon-composite electrodes has been examined. 6 It was pointed out that careful background studies are critical in ruling out contributions from components other than the zeolite in the ZME. The protocol to compare different experimental results includes the reporting of the specific sources of all components in the ZMEs, which is particularly important for carbon cohering components. 6,18 Ground or pressed ZMEs, in which mechanical force can fracture supercages and lead to loosely associated transition metal complexes, 6 has been found to markedly influence the observed electrochemical response. 14 Stirred zeolite suspensions at a large area electrode, 14 if applicable, eliminate the complications of electrode binders, carbons , and mechanical handling necessary to prepare a ZME. This paper describes a new method to fabricate ZMEs using elec-trophoretic deposition (EPD) of the zeolites. During EPD, charged particles suspended in a liquid are deposited onto a conductive sub-strate under the influence of an electric field. The advantages of this process are the uniformity of deposition even for complex shapes, control of deposit thickness, reduction of waste often encountered in dipping or spraying techniques, low levels of contamination, and continuous processing. EPD has been used for a variety of materials and applications, including the EPD of phosphors in the manufacture of screens for advanced information displays. Our previous studies have focused on the fundamentals of the EPD process for the deposition of phosphor particles: charging of the particles, 19 transport of the particles to the electrode, deposition conditions , 20 and more recently, adhesion strength. 21,22 The EPD bath of interest in these studies was a suspension of phosphor particles in iso-propyl alcohol (IPA) which contains dissolved Mg(NO 3) 2 and small amounts of water. The Mg(NO 3) 2 dissociates slightly, providing ions to charge the particles positively. Hydroxides and alkoxides form via cathodic precipitation and are the binder materials for the phosphor particles in the deposited layer. 22,23 It has been shown that a minimum amount of binder is necessary for particle adhesion. 20,21 Zeolite-modified electrodes (ZMEs) have been fabricated via electrophoretic deposition (EPD) onto a platinum rotating disk electrode. A polymer (polystyrene or Nafion) was coated over the zeolite deposit to impart mechanical stability. While the EDP zeo-lite-Y/polystyrene electrodes showed poor mechanical stability, the EPD zeolite-Y/Nafion electrodes exhibited a high level of mechanical stability in an aqueous solution, even after many hours of stirring. The electrochemical properties of the electrodes were tested by reproducing the cyclic voltammetry experiment previously reported for a zeolite-Y/polystyrene slurry electrode, using Cu 2 as a probe species. The EPD zeolite-Y/polystyrene electrode showed a 44% increase in analyte preconcentration, due to the increase in zeolite density near the electrode surface. The EPD zeolite-Y/Nafion electrode showed only a 7% increase in analyte preconcentration compared to the original report, with the lower level in comparison to the polystyrene electrode due to competing ion exchange reactions in the Nafion layer.