Functional Hybrid Materials Chapter 6

6 Electrochemistry of Sol-Gel Derived Hybrid Materials Pierre Audebert and Alain Walcarius 6.1 Introduction Sol-gel derived hybrid materials have attracted substantial attention because they present special challenges and opportunities with respect to potential applications in various fields, including optical and electronic materials, solid electrolytes, pro- tective coating technology, sensor devices, catalysis, separation science, biology, and electrochemistry [1, 2]. Hybrid materials lie at the interface of the organic and inor- ganic realms. By combining organic and inorganic components into a single com- posite, the versatility of sol-gel processes provides a rather straightforward way to produce a wide range of organic-inorganic hybrid materials with numerous pro- mising applications [2]. Several well-documented reviews are available, dealing with their use in connection with electrochemistry [3–7], their application for electroa- nalytical purposes [7–11], and their exploitation in analytical chemistry [12–14]. Organic-inorganic hybrids are often classified into two main categories: (i) “class I materials”, in which the organic and inorganic components are weakly linked through hydrogen bonding, van der Waals contacts, or electrostatic forces, and (ii) “class II materials”, in which the constituents interact strongly through ionic or covalent bond formation [1]. Four different routes are mainly used to prepare orga- nic-inorganic hybrid materials [2]: ∑ Impregnation of a porous inorganic matrix (most often silicon dioxide) by orga- nic components, which can even be polymerized in situ, displaying a particular affinity for the host structure (mainly class I hybrids); ∑ Dispersion or solvation of the organic compound in a sol-gel mixture, which is often referred as sol-gel doping, where the organic component is physically ent- rapped in the three-dimensional inorganic structure during its formation (class I hybrids); ∑ Use of adducts with at least one direct, non-hydrolyzable heteroatom-carbon bond, that can be either fixed by post-synthesis grafting onto an as-synthesized inorganic support, or alternatively, be introduced as an organofunctional pre- cursor in the starting sol and polycondensed with the other sol-gel precursor(s), 172 Functional Hybrid Materials. Edited by Pedro Gómez-Romero, Clément Sanchez Copyright © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 3-527-30484-3 or even prepared by combining these two approaches (class II hybrids). Both amorphous and mesoporous crystalline materials can be obtained in to these ways. These processes can even be associated with impregnation or sol-gel doping to produce hybrids of classes I & II in a single material; ∑ Intercalation compounds and interpenetrating organic-inorganic polymers can be obtained by combining layered materials with organic polymer chains (class I and class II hybrids). In principle, these approaches that exploit most often the versatility of sol-gel pro- cessing are offering exceptional opportunities to create a wide range of new pro- ducts displaying tailor-made composition, structure, and properties, which can be tuned to fit the desired target application. These materials are not only combining the distinct properties of organic and inorganic components within a single com- posite, but new or enhanced phenomena as well as novel truly unique properties may arise as a result of the interface between the organic and inorganic worlds. The full control of all the experimental parameters affecting the synthetic pathways, however, will still require much work in the future, even though considerable advances have been performed recently [2]. This emerging field of materials science has generated considerable and increa- sing interest from the electrochemical community during the past decade [3–11]. On the one hand, electrochemistry was applied as a characterization technique to describe the basic behavior of sol-gel processed materials and to characterize mass transfer reactions in xerogels and, on the other hand, the intrinsic properties of organic-inorganic hybrids (mainly those based on silica) were exploited in various electroanalytical and sensor applications as well as in power source technology as solid electrolytes. Although gelled solvents have been known for years (Napalm is a regrettable example of this), electrochemical investigations into gelled systems have been very slow to start, especially molecular electrochemistry, the main problem being the volume of the solution/gel to deal with, as well as the difficulty of gelling polymer- solvent mixtures in confined vessels of unspecified shape. This has been overcome with the use of hydrophobic Nafion gels [15]. Performing electro-chemistry in such gels requires dropping an aliquot of the hydrophobic gel on a millimeter size elec- trode and allowing it to dry, and using the resulting device as a working electrode in an aqueous electrolyte [16, 17]. Such systems have been shown to operate well, including in a few analytical applications [17]. In such systems, it was shown for the first time that, contrary to classical Nafion modified electrodes with ionically bounded redox sites [18], the included redox compounds were transported with a diffusion coefficient characteristic of the solvent used to prepare the gel. Another original exploitation of electro-chemistry was the determination of the glass transi- tion temperature of a concentrated acrylate gel through the time dependence of the diffusion coefficient of included electroactive species [19]. This latter work has ope- ned the way to fundamental investigations in oxide gels, and therefore the use of electrochemical techniques as a “spectroscopy of gelled matter”. This aspect will be presented in the first part of this chapter. 6.1 Introduction 173 In parallel to the fundamental investigations, the numerous intrinsic properties of sol-gel-processed hybrid materials, sometimes unique, are very attractive in elec- troanalytical chemistry for designing new modified electrode devices with impro- ved performance in a continuously expanding frontier at the interface of electro- chemistry and materials science. The desire to combine the specific properties of a particular material with an electrochemical reaction has generated an extraordina- rily wide development of so-called “chemically modified electrodes” (CMEs), which have found numerous applications in electroanalysis [20]. As a general rule, the urge to modify the surface of conventional electrodes makes each modifier with potentially interesting features for a target application very welcome, provided it can be associated with an electrode material as an integrated chemical system. Electro- des modified with sol-gel derived materials constitute a sub-class of CMEs, with the advantage of bringing new or enhanced properties at the electrode surface, such as mechanical stability and durability, the possibility for molecular recognition or discrimination, a spatially-organized porous structure with well-defined organiza- tion, catalytic activity, and selective binding ability. Moreover, the versatility of sol-gel chemistry for producing a wide range of conductive composite materials and for allowing convenient film deposition strategies features an additional advantage in designing CME devices with tailor-made composition, structure and proper- ties. This chapter aims at highlighting the major advances that have been achieved at the intersection of electrochemical science and chemistry of sol-gel derived hybrid materials. It does not aim to be comprehensive (more than 1800 papers have appe- ared up to now on topics combining sol-gel processes and electrochemistry), but is rather focussing to give a rapid survey of the field by way of some striking funda- mental investigations and via some illustrative examples of application; the reader interested in a more in-depth coverage of special topics is directed to the related well-documented reviews [3–14]. The chapter is organized into two main parts: (i) basic electrochemical investigations of hybrid gels and xerogels, and (ii) advanced applications of electrodes modified with sol-gel derived materials, which have been largely described in the fields of electroanalysis. Most work has been devoted to sili- ca-based materials. 6.2 Fundamental Electrochemical Studies in Sol-Gel Systems Beside the widely explored electroanalytical applications, a certain amount of fun- damental studies devoted to the analysis of sols or gels, either in the wet gel or the xerogel state, has been published. The distinction may appear subtle, as since most of the time electrochemistry requires the presence of a solvent and supporting elec- trolyte [21], so even xerogels have to be wetted by an electrolyte solution. However, this allows discrimination between studies in which a xerogel state has been rea- ched before the electrochemical study, and the other case where the gel solvent acts as the supporting electrolyte. 6 Electrochemistry of Sol-Gel Derived Hybrid Materials 174 A large number of basic studies dealing with the electrochemical behavior of a number of oxides or oxide films prepared via a sol-gel process have been publis- hed. Since the purpose of this review is to detail especially the work done on gels, these works will be excluded, the interested reader being directed towards comple- te reviews on this topic [22]. The works involving electrochemistry in sol-gel systems concern almost exclusively silica and silica-based hybrid systems. This is quite understandable, since silica and its corresponding hybrids are much more processable, much more widespread than transition metal oxide gels or xerogels and, in addition, there is a large amount of technical knowledge, which makes easier the choice of the systems that should be more suited to any electrochemistry oriented investigation. However, since electrochemical methods can be a useful tool towards investigating the inner nanostructure of silica gels [23], it should also help in the determination of the nanoscale structure of several non-silica gels, and there is an important need of work in this area. 6.2.1 Electrochemistry into Wet Oxide Gels The works dealing with electrochemistry of wet oxide gels (WOG) aimed mainly at three targets. First of all, it had to be shown that electrochemistry could be perfor- med in a medium such as WOG, although previous works on solvent-wetted orga- nic polymer gels had opened the way [24]. Then two separate directions emerged. Following the first works on polymer gels, it was interesting to check if and how electrochemistry could help to analyze the structure of the gels. In particular, since sols and gels are in constant evolution due to the occurrence of continuous poly- merization and sometimes depolymerization processes, it was of great importance to check how these processes could be followed through the electrochemical response of embedded electroactive probes, either free or linked to the oxide net- work. Second, a goal of special interest was to look at the possibility of taking advan- tage of the electrochemical growth of conducting polymers, to try to polymerize them into WOGs, and prepare interpenetrated networks of oxide(s)-conducting polymers. In addition, a few other works were dealing with various composites involving other polymers. 6.2.1.1 Electrochemistry as a Tool for the Investigation of Sol-gel Polymerization The first fundamental analysis of the electrochemical behavior of free electroactive probes in WOGs was performed by the groups of Audebert and Sanchez [25]. They in particular demonstrated that ferrocene and ferrocene-methanol moieties incor- porated in these media were totally free of motion, by monitoring the microvisco- sity of the wetting solvent through the classical Stokes-Einstein law. In fact, three stages have been recognized in the polycondensation process: (i) the gelling peri- od, during which the probe diffusion remained precisely constant; (ii) aging of the gel, during which the diffusion of the probe was first slowed down, because of the densification of the network, then increased again, because of the decrease of the microviscosity of the interstitial liquid phase; (iii) a period during which the probe 6.2 Fundamental Electrochemical Studies in Sol-Gel Systems 175 motion was extremely stable and through which the gel could be dried up to one third of its initial weight (Fig. 6.1). Recently, these results were confirmed and extended with further details by Collinson et al., by using exclusively the newly developed experimental technique of ultramicroelectrodes (Fig. 6.2), although their studies were centered on the final drying period, disregarding the first steps of the sol-gel process [26]. A striking feature of their results is that the variation of the electroactive probe diffusion coefficient is strongly dependent upon the probe char- ge. While diffusion of cationic and, to a much lesser extent, neutral species was found to decrease during the drying step, there was no noticeable variation of the behavior of the anionic ones. This result can probably be explained by the existen- ce of much stronger interaction between the silica pore walls with cations than with the neutral species, or above all with the anions. Up to now, the only published works focused on pure oxide gels, and no report is available to demonstrate if this is a special feature of silica, or on the contrary a more general behavior extendable to the wider family of oxide gels. In a different approach, Cox et al. have used cyclic voltammetry to demonstrate that diproportionation of uranium(V) in the gels is very sensitive to the local environment and could be a tool for testing the presence of anionic sites in the pores [27]. Finally, a recent study describes the electroche- mically assisted deposition of a hybrid silica film, from the electrooxidation of a tetrafunctionalized phenothiazine [28]. The sol-gel polymerization in this case ari- ses from the electrochemically-induced precipitation of the cation radical on the electrode surface. The triethoxysilane endgroups can then condense because of their high local concentration on the electrode, in the presence of the electrolyte solution. 6 Electrochemistry of Sol-Gel Derived Hybrid Materials 176 Fig. 6.1 Dependence of reduced diffusion coefficient of ferrocene in the gel on reduced gelation time for silica gel. From [25, first], reproduced by permission of The Royal Society of Chemistry 6.2 Fundamental Electrochemical Studies in Sol-Gel Systems 177 In parallel to the efforts directed to the examination of the behavior of “free” molecules embedded in a gel, it was of still greater interest to look at the motion of redox probes anchored within the sol-gel polymer, both in the course of their for- mation/growing, as well as in the final state of the gel. Actually, the only reported work was aimed at investigations of the polymerization of zirconium and (to a les- ser extent) titanium propoxide, in the presence of various chelating agents [29]. It was shown that the degree of polycondensation that can be attained was relatively low (depending however mainly on the chelate content). More impor- tantly, it was confirmed that only weak bonds remained in the wet gel, in the bulk oxopolymer, so that the gel may be considered in fact as a kind of “dynamic solid”, which is of relatively rare recognized occurrence. First results show that there is a similar possibility in the field of silica WOGs, and further work is in progress [29]. 6.2.1.2 Conducting Polymers – Sol-gel Composites The polymerization of conducting polymers into WOG structures is not an easy task, because the cation-radical precursors of conducting polymers are very often sensitive to weakly nucleophilic functions present in WOGs. An early partially successful attempt was made seven years ago by Sanchez et al. who polymerized pyrrole into a hybrid gel [30]. However, beyond the observable electrochemical response of the polymers, the other properties were poor or unreported. In fact, an easier approach was followed by other groups who proceeded by performing sol- gel polymerization and either adding a soluble conducting polymer to the start- ing sol (which is feasible in the case of polyaniline and some substituted poly- thiophenes), or, more often, performing the chemical polymerization of the hetero- cycle in the presence of a chemical oxidant, simultaneously to the sol-gel process [31]. (A) (B) Fig. 6.2 Variation in the diffusion coefficient of gel-encapsulated ferrocene methanol (FcCH 2 OH) (A), or Fe(CN) 6 3– (B), with gel drying time. Single measurements on three different monoliths are shown. Inset: variation in the concentration of gel-encap- sulated electroactive species with drying time. Reprinted with permission from [26, first]. Copyright (1999) American Chemical Society Polymerization of pyrrole and aniline into sols was also described [30], but the properties of the resulting polymer films were not substantially different from those of classical electrochemically-grown liquid electrolyte films, although the authors reported that the films had incorporated some silica during the growth process. In a recent and noteworthy work, the same group claimed that pyrrole functionaliza- tion was achieved by in situ nucleophilic attack of the pyrrole cation-radical by sil- anols, leading to an electrogenerated hybrid film (Fig. 6.3) which in turn exhibited much better mechanical properties [32]. However, the electrochemical growth of materials in sols or WOGs appears less attractive than in xerogels and has been the- refore less investigated. 6.2.2 Electrochemical Behavior of Xerogels and Sol-gel-prepared Oxide Layers It is not obvious how to separate the field of xerogels (i.e. dried gels from which the gelation solvent has been extracted) from the field of oxides, since a whole fami- ly of materials exists in between. A xerogel is normally obtained upon simple dry- ing, requiring sometimes a mild thermal treatment, while an oxide layer requires always a thermal treatment at a relatively elevated temperature. This latter depends upon the nature of the precursor and the type of xerogel initially prepared. How- ever, if the curing step is performed at a lower temperature, or is not long enough, an intermediate state (oxopolymers) is obtained. This is often the case for electro- chemical experiments, since pure oxide layers are sometimes too compact and hin- der diffusion of the reactants or the electrolyte species through the film, while as- prepared xerogels are sometimes characterized by a lack of mechanical stability and strip off the electrode. 6 Electrochemistry of Sol-Gel Derived Hybrid Materials 178 Fig. 6.3 Schematic diagram of the interpenetrating network system formed from annealing of the silanol-functionalized polypyrrole. Reprinted with permission from [43]. Copyright (2000) American Chemical Society Beyond the fundamental work, a recent challenge is to use the versatility of sol- gel chemistry to realize new electrochemical electroluminescent devices. Electro- chemical luminescence is light generation from in situ electrogenerated species. This phenomenon can also be used for electroanalytical purposes. Again the different concerns exposed previously can be found in this research area. First, fundamental studies have been reported on xerogels that have been tur- ned electroactive by insertion/functionalization of redox systems. Second, compo- sites made by combining a ceramic component and conducting polymer