Colloidal metals electrochemical preparation

There are several bottom-up methods for the preparation of nanoparticles and also colloidal nanometals. Amongst these, the salt-reduction method is one of the most powerful in obtaining monodisperse colloidal particles. Electrochemical methods, which gained prominence recently after the days of Faraday, are not used to prepare colloidal nanoparticles on a large scale [26, 46], The decomposition of lower valent transitional metal complexes is gaining momentum in recent years for the production of uniform particle size nanoparticles in multigram amounts [47,48],... 

SERS active structures can be prepared by a variety of chemical physical and electrochemical methods described in Sect. 4.1. The chemical preparation of colloidal nanoparticles is frequently used (Sect. 4.1.1). An interesting electrochemical preparation procedure is the so-called double-pulse technique. This method is an electrochemical tool for controlling the metal deposition with respect to particle size and particle density (Sect. 4.1.2). 

Subsequently, this important insight led to another serendipitous discovery. We had previously shown that the electrochemical preparation of colloidal metal nanoparticles, usually carried out in organic solvents such as THF, can also be performed in water, provided H20-soluble ammonium salts of the betaine type are... 

The radiation method was described by Rogninski and Schalnikoff for the first time and is based on condensation of the metal atoms after collision [154]. Reetz et al. prepared nanoparticles via electrochemical synthesis [155]. Salt reduction was developed by Bonnemann to obtain mono- and bi-metallic nanoparticles in solution [156]. Salt reduction is the most widely practised method for the synthesis of colloidal metal suspensions. Faraday synthesised gold particles by the reduction of HAuCb [157]. 

Enhancement of absorption bands in the IR spectra of ultrathin films in the presence of discontinnons (islandlike) nnder- and ovemanolayers of Ag and An was discovered by Hartstein et al. [356] in the early 1980s. Although these researchers believed that they observed an increase in the vCH band intensities for p-nitro-benzoic acid (p-NBA), benzoic acid, and 4-pyridine-COOH films, it was recently shown [350] that the spectra reported are in actual fact due to fully saturated hydrocarbons (possibly vacuum pump oil). In any case, this discovery has stimulated various research activities and led to the development of surface-enhanced IR absorption (SEIRA) spectroscopy. To date, the SEIRA phenomenon has been exploited in chemical [357] and biochemical IR sensors (see [357-360] and literature therein), in studying electrode-electrolyte interfaces [171, 361-365], and in LB films and SAMs [364, 366-370]. Other metals that demonstrate this effect are In [371] and Cu, Pd, Sn, and Pt [372-375]. The metal films can be prepared by conventional metal deposition procedures such as condensation of small amounts of metal vapor on the substrate, spin coating of a colloidal solution, electrochemical [388], or reactive deposition [299] (see also Section 4.10.2). 

The electrochemical and electroflotation methods are widely used to prepare of chemisorbed macromolecules bound to colloidal metal particles generated in situ. Electrochemical polymerization reactions are heterogeneous They are initiated on the electrode surface, while other stages (chain growth or termination) occm, as a rule, in the liquid phase. The yield of a polymer depends on the chemical and physical nature of the electrodes and their surface, electrode overvoltage, potential rmder which the reaction occurs, and electrical current density. The nature of the electrode material (metals or alloys, thin metallic coats, etc.) determines the characteristics of electron-transfer initiation and polymerization. Direct electron transfer between the electrode and monomer, cathodic deposition, and anodic solubilization of metals are optimum for electrochemical polymerization. Metal salts are the precursors of nanoparticles, which may act as specific electrochemical activators. Nanoparticles can influence activations through direct chemical binding to the monomer and by virtue of transfer, decomposition, or catalytic effects. Nonetheless, electrochemical polymerization has found only limited use in the preparation of polymer-immobilized nanoparticles. 

The preparation of metal organosols by electrolysis in a two-layer bath proved to be more suitable than electrochemical polymerization. The upper organic layer of the electrolytic bath is a dilute solution of a polymer in an orgaific solvent, sometimes, supplemented with a small amormts of surfactant. The polymer interacts with the nascent colloidal metal particles near the interface between layers. 

Transition-metal nanopartides are of fundamental interest and technological importance because of their applications to catalysis [22,104-107]. Synthetic routes to metal nanopartides include evaporation and condensation, and chemical or electrochemical reduction of metal salts in the presence of stabilizers [104,105,108-110]. The purpose of the stabilizers, which include polymers, ligands, and surfactants, is to control particle size and prevent agglomeration. However, stabilizers also passivate cluster surfaces. For some applications, such as catalysis, it is desirable to prepare small, stable, but not-fully-passivated, particles so that substrates can access the encapsulated clusters. Another promising method for preparing clusters and colloids involves the use of templates, such as reverse micelles [111,112] and porous membranes [106,113,114]. However, even this approach results in at least partial passivation and mass transfer limitations unless the template is removed. Unfortunately, removal of the template may re-... 

Water. A laboratory engaged in careful electrochemical work with aqueous solutions or in trace analysis will need facilities for the preparation and storage of highly purified water. Water commonly is contaminated with metals in both dissolved cationic form and in the form of colloidal or particulate matter that is not ionized appreciably.70 Frequently it also is contaminated by bacteria and by organic impurities that cannot be removed by ordinary or oxidative distillation because of the steam volatility of the impurities.71... 

Interest in the application of nanostructured catalysts stems from the unique electronic structure of the nanosized metal particles and their extremely large surface areas. Nanostructured metal colloids can be defined as isolable particles between 1 and 50 nm that are prevented from agglomerating by protecting shells. They can be prepared to be redispersed in both water ( hydrosols ) and organic solvents ( organosols ). Here we hope to provide a synopsis of the wet chemical syntheses of these materials and their application as precursors of electrochemical catalysts. 

Electrochemical studies with nanosized particles require them to be accessible to the electrode surface where the potential can be controlled. This can be accomplished either by using the metallic colloid along with an inert, planar electrode or by attaching the nanopartide onto a conducting matrix. Both these methods have been demonstrated in the literature [21-25]. In this section, some of the anchoring methods that are available for this purpose are briefly explained. A few other methods of preparation are explained wherever appropriate. 

This review will emphasize SERS in the context of electrochemical systems. The liberty has been taken of including in this category work done on colloids suspended in (mostly aqueous) solutions. Colloids, anyway, have many common features with systems in electrochemistry. Thus SERS at the solid-electrolyte interface is the main question of interest here. Of course, one cannot ignore the work on other systems, nor does one want to. Therefore we will also discuss the other systems, such as various films in ultrahigh vacuum, in air or in tunnel junctions, on specially prepared lithographic structures, on metal clusters trapped in a noble-gas matrix, or on an oxide in catalytic systems, though they will not be at the main focus of this review. 

In the standard chemical preparation methods, the properties, especially the size and size distribution of the nanoparticles, are defined by the choice of the reaction conditions, reactant concentrations, etc. The use of electrochemical techniques to generate nuclei has the advantage that the supersaturation is determined by the applied potential or current density. Thus, the size of the particles can be controlled by electrochemical instrumentation rather than by changing the experimental conditions. Reetz and Helbig [115] demonstrated how electrochemical methods can be used to produce metal colloids of nanometer size and more importantly how particle size can be controlled in a simple manner by adjusting the current density [159]. First, a sacrificial anode was used as the source of the metal ions, which were then reduced at the cathode. Later, a more general approach was introduced, where metal salts were used as the starting material [160]. The particles were stabilized by alkylammonium or betaine salts. With a suitable choice of surfactants, the electrochemical method can be applied in the preparation of different shapes of particles, e.g., nanorods [161]. 

As described at the end of the last section, we came upon the study of metal oxide nanoparticles in aqueous solution accidentally. Our only earlier experience with this class of nanoparticles, although in organic solvents such as THF, was the investigation of the controlled 02-mediated oxidation of tetraalkylammonium bromide stabilized cobalt colloids prepared size-selectively by the electrochemical method (Fig. 8.5) [53]. 

The Miilheim electrochemical method of producing R4N+X -stabUized transition metal colloids is a viable alternative to the traditional chemical process. The preparation of aqueous colloidal solutions of nanosized transition metal oxides or multimetal oxides (1-3 nm) is possible by an unusually simple procedure, namely hydrolysis of the corresponding metal salts under basic conditions in the presence of water-soluble stabiUzers. High concentrations (0.1-0.5 M) are usually possible, which is crucial for industrial applications. In many cases an in situ method for immobUization on a soUd carrier such as carbon black is possible in the absence of a stabUizer. In the rare case of IrO, the colloidal soluhon (2nm) is stable for... 

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