Solutions, colloidal preparation

The colloidal palladium solution is prepared as follows A solution of a palladium salt is added to a solution of an alkali salt of an acid of high molecular weight, the sodium salt of protalbinic acid being suitable. An excess of alkali dissolves the precipitate formed, and the solution contains tine palladium in the form of a hydrosol of its hydroxide. The solution is purified by dialysis, and the hydroxide reduced with hydrazine hydrate. On further dialysis and evaporation to dryness a water-soluble product is obtained, consisting of colloidal palladium and sodium protalbinate, the latter acting as a protective colloid. 

We prepared ceria on Ni substrate by sol-gel coating method. Ceria sol solution was prepared with ceria sol solution (Alfa, 20% in H2O, colloidal dispersion) mixed with ethanol (99.9%, Hayman) with weight ratio (1 2) and stirred. Ceria was deposited on Ni substrate by dip coating method. The variation number of dipping was carried out to obtain different coating ratio. The anode was completely dipped into the ceria sol solution for several seconds and dried at a temperature of 50 C for 24 hours in air atmosphere followed by calcination at 700 C for 30 minutes in 5%H2-N2 atmosphere. 

The photoelectrochemical properties of 283 colloids prepared by chemical solution growth [193] have been demonstrated by carrying out oxidation and reduction processes under visible light irradiation. Charged stabilizers such as Nation were found to provide an effective microenvironment for controlling charge transfer between the semiconductor colloid and the redox relay. 

Studies performed on CdS [282, 283] have revealed the importance of the microstructure, i.e., crystal structure, crystallite size, and geometrical surface area, in both the control of band structure and the concentration and mobility of charges, in relation to the photocatalytic performance of the photocatalyst. It has been shown also that the solubility product of CdS colloids prepared from acetate buffer aqueous solutions of suitable precursors increases from 7.2x 10 for large particles to about 10 for small (< 2.5 nm) particle colloids, this increase invoking a positive shift on the cathodic corrosion potential [284]. 

Thus, we see that the digestive ripening process leads to highly monodispersed nanoparticles that can come together to form ordered superstructures similar to atoms or molecules that form crystals from a supersaturated solution. Then if the superstructure formation can indeed be related to atomic/molecular crystallization, it should also be possible to make these supercrystals more soluble in the solvent with a change of temperature. Indeed, the optical spectra of the three colloids prepared by the different thiols discussed above exhibit only the gold plasmon band at 80 °C suggesting the solubilization of these superlattices at the elevated temperatures [49]. 

The dye-sensitised solar cell (DSSC) is constructed as a sandwich of two conducting glass electrodes filled with a redox electrolyte. One of the electrodes is coated, using a colloidal preparation of monodispersed TiOj particles, to a depth of a few microns. The layer is heat treated to rednce resistivity and then soaked in a solution of the dye until a monomolecnlar dispersion of the dye on the TiO is obtained. The dye-coated electrode (photoanode) is then placed next to a connter electrode covered with a conducting oxide layer that has been platinised , in order to catalyse the reduction of the mediator. The gap between the two electrodes is filled with an electrolyte containing the mediator, an iodide/triodide conple in acetonitrile. The structure is shown schematically in Fignre 4.29. 

We point out here that the colloid prepared by these methods is very clean, because the carrier gas used is usually high-purity grade at six-nine, the chamber is once evacuated to depress the extent of contaminating oxygen and moisture, and the liquids themselves are always purified by sublimation process except for the solution trap method. To transfer the colloidal suspension after preparation, a specially designed stock bottle with a Luer-lock syringe is normally used in order to enable the operations under Ar flow to avoid unexpected air contamination. Therefore, we can carry the suspension liquid away from the production chamber without exposure to air, which means that the surface of colloidal metal is very clean if it does not react with suspension liquids. 

Fig. 9.4.25 Optical absorption spectra of copper colloid prepared by the gas flow-solution trap method as a function of lime development. The numbers in the figure are the time after the preparation of Lhe sample. The spectrum of sodium eihoxidc in ethanol (authentic sample) is also shown in the same figure, marked by b. The insertion is the expansion of the region of the isosbestic point. The deviation from the isosbestic point at 10 h after the preparation of colloids is shown by a in the insert. (From Ref. 26.)...

The number of studies on the health effects of fullerenes and carbon nanotubes is rapidly increasing. However, the data on their toxicity are often mutually contradictory. For example, the researchers from universities of Rice and Georgia (USA) found that in aqueous fullerene solutions colloidal nano-C particles were formed, which even at low concentration (approximately 2 molecules of fullerene per 108 molecules of water) negatively influence the liver and skin cells [17-19]. The toxicity of this nano-C aqueous dispersion was comparable to that of dioxins. In another smdy, however, it was shown that fullerene had no adverse effects and, on the contrary, had anti-oxidant activity [20]. Solutions of prepared by a variety of methods up to 200 mg/mL were not cytotoxic to a number of cell types [21]. The contradiction between the data of different authors could be explained by different nano-C particles composition and dispersion used in research. 

The slip coating-sintering procedure can be used to make membranes with pore diameters down to about 100-200 A. More finely porous membranes are made by sol-gel techniques. In the sol-gel process slip coating is taken to the colloidal level. Generally the substrate to be coated with the sol-gel is a microporous ceramic tube formed by the slip coating-sintering technique. The solution coated onto this support is a colloidal or polymeric gel of an inorganic hydroxide. These solutions are prepared by controlled hydrolysis of metal salts or metal alkoxides to hydroxides. 

Zeolite ZSM-5. Zeolite samples were crystallized from a gel containing colloidal silica (Ludox AS-40), deionized water, aluminiumtriisopropylate (ATIP, Merck), as source of aluminium, and tetrapropylammonium bromide, (TPABr, Aldrich). Ammonia solutions were prepared by saturating a thermostated aqueous solution with... 

The metal colloid solutions were prepared by the condensation of the metal vapors into methylcyclohexane solutions of i obutylaluminoxane at -120 C, a general method we have developed for the preparation of stable colloidal transition metals in non-polar organic solvents (3). The aluminoxane, ( C4H9A10)n which is a low... 

The colloidal metals prepared in the manner described above were characterized by transmission electron microscopy. The solutions, as prepared, were diluted to a concentration adequate to allow for the formation of a film of aluminoxane on a sample grid of sufficient thinness for adequate imaging of the metal particles. 

It is intuitively more probable that the polar inorganic backbone of the oligomer interacts in some fashion with the surface of the metal. This speculation is supported by the fact that polyisobutylene, a purely paraffinic polymer, is ineffective in stabilizing the colloids in methylcyclohexane. Although we have not made a thorough investigation of this aspect of the chemistry of the colloid preparation, we have observed that a minimum ratio of aluminoxane (as monomer equivalents)to metal of ca. 5 is necessary to obtain a stable solution. Given this condition, the colloidal metal solutions are stable for months at room temperature, and can be heated to moderate temperatures without precipitation of bulk metal. 

See also Sabaaieeff, Chem. Zentr., 1891, i., 10 UUik, Annalen, 1867, 144, 329 1870,153, 373. Rosenheim and Davidsohn (loc. cit.) consider that the solutions previously-prepared by Graham s method, since not precipitated by electrolytes, in all probability contained no colloidal acids. 

These films can be prepared by a variety of routes, only a few of which are mentioned here. The original references should be consulted for more practical details. Titanium dioxide is used as an illustrative example below. First, colloidal solutions are prepared, e.g., from titanium isopropoxide. The resultant sol is concentrated under vacuum at room temperature until its viscosity increases. Then it is spin-coated on to suitable supports (e.g., conducting glass) and fired in an oven. The firing temperature critically controls the morphology of the resultant film as discussed elsewhere [300-303]. Films up to several micrometers thick can be prepared by this simple version of the sol-gel technology [304]. Aerosol or spray pyrolysis is a somewhat related approach [305, 306]. 

The tendency of Pu(IV) to hydrolyze and to form real solutions, colloid solutions, or insoluble preeipitates has been known sinee the Manhattan Projeet. The main results of the earlier work in this field were summarized by Seaborg and Katz [19]. Sinee then, studies have been performed to examine in detail the equilibrium of Pu(fV) hydrolytie reaetions in different media [20,21]. Great attention has also foeused on the preparation, strueture and properties of Pu(IV) polymers or colloids [22-27]. These eompounds have found an important application in sol-gel teehnology for the preparation of nuclear fuel materials [28]. However, studies of the properties and behavior of solid Pu(IV) hydroxide in complex heterogeneous systems are rather rare. A most important result of these studies was the eonelusion that Pu(fV) hydroxide, after some aging, consists of very small Pu02 erystallites and, therefore, should be considered as a Pu(IV) hydrous oxide [23,24]. 

Golander, C.-G. Caldwell, K. Lin, Y.-S., A new technique to prepare gradient surfaces using density gradient solutions, Colloid Surf. 1989, 42, 165-172... 

Colloidal CaHAP and SrHAP particles were prepared by the method described in papers [3, 25]. Ca(OH)2 (3.00 g) or Sr(OH)2 (4.39 g) was dissolved in 20 dm of deionized and distilled water free from CO2 in an N2 atmosphere by stirring overnight at room temperature. Various amounts (75 - 85 cm ) of 10 % mass H3PO4 were added to the Ca(OH)2 or Sr(OH)2 solutions to prepare the samples with different Ca/P or Sr/P molar ratios. The resulting suspensions were stirred for 24 h at room temperature and then aged at 100°C for 48 h in a capped Teflon vessel. The white precipitates formed were filtered off, washed with deionized and distilled water and finally dried in an air oven at 70°C for 24 h. 

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. 

Colloidal solutions of most all the metals can be obtained by forming an arc under water using an electrode of the metal of which the colloidal preparation is desired. For instance, if we desire to prepare a colloidal solution of silver we would draw an arc between two silver wires under water. It will be found difficult to maintain this subaqueous arc for any length of time. However, every time the wires are touched a small cloud of colloidal silver will be produced in the water. The water used should be as pure as possible as the colloidal particles are extremely sensitive to foreign matter. 

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