A Pt sol was formed using a sol-gel derived methodology. This sol, and films formed from it, were characterized using various techniques. The nanoparticulate phase, seen by transmission electron microscopy ͑TEM͒, exhibited a notable temperature dependence, with the particles averaging 1-3 nm diam when dried at room temperature and 3-6 nm diam when dried at 400°C. X-ray photoelectron spectroscopy ͑XPS͒ confirmed the presence of metallic Pt in these films. The charge density of films deposited on Au also exhibited a temperature dependence, with maximum charge densities being exhibited at ca. 200°C. The increase from room temperature to 200°C can be attributed to thermal conversion of residual oxidized Pt in the film to metallic Pt, as well as improved electrical connectivity between the particles due to sintering. Above 200°C, sintering becomes a major contributor to the loss of charge density, as it reduces the electroactive surface area. The efficiency of use of the metallic Pt was examined using electrochemistry and the quartz crystal microbalance technique, and was found to be 20-25%. A second phase, consisting of larger crystallites, was also found in as-formed films by both TEM and scanning electron microscopy. While this phase could be removed by rinsing with acid, it could not be identified by either X-ray diffraction or XPS. Platinum is of great interest due to its use as an electrocatalyst for numerous reactions, examples including isobutane dehydrogenation, 1 methanol oxidation, 2,3 oxidation of methane to methanol, 4 and oxygen reduction. 5 The use of Pt as a catalyst requires that the active area remains stable with time of operation and that the expensive Pt metal is efficiently used. Thus, numerous methods to produce and utilize high surface area ͑usually nanoparticulate͒ forms of Pt have been developed, including the deposition of Pt onto porous substrates, such as zeolites or gas diffusion backings, 1,6 reduction of Pt salts using various reducing agents to produce a nanoparticulate powder, 7-11 mixing Pt black with carbon powder and/or Nafion 3,12-16 and its incorporation into polymeric matrices. 17In our previous work with Ir, it was found that using the same approaches as employed in sol-gel ͑SG͒ synthesis of metal oxides allowed for the preparation of highly porous, nanoparticulate ͑down to 1 nm diam͒ metallic Ir films 18 through an electrochemical reduction process. Thus, our goal has been to achieve the same outcome with Pt. In this work a derivative of the alkoxide route is used, resulting in the formation of colloidal Pt particles in situ. This alkoxide route has several benefits, due to its liquid phase synthesis, including easy mixing of other components into the solution at the atomic level. The synthesis of solutions using SG-derived methodologies is also relatively facile, involving several variables which can be readily manipulated to optimize the films produced. Some of these include the amount of water in the synthesis, the ratio of reactants, the thickness of the film and the film...
Identification of the species formed during the in situ reduction of hexachloroplatinic acid by sodium ethoxide, forming a Pt sol, is made. The solution phase is shown to consist of suspended metallic Pt nanoparticles (1-3 nm in diameter), acetaldehyde, and a Pt(II) species, identified by NMR and X-ray adsorption near-edge spectroscopy (XANES) to be NaPtCl3(C2H4), a sodium analogue of Zeise's salt [KPtCl3(C2H4)]. The NaPtCl3(C2H4) product exhibits greater stability in both ethanol and air than the conventional Zeise's salt, providing a means of storing the useful Zeise's anion [PtCl3(C2H4)-]. Electrochemistry, X-ray diffraction (XRD), and transmission electron microscopy (TEM) analyses have shown that the precipitate phase formed during the synthesis is composed solely of Pt particles approximately 6 nm in diameter and NaCl. Thermal gravimetric analysis/differential scanning calorimetry (TGA/DSC) showed that the color of the precipitate is an accurate gauge of the ratio of Pt to NaCl, with the lightest to darkest precipitates containing from 1% to 40% nanoparticulate Pt by mass, respectively. A comprehensive characterization of both phases formed has allowed us to propose a mechanism for the conversion of hexachloroplatinic acid to Pt nanoparticles.
There has been much interest in the practical applications of Ir oxide films in supercapacitive, 1-3 electrochromic, 4-6 and energy storage devices, 6 as interneural stimulating electrodes, 7,8 electrocatalysts, [9][10][11][12][13] and pH electrodes. 14-16 In many cases, Ir oxide films have been formed electrochemically, i.e., by cycling or pulsing the potential of an Ir metal electrode between critical limits in various aqueous solutions. [4][5][6][17][18][19][20][21][22][23][24][25][26][27] This can result in the formation of relatively thick films (up to several micrometers in thickness), which are hydrous in nature (coded as activated iridium oxide films (AIROFs) in the prior literature 2,15,28-30 ) and exhibit very rapid oxidation/ reduction kinetics when the films are switched between their conducting Ir(IV) and insulating Ir(III) states. However, this method of film growth is both time and energy intensive, requires a metallic Ir substrate (often not fully reacted), and also usually involves some simultaneous loss, through dissolution, of the Ir metal substrate.Other methods 2,15,28,29 which have been used to form Ir oxide films have involved the thermal oxidation of iridium salts, or the sputtering of Ir onto a conductive substrate in an oxidizing plasma environment. Sputtered iridium oxide films (SIROFs) are known to be less hydrous in nature, have a more featureless cyclic voltammetric (CV) response, and yield lower maximum charge densities than do AIROFs. 2,15,28,29 Ir oxide films have also been formed by induction heating 31 and by electrodeposition at constant anodic potentials from alkaline Ir-containing solutions, 32 although insufficient data are available for the properties of these films to be assessed relative to AIROFs and SIROFs.The present paper is a continuation of our previous work, 33 involving the use of the sol-gel (SG) technique for the preparation of Ir oxide films. The SG method has been employed before to prepare IrO 2 34 and mixed oxides such as IrO 2 -Ta 2 O 5 , 34 IrO2-SnO 2 , 35 and RuO 2 -IrO 2 , 36 in powdered form. IrO 2 coatings were also obtained by painting 37,38 and dip-coating 38 techniques, using substrates such as amorphous silica microbeds 37 and glass, 38 followed by drying at temperatures between 350 and 600ЊC. However, only compositional/structural studies of these materials were performed, 34-38 and no prior electrochemical characterization has been reported.In our previous paper, 33 it was shown that the Ir-containing films formed by the SG method actually consist of Ir metal nanoparticles. These can be transformed easily into the oxide by potential cycling within the limits normally used for the electrochemical growth of IrOx films at bulk Ir electrodes. The resulting films, which were not studied in detail previously by us, 33 were shown to exhibit the general properties of AIROFs. In the present work, SG-formed Ir oxide films are demonstrated to be electrochromic, to yield high charge densities, as well as to exhibit excellent (Ir(III)/(IV)) oxidation/ reducti...
This work is focused on the optimization of the synthesis conditions of a Pt sol phase containing suspended metallic Pt nanoparticles with the primary goal being to produce thin (ca. 1 monolayer) Pt films having the highest possible electroactive surface area per gram. This is gauged here by the surface roughness factor, determined from the magnitude of the Pt electrochemical response in sulfuric acid solution. Two Pt(IV) chloride compounds (H 2 PtCl 6 , Na 2 PtCl 6 ) are shown to be the best Pt precursors, producing stable Pt nanoparticles with an average particle diameter of 1-3 nm. Sodium ethoxide and formic acid are found to be excellent reducing agents of the PtCl 6 2-anion, although formaldehyde results in a lower yield of Pt nanoparticles. A ratio of sodium ethoxide to H 2 PtCl 6 of 2:1 and a 72°C reflux in ethanol between 30 min and 5 h resulted in the highest Pt roughness factor (ca. 8). Transmission electron microscopy analysis has verified that all of the reducing agents produce Pt particles of a similar size and that the higher roughness factors are the result of a higher yield of Pt nanoparticles. The effect of time of storage of Pt sols formed using sodium ethoxide showed that only a minor aging effect is observed over long periods of time, likely minimized by the stabilization offered by PtCl 3 (C 2 H 4 ) -, a species formed as a byproduct during the synthesis.
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