A New Route for The Synthesis of Reduced Transition Metal Oxides Using Borohydrides
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grinding and firing of the components at elevated temperatures. Such a "brute force" approach not only results in products with larger particle size, but also leads to an inaccessibility of metastable phases that may otherwise be unstable at higher temperatures. There is growing interest among the materials community during the past several years to design and develop synthetic routes that can bring down the reaction temperatures [1]. Low-temperature preparative techniques such as sol-gel processing [2], design of molecular precursors [3), coprecipitation [4], chemical and electrochemical lithium insertion/extraction reactions [5], and ion-exchange reactions [6] are becoming increasingly important in the synthesis of simple and complex inorganic materials. These techniques offer several advantages over the conventional high temperature techniques. An intimate or atomic-scale mixing of the components or the availability of the structural framework lowers the processing temperatures. The lower processing temperatures, in some cases, may allow the stabilization of unusual valence states or metastable phases with unusual atomic arrangements that are otherwise inaccessible by conventional high-temperature procedures. They can also provide better homogeneity and higher densities. In addition, the rheological properties of sols and gels might provide important advantages in the formation of films or fibers by spinning, dip-coating or impregnation techniques [2]. We present a novel approach for the synthesis of reduced transition metal oxides using borohydrides as reducing agents in aqueous solutions. 69 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
BOROHYDRIDES AS REDUCING AGENTS Sodium borohydride, NaBH 4 , was recognized forty years ago as an effective reducing agent in aqueous solutions [7]. The borohydride ion hydrolyzes in aqueous solutions to give hydrogen, BH4 " + 2H2 0
BO2
--
+ 4 H2
(1)
The hydrolysis is facilitated by acidic condition or by certain metal ions like Co 2+, Fe 2+ or Ni2+. The reaction of these metal ions with aqueous borohydride gives finely divided black precipitates of metals or metal borides, which seem to catalyze the hydrolysis reaction 1. Borohydrides have since then been used extensively for the generation of metallic particles and for the hydrogenation reactions in organic synthesis [8]. Use of sodium borohydride to generate metallic particles gained enhanced interest in recent years with respect to the formation of amorphous or nanophase magnetic particles. For example, ultrafine metallic magnetic particles such as Fe, Co or Ni with or without boron can be generated by a reduction of aqueous solutions of Fe 2 +, Co 2 + or Ni2 + with sodium borohydride [9]. Aerobic conditions give metallic particles while anaerobic conditions give metal borides [10, 11]. However, borohydrides remained to be explored for the synthesis of reduced transition metal oxides. REDUCED TRANSITION METAL OXIDES Transition metals such as tungsten, molybdenum and vanadium form stable tetrah
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