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Small particles with high surface-to-volume ratio composed of bimetallic alloys are extensively used as catalysts and magnetic recording media in the chemical and information technology industries. Current research has been focused on reducing particle size, preferably to the nanometer scale, to achieve both improved catalytic activity and higher areal density for information storage. Both physical and chemical methods have been developed for such purpose, albeit with many shortcomings. Gonsalves et al. have provided an excellent review of those methods, with emphasis on chemical approaches. Physical methods such as gas condensation or mechanical deformation are very energy-intensive and tend to produce heterogeneous size populations. Chemical approaches offer better control over alloy composition and homogeneity. Chemical reduction of metallic salts is the most generally applied approach. Reducing agents based on borohydrides have been extensively used, while recently, organic reducing agents are receiving more attention. Nanosized alloys can also be prepared through thermal decomposition reactions. In its finer form, the thermal treatment is performed sonochemically in numerous minute reactors created through supersonic radiation. Chemical preparations of nanoalloys usually suffer from tough, demanding environments (airless and water-free conditions in some reactions), contamination from reducing agents, and the limited choice of starting agents (metal carbonyls for thermalization and supersonication). In addition, chemical reduction routes are flawed by an intrinsic limit that the reduction rates of both metal cations need to be similar; otherwise, a core-shell structure rather than real alloys will prevail.
Sol–gel processing has been widely used to prepare nanomaterials, especially composites. However, to date, this approach has not been applied to the formation of nanoalloys. This method offers the advantage of high chemical purity as well as product homogeneity. Moreover, this technique excels in fabricating thin films, which are actually preferred in the aforementioned applications. However, current sol–gel processing is based on the hydrolysis of metal alkoxides, a method applicable to only d0 transition metal cations. More importantly, a large amount of oxygen exists in the backbone of the gel polymer, from which only ceramics can be prepared through subsequent sintering treatment. We have found two cases that consider the synthesis of nanometal alloys through sol–gel processing. In fact, in both studies, it was the silica matrix that underwent the sol–gel process and the alloys were prepared through hydrogen reduction.