Aerogel-Like Metals Produced Through Physical Vapor Deposition

Since the pioneering set of aerogels created by Kistler in 1931, this fascinating class of materials has evoked a myriad of scientific and technological possibilities and inspired an ever-growing and diverse research community. Within each aerogel lies an inner nanonetworked solid architecture that imparts it with novel physical and chemical properties; hence, scientists have continued to seek synthetic routes for preparing aerogels of an ever-widening variety of substances, including inorganic oxides, synthetic and biological polymers, metal chalcogenides, and, recently, metals. Metal aerogels in particular combine the unique catalytic, electrical, optical, and chemical functionalities of metals with the high surface area, high porosity, and low density typified by aerogel architectures. However, preparation of metallic aerogels via traditional sol–gel synthesis and supercritical drying is challenging, highly dependent on the chemistry of the metal of interest, and unachieved as of yet for some metals. As such, alternate synthetic strategies for synthesizing metal aerogels are highly desirable. Herein we present the synthesis of aerogel-like metals grown by direct physical vapor deposition (PVD) in the free space above a silica aerogel substrate, guided by electrostatic field lines extending off the aerogel. The technique is generalized for a variety of disparate metals as well as metal oxides and enables tuning of the network’s architectural parameters through adjustment of the PVD parameters and substrate surface chemistry. The combination of nanosized metallic elements with three-dimensional (3D) organization over macroscopic dimensions of these materials shows a unique promise for applications including catalysis, light harvesting, photonics, optoelectronics, chemical purification, sensors, and batteries. This chapter provides a comprehensive overview of the various preparation techniques for synthesizing metallic aerogels that have been demonstrated to date with a focus on aerogel-like networks synthesized through PVD. Characterization of PVD-derived metallic networks including linear and nonlinear optical properties and potential applications are discussed. Finally, a glimpse into the future of 3D hybrid materials based on molecular plasmonics enabled by PVD-derived nanostructured metal networks is presented.