Ultrathin Silica Layers as Separation Membranes for Artificial Photosynthesis

Abstract

Efficient artificial photosystems for the conversion of carbon dioxide and water to fuel require the separation of the incompatible oxidation and reduction catalysis environments by a membrane. Of particular interest are complete photosystems of nanoscale dimensions, a key design feature of natural photosynthesis, which is the only known system for making chemical compounds at the terawatt scale, the level required for impact on fuel consumption. Ultrathin amorphous silica layers with embedded molecular wires provide a means for integrating the water oxidation and carbon dioxide half-reactions into nanoscale units under separation while enabling electronic and protonic coupling between them. This approach affords optimization of electronic charge transfer independently from optimization of proton transport and separation properties. Synthetic methods are introduced and the structural characterization of nanomembranes based on surface-sensitive vibrational spectroscopy is presented. The charge transfer, proton transport, and photocatalytic behavior of silica nanomembranes coupled to light absorbers and catalysts are quantitatively evaluated and optimized by photoelectrochemical, ultrafast optical, and infrared spectroscopic methods. By selecting nanotube morphology for photosynthetic units with a built-in ultrathin membrane, square inch-sized nanotube arrays are fabricated as artificial photosynthetic systems that extend the membrane function from the nano- to the macroscale. Ultrathin silica membranes open up opportunities for interfacing a wide range of incompatible reaction environments on the nanoscale for energy applications, such as the coupling of microbial and inorganic catalysis in the form of nanobiohybrids.