Germanium (Ge)-based semiconductors have attracted extensive interest in many applications for optoelectronics, detectors, and energy storage. One of the most important potential applications of the Ge materials is as efficient anode materials for lithium-ion batteries (LIBs). In terms of the efficiency and cost-effectiveness, developing new Ge-based semiconductor thin films with engineered porous nanostructures from sustainable materials is an interesting synthetic route for practical technologies. Recently, we used biopolymeric cellulose nanocrystals (CNCs) prepared from plants to produce a new family of Ge-based semiconductors that are available as freestanding thin films and have special spiral nanoporous structures.
In 2010, our group used CNCs to produce photonic silica films where we simply combined silica precursor with cellulose liquid crystals in water. We obtained mesoporous silica films with a twist after calcining the resulting silica/cellulose composites in air to burn away the cellulose template. These new materials are colorful glasses that are useful for photonic and optical technologies (Nature, 2010, 468, 422). We know that Ge is silicon’s neighbor in periodic table, so the question was could we apply this synthetic procedure for germania materials? We first attempted to combine Ge precursor with CNCs in water, but we always obtained cloudy inhomogeneous composite films as a result of strong hydrolysis and condensation of highly reactive germanium alkoxides in water. This means that germanium is very reactive with water compared to stable silane. Consequently, we found it difficult to control the twisting organization of cellulose liquid crystals in the presence of Ge precursor during the air-drying of water.
To address this issue, we developed a new technique for these Ge-based materials where we primarily used a mixed solvent system of water and organic solvent (e.g., DMF). The presence of the organic solvent in the aqueous suspension of CNCs allowed for flexible control of the slow hydrolysis and condensation of germanium precursors. As a result, we can produce large, crack-free freestanding GeO2/cellulose films with brilliantly iridescent colors after air-drying of the mixed suspension. We subsequently pyrolyzed the GeO2/cellulose composites under different calcination conditions to produce twisted nanoporous semiconductor films of GeO2, GeO2/C, and Ge/C. This work was done by Jing Xu, a visiting Professor at UBC and Thanh, currently a post-doc in our group who determined the structures and helped her prepare the paper.