Self-assembled silicon nanotubes (SiNTs) are synthesized by a hydrothermal method, with silicon monoxide powder as a starting material and de-ionized water as a reaction medium. No metallic catalysts are added during the process. Unlike the disordered, aggregated SiNTs grown by the template method, the SiNTs synthesized here are self-assembled. The SiNTs have hollow structures with crystal silicon wall layers surrounded by an amorphous silica shell, as observed by transmission electron microscopy (see Figure).