Submicron rectangular hollow tube crystals of rutile-type GeO2

Single crystals of rutile-type GeO2 having a structure equivalent to that of TiO2, a well-known photocatalyst, have been grown for the first time in supercritical oxygen at approximately 5 GPa and 3000 K. The obtained crystals exhibit a rectangular hollow tube structure with submicron size (cross section with sides of ∼500 nm, wall thickness of ∼20 nm, and longitudinal length of ∼5 μm). These single crystals were grown within 1 s and along the c-axis surrounded by the (1 1 0) faces. The crystal growth mechanism strongly depends on the growth mechanism of rutile-type oxides, and the extremely short growing time is an important factor in the formation of hollow tube crystals.