Synthesis of birnessite structure layers at the solution-air interface and the formation of microtubules from them

H(x)MnO2·nH2O layers have been successfully produced through a facile low-temperature process at the solution-air interface without using any templates. The crystalline structures of layers can be tuned by the compositions and the pH of the growth solutions. The analysis of birnessite-like layers indicates that they are formed by nanosheets approximately 4-6 nm thick that are oriented for the most part normally to the interface. Our results demonstrated that 1-3-μm-thick layers can roll up into microtubules 20 to 100 μm in diameter and up to 10 mm long. The hypothesis explaining the formation of the microtubular structures is assumed.