Sol-gel chemistry is widely used in the synthesis of metal oxide nanotubes, a good example being that of silica and TiO2. Oxide gels in the presence of surfactants or suitable templates form nanotubes. For example, by coating CNTs with oxide gels and then burning off the carbon, one obtains nanotubes and nanowires of a variety of metal oxides including ZrO2, SiO2 and MoO3. Sol-gel synthesis of oxide nanotubes is also possible in the pores of alumina membranes. MoS2 nanotubes may also be prepared by the decomposition of a precursor in the pores of an alumina membrane. Metal-oxide nanotubes have properties quite different from those of carbon nanotubes. They can be made more hydrophilic than carbon and can load nearly 50% percent of their weight with water. Controlling dimensions of nanostructures is critical because properties such as electronic band-gap largely depend on dimensions. Dimension control has proven difficult in carbon nanotubes, but control is possible with different chemical-process conditions across a broad range of metal-oxide materials according to researchers.