A method of forming a graphene oxide based layer includes preparing a dispersion of graphene oxide and nanostructures, and spin coating the dispersion on a surface of a substrate to form a spin coated film thereon; and thermally annealing the spin coated film to form the graphene oxide based layer, where the mass ratio of the graphene oxide and the nanostructures in the graphene oxide based layer is in a range of about 1:0.01 w/w to 1:0.8 w/w. The nanostructures are functionalized with carboxylic acid. The nanostructures include carbon nanotubes, or nanofibers. The carbon nanotubes include single walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs).