There are so many oxygen groups on the GO surface and these groups intract with metal cations and act like as nucleation center. Then you have so many nucleation and they pin on the surface and can't have surface diffusion and you will have nanoparticles not any other shape. If you want to have different morphologies you should control the chemistry of GO surface. Please look at this article it can answer you http://pubs.acs.org/doi/abs/10.1021/ja100329d
@Effat Jokar, thank you very much. do you know any mechanism for MnO2 to MnO. I have prepared MnO/GO by using MnO2 nanoparticle as Mn source. I need to know how MnO2 converted into MnO. any equations, literature support please