Comparative study on the formation mechanism of graphene oxide-derived carbon/Pd composites

Na2PdCl4 and Pd(en)2Cl2 were used as Pd precursors to prepare carbon/Pd composites by chemical reducing exfoliated graphite oxide (GO) and Pd precursor mixture solution according to similar procedures. The obtained samples were examined by X-ray diffraction (XRD), N2 adsorption tests at 77 K and a scanning electron microscope (SEM). It was found the samples prepared from Pd(en)2Cl2 have a relatively large Brunauer–Emmett–Teller (BET) surface area with little graphitic structure while the samples obtained from Na2PdCl4 have a low BET surface area with considerable graphitic structure. SEM tests showed that the Pd nanoparticles (NPs) are decorated in between or on some edges of curved graphene sheets in the sample from Pd(en)2Cl2 while Pd NPs are located on the surface of the reduced graphite substrate in the sample from Na2PdCl4. XRD proved that GO is reduced to graphite in the sample from Na2PdCl4 and it is reduced to graphene sheets in the sample from Pd(en)2Cl2. The Pd(en)22+ can adhere to the graphene oxide sheets, preventing the graphene sheets restacking during reduction and there is no obvious interaction between PdCl42− and the graphene oxide sheets, thus leading the graphene sheets to aggregate to form the graphite structure. The great structure differences between the samples are caused by different ratios of the two contradictive roles: the graphene sheets' tendency to restack to form the graphite structure and the reduced metal nanoparticles' pillared role to separate graphene sheets as spacers. The formation mechanism of carbon/Pd composites is proposed, which is of a theoretical guide for related research.