Synergistic photo optical and magnetic properties of a hybrid nanocomposite consisting of a zinc oxide nanorod array decorated with iron oxide nanoparticles
Multifunctional nanocomposite materials recently emerged as a powerful concept because they offer to combine physical properties of different inorganic materials. Besides the simple combination of physical properties, the real challenge is to provide high control of the structure of nanocomposites (i.e. the spatial arrangement of the two entities, interface or separating distance, etc.). In this case, synergy between the two entities may be favoured in order to dramatically enhance the physical properties. Herein, we report on a multifunctional hybrid nanocomposite material consisting of a zinc oxide (ZnO) nanorod array decorated with iron oxide (Fe3−δO4) nanoparticles through a 1,4-phenylenebis(phosphonic acid) (PBA) linkage. We report on an original, easy to process, and versatile preparation method based on self-assembly mediated by specific interactions between metal oxides and phosphonic acid groups. This strategy allows grafting irreversibly Fe3−δO4 nanoparticles to ZnO nanorod arrays while preserving the pristine crystal structure of both inorganic entities. The structure and physical properties of such hybrid nanocomposites were investigated by means of a large panel of characterisation techniques: SEM, TEM, FTIR, XPS, SQUID and PL. The assembly mechanism was studied by varying several experimental parameters, such as the concentration of the ligand solution and nanoparticle suspension as well as the reaction time of ZnO arrays in the aforementioned suspensions. Finally, we show for the first time that the ZnO/PBA/Fe3−δO4 nanocomposite displays enhanced magnetic and optical properties as a result of dual synergy. We show unambiguously that a ZnO nanorod array acts as a very efficient anisotropic scaffold which favours unidirectional dipolar interactions and enhances the in-axis collective magnetic properties of Fe3−δO4 nanoparticles (a 10-fold increase of the coercive field in comparison to the same magnetic nanoparticles in the powder state). ZnO nanorods also benefit from Fe3−δO4 nanoparticles by enhancement of their optical properties in the visible range (a 2-fold increase of PL intensity with respect to bare ZnO nanorods) which we propose to result from photon energy transfer.