46443-AC10
Synthesis of Novel Heterostructured Catalyst Materials with Nanoscale Precision
Anisotropic nanocrystal heterostructure (NCHs) containing two or more distinct crystalline phases provide new opportunities to combine unique properties of nanoscale materials. However, there is no well-established understanding of how the interfaces form and the details of the nature of the heterojunctions (e.g. orientations of the facets at the interfaces, interfacial strain, etc.) are lacking. Such information is critical in developing routes to optimizing morphologies of NCHs in a desired manner. Our approach exploits coincidence site lattices of Fe3O4 and CdS to achieve anisotropic shapes. Only the favorable interfaces - mainly (111)/(111) for zinc blend CdS or (111)/(0001) for wurtzite CdS - where there are coincidence site lattices form the heterojunction interfaces. Even in these coincidence site lattices, there is a residual lattice mismatch and the interfacial strain increases with the size of each of the components. This increasing strain leads to limitations on the achievable size of nuclei of the second epitaxial phase. This limitation then in turns provides a new handle on control over the morphology of NCHs nearly at the single particle level. The underlying interfacial strain also provides some degree of control over the number of CdS nanocrystals that nucleate on the initial Fe3O4. At the very small sizes of ~3 nm or less (for the initial Fe3O4), the number of second phase heterogeneously nucleated is limited to one but the size that the second phase can reach is relatively large (> 10 nm). At our large size limit of ~20 nm of the initial Fe3O4, what appears to be a saturation of all favorable coincident site lattices with CdS nucleation is observed. But in this case, the maximum size of each CdS component is only ~3nm. At the intermediate sizes, variations in the number of CdS per Fe3O4 nanocrystal are often seen. To better understand and to better control the heterojunction formation especially in the intermediate size regime, a pseudo-separation of nucleation and growth of CdS on Fe3O4 has been achieved by multiple Cd/S reagent injection steps. At higher concentrations of reagents and lower temperatures of initial nucleation step, we have been able to achieve what appears to be a saturation of CdS on Fe3O4 nanoparticle surfaces. At lower concentrations of Cd/S reagents, high yield of dimer-type NCHs can be obtained. Additionally, by varying the CdS growth rate, we have been able to synthesize rod(s)-on-dot structures which enhance the spatial anisotropy of the NCHs. Interestingly, we have been able to obtain rod structures without the presence of capping molecule mixtures (e.g. trioctylphosphine oxide with hexylphosphonic acid) that are known to promote 1D growth of II-VI semiconductor nanocrystals. These results lead to new guidelines in tailoring morphology of NCHs providing versatility in their applications.
