Manipulation and Characterization of Morphology and Composition in Mono- and Heterometallic Nanoparticles
This thesis investigates the use of simple synthetic manipulations to achieve control over morphologies and compositions within a range of metal nanoparticle (NP) systems. The materials of interest are relevant to ultra-high-density data storage devices, plasmonics, photothermal therapy and (electro)catalysis. This thesis is divided into two distinct sections.
The first section reports the synthesis of core@shell NPs for applications in data storage or plasmonics. The synthetic development of FePt@Fe3O4 by the thermal decomposition of Fe(CO)5 upon a preformed FePt seed with enhanced Fe3O4 shell thickness control whilst avoiding the primary-nucleation of Fe3O4 impurities was achieved. The coating of non-magnetic Pt cores established that a magnetic seed was not necessary for shell growth. A synthetic strategy was then developed to coat Cu0 plasmonic NPs with an Fe3O4 shell. This prevented oxidation of the particle core for up to four months, as established using powder X-ray diffraction, advanced electron microscopy and micro-extinction spectroscopy. Measuring a plasmonic response from Cu NPs after four months of benchtop storage either dispersed in wet hexane or as a powder in air is unprecedented. The relationship between Cu size and morphology in addition to Fe3O4 coating success during temperature and stabiliser variation were studied. Trends between coating success and stabiliser mixtures were uncovered and an oleic acid (OA)-oleylamine (OAm) mixture for thin and smooth Fe3O4 shells was optimised. By manipulating Cu-precursor reduction kinetics, larger or faceted Cu cores were prepared and coated. Advances highlighted the versatility of this coating method, which indiscriminately provides a thin and uniform Fe3O4 shell to a range of core sizes and morphologies.
The second section of this thesis focuses on branched NP morphologies with mono-, bi- and trimetallic compositions for applications in electrocatalytic oxygen reduction reaction (ORR). Monometallic Pt nanopods were achieved through simple high temperature polyol reduction, where the morphology could be controlled by the OA:OAm stabiliser ratio. In contrast to the prior art for this morphology, noble metal structure-directing agents (SDAs) such as silver and gold were not required, nor were temperature restrictions; SDA-free nanopods could be heated to 250 °C for >1 hour whilst maintaining the nanopod morphology. Time-dependent electron microscopy provided insight into the nanopod growth mechanism. The simple and reproducible methodology was successfully developed to form bimetallic MPt (M= Co, Ni, Fe) equivalents. An energy dispersive X-ray spectroscopy investigation of the effects of OA and OAm stabiliser mixture on single-particle NP composition was done. Whilst maintaining an equimolar mixture of 1st row transition metal (TM) and Pt precursor, nanopod composition was heavily dependent upon the relative concentration of stabilisers used. The at% of Pt could vary from 96-47 %, where high OA compositions favoured Pt-rich structures. The ability to prepare equivalent branched morphologies of FePt, CoPt and NiPt with comparable compositions enabled the influence of elemental composition upon ORR activity within non-pseudospherical materials to be elucidated. In contrast to pseudospherical NPs, where prior art suggests a preference for NiPt bimetallic alloys for boosting ORR activity, the observed trend was; FePt nanopods > CoPt nanopods >> Pt spheres > NiPt nanopods > NiPt polypods. The simple synthetic methodology was successfully applied to trimetallic nanopod equivalents, with the NP morphology controlled by stabilizer mixtures and also elemental composition. Trends in trimetallic nanopod composition mirrored those in bimetallic equivalents, where TM composition increased with decreasing OA fraction. The nanopod morphology for trimetallic equivalents was highly dependent upon relative TM composition. As an example, a Fe35Co15Pt50 trimetallic nanopod followed the same morphology trends as its FePt bimetallic equivalent as a consequence of its superior Fe composition compared to Co.
Finally, FePt octopods were prepared showing up to a 150 % increase in Fe composition whilst maintaining the octopod morphology by reducing the molar fraction of OA. This concept was also applied to FePt octopods with higher Fe compositions, leading to a previously unreported octopod-dumbbell (OPDc) FePt-Fe3O4 morphology through a one-pot route. Manipulating the temperature profile for the OPDc synthesis enabled the morphology of the FePt phase to be selectively altered, leading to another previously unreported dendrite-dumbbell FePt-Fe3O4 morphology. Structures were thoroughly characterised by advanced electron microscopy. The OPDc sample outperformed comparable FePt octopods and Pt-Fe3O4 or FePt-Fe3O4 spherical dumbbells by a minimum of 300% in ORR activity in alkaline media.