58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

Stainless steel is known for its resistance to oxidation and many forms are based on FeCr alloys. A characteristic of this material is the formation of a thin Cr₂O₃ passivating layer upon oxidation, which limits oxygen transport. Like bulk materials, the oxidation of nanomaterials is a critically important phenomenon, and the extent or type of oxide can determine function. This is especially true for nanomaterials made from first row transition metals. While the synthesis of such oxide nanomaterials is well established, approaches to passivate oxidation have varied, and recent studies have even welcomed oxidation as a synthetic tool to manipulate structure.

Figure 1: An idealized reaction scheme for the synthesis of crystalline a-Fe and the formation of a-Fe/Cr, a-Fe/Fe_xCr_1-x core-alloy NPs.

Building on our success in year 1, we focused our attention on this never before made nanoparticle. In year 2, we successfully synthesized these materials. Figure 1 shows a general strategy. First, iron nanoparticle cores (Fe) were synthesized via the well established thermal decomposition of iron(0)pentacarbonyl (Fe(CO)₅). Next, chromium shells (Cr) were deposited in a layer-by-layer process, which resulted in a shell thickness (t_S) that was controlled from t_S = 0.5 – 3.0 nm. We chose this layer-by-layer deposition in order to promote alloying by depositing an either thin layer (ca. ~0.5 nm/step) or a sub-monolayer of Cr, as described in year 1 and our recent work. The alloying was also favored because of the miscibility of binary FeCr, the known promotion of lloying at the nanoscale, and the thermal annealing steps.

Figure 2. Representative TEM micrographs for the a-Fe core (a), the Fe/Fe_xCr_1-x before oxidation (b), and after oxidation (c).

Figure 2a shows a set of TEM micrographs. The a-Fe core (a) was uniform with a diameter of d_C = 13.2 ± 1.0 nm. Figure 2b shows the TEM image for the Fe/Fe_xCr_1-x NPs after 16 Cr-depositions, and before oxidation. The NP grew to a new core + shell diameter (d_C+S) of 15.1 ± 1.5 nm, indicating a shell thickness of t_S Å 1 nm. Up to this point in the synthesis care was paid to limit any oxidation of the NPs in solution. After purification, the HDACl/OAm-capped Fe/Fe_xCr_1-x NPs were oxidized by opening the solution dispersed in ODE to air at T = 100 ^oC for 12h. After oxidation, the morphology of the nanoparticle consistend of a distinct inner core, surrounded by an area of decreased contrast, then subsequently by a thin shell of increased contrast. This core-void-shell morphology is shown in Figure 2c. The composition of these core-void-shell nanoparticles was confirmed by XPS as well as HRTEM and EDS analysis. The structural transformation was characterizated by powder XRD analysis. The novelty of this system, and the reason why we pursued it was the finding that the internal voids were controllable (in terms of thickness and position), which would make these materials novel new materials for gas absorption, catalysis, or even lithium ion battery materials. Within the next few years we believe that synthetic routes like these will become increasingly relevant for creation of new performance gains by design.

Investigating the Role of Polytypism in the Growth of Multi-Shell CdSe/CdZnS Quantum Dots

In a second project that derived from our first year work, we explored the role that polytypism has on the growth of multi-shelled, giant quantum dots (gQDs). Multi-shell growth was initiated at CdSe cores with either Zinc Blende or Wurtzite crystal structures. The shells consisted of a Cd_xZn_1-xS gradient that was deposited in a slow layer-by-layer SILAR process. The final gQDs had sizes of >15nm, with shapes and symmetry that were influenced by core type, and polytypic growth conditions. A systematic study of morphology and crystal structure change at each stage of shell growth was carried out by powder XRD. In both types of cores, shell growth was found to transition to Wurtzite, whereas the percentage of polytypism was shown to alter both morphology and optical properties.

Figure 3: The powder XRD results for shell growth at ZB-cores at shell layers (n) of 4 (i), 8 (ii), 12 (iii), and 18 (iv).

Using powder XRD we observed a considerable amount of polytypism occuring during the growth of giant quantum dots (gQDs). These gQDs are important for a number of energy transfer phenomona, like charge transfer in photovoltaics. Figure 3 shows the XRD patterns during CdZnS shell growth at layers of 4 (i), 8 (ii), 12 (iii), and 18 (iv). An interesting development can be observed. We find for instance a diffraction pattern that suggests a transformation from a ZB-type to a W-type shell, as indicated by the emergence of the <103> reflection. Additional analysis of the XRD shows that the <110> and <112> reflections (using either ZB- or W-cores) are exceptionally prominent. This suggests that crystal growth in either case is not proceeding in a purely crystalline manner ( i.e. solely W), and that ZB domains must be forming within the shell growth despite W-domains being the preferred crystal type. This phenomenon of coexistence of ZB and W domains in the same crystal has been reported earlier in core/shell QD systems as investigated by HRTEM, where it was shown that the core and shell have different crystal structures. However in this case the polytypism seems to be continuing throughout shell growth, and is a new finding.

Summary

These results demonstrate that we made considerable advances in this year. In the first system, we showed that the core-alloy approach can be extended to nanoparticles with 'stainless' interfaces for the first time. In the second system, we focused on the fundamental understanding of crystal evolution during the growth of giant quantum dots. Both of these materials have considerable futures in basic and applied energy research. The stainless nanoparticles with internal voids can be used in gas storage and catalysis, as well as a lightweight/stainless coating material, whereas the giant quantum dots will have their photophysical properties correlated with internal crystal structure and defects.