Hematite nanocrystals

Morris, R.V. Agresti, D.G. Lauer, H.V.Jr. Newcomb, J.A. ShelfepT.D. Murali, A.V. (1989) Evidence for pigmentary hematite on Mars based on optical, magnetic, and Mossbauer studies of superparamagnetic (nanocrystal-line) hematite. J. Geophys. Res. 94 2760-2778... 

A dielectric oxide layer (eg. silica) is useful as a shell material because of the stability it lends to the core and its optical transparency. The classic method of Stober for solution deposition of silica are adaptable for coating of nanocrystals with silica shells.111 This method relies on the pH and the concentration of the solution to control the rate of deposition. The natural affinity of silica to oxidic layers has been exploited to obtain silica coating on a family of iron oxide nanoparticlcs including hematite and magnetite111 Such a deposition process is not readily extendable to grow shell layers on metals. The most successful method for silica encapsulation of metal nanoparlides is that due to Mulvaney and co-workers.114 In this method, the surface of the nanoparticles is functionalized with aminopropyltrimethylsilane. a bifunctional molecule with a pendant silane group which is available for condensation of silica. The next step involves the slow deposition of silica in water followed by the fast deposition of silica in ethanol. Fig. 13 shows the TEM images... 

Figure 27. Analysis of polarized EXAFS data, (a) EXAFS radial structure functions for irom oxide precipitates on quartz single crystal surfaces, r and m refer to the (1011 )and (1120) surface planes of quartz. The parallel and perpendicular refer to the polarization direction of the X-ray beam and thus the probe direction of the EXAFS scattering process, (b) Raw polarized EXAFS and fits for the same samples in (a), (c) Polarized stracture function simulations. Top radial stracture function for a single Fe atom within a 50-atom hematite crystal with [0001] orientation. Middle Same for 20-atom crystal. Bottom Weighted average of all Fe stracture func-tions in the 20-atom crystal. The analysis suggests highly textrrred hematite-like nanocrystals on the quartz surface but no epitaxial relationship. From Waychunas et al. (1999).

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